Troubleshooting and Repair of
Consumer Electronic Equipment

Preface

Author and Copyright

Author: Samuel M. Goldwasser

For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page.

Copyright © 1994-2004
All Rights Reserved

Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:

1.This notice is included in its entirety at the beginning.
2.There is no charge except to cover the costs of copying.

DISCLAIMER

Introduction

Getting Into Troubleshooting

Mostly, you will learn by doing. However, you do need to prepare.

There are many schools dedicated to electronics repair. Some of these are quite good. Many are not. This document, however, is written from the perspective of the motivated do-it-yourselfer, hobbiest, and tinkerer.

The Repair FAQs usually list suggested references for each area. Your local public or university library will probably have some of these or other repair oriented electronics books.

Above all read and understand the document: Safety Guidelines for High Voltage and/or Line Powered Equipment. Your life may depend on it. That fabulous large screen won't be of much use to you if you are dead.

Collect broken electronics and appliances from your friends, relatives, the dump, garage sales and flea markets, etc. Start on those that have been written off - you will screw up at first. We all did. As times passes, your batting average will improve. It may not happen overnight but it will happen if you apply yourself. There will be many relatively easy successes but the 'tough dogs' may make up for these triumphs. Don't let them get to you - not everything can be repaired. Sometimes, the basic design is flawed or someone before you messed up royally. Troubleshooting is like being a detective but at least the device is generally not out to deceive you.

Experience will be your most useful companion.

If you go into the profession, you will obtain or have access to a variety of tech tips databases. These are an excellent investment where the saying: 'time-is-money' rules. However, to learn, you need to develop a general troubleshooting approach - a logical, methodical, method of narrowing down the problem. A tech tip database might suggest: 'Replace C536' for a particular symptom. This is good advice for a specific problem on one model. However, what you really want to understand is why C536 was the cause and how to pinpoint the culprit in general even if you don't have a service manual or schematic and your tech tip database doesn't have an entry for your sick TV or VCR.

While schematics are nice, you won't always have them or be able to justify the purchase for a one-of repair. Therefore, in many cases, some reverse engineering will be necessary. The time will be well spent since even if you don't see another instance of the same model in your entire lifetime, you will have learned something in the process that can be applied to other equipment problems.

As always, when you get stuck, the sci.electronics.repair newsgroup will still exist!

Happy repairing!

Comments on How to Learn Repair

Here's how I see it:

By all means, do what you can to understand basic principles first. Your success will be much more likely when you understand how a device works. If you can, read Electronics Now and Popular Electronics, as well as Nuts and Volts (http://www.nutsvolts.com). Also have a look at the Radio Amateur's Handbook.

These periodicals are not carefully edited, unfortunately, and now and then things get into print that are simply wrong or misleading, but they are still useful; I learned quite a bit from their predecessors (Radio Craft and Radio News!).

I can't speak firsthand, but it might be a very good idea to become (eventually) a Certified Electronic Technician. Look up the I.S.C.E.T.

Hearsay and folklore sometimes indicate that you should replace a given part when certain symptoms occur, and in the case of frequent failures of such parts, this information might even be true. But that's no way to become a competent technician.

My personal take is that you have to know when to 'let go' of an hypothesis about what the cause of the trouble is. A tech. who persists beyond a certain point in his belief that such-and-such is causing the problem is stuck and spinning his wheels. (I'm sexist; I think women are far less likely to get stuck this way! I think it's a male trait. :)

Troubleshooting is a special field of knowledge and has its own special outlook on things. The device did work, after all.

Production testing and troubleshooting is different; you are likely to be the first person to apply power to a device, and the device has never worked before. If the assemblers aren't giving you excellent quality, you can have some remarkably-bizarre symptoms with a poorly laid-out board from solder shorts, for instance.

A variable toroidal autotransformer (universally known by what used to be a General Radio trade-name, Variac) is priceless for troubleshooting circuits that handle any amount of power and which are powered by the AC line. (Not all devices function at all at, say, half of rated AC input; I work on a poorly-designed amplifier that draws many amps at something like 70 volts with no signal and no load. Unfortunately, Variacs and their equivalents are horribly expensive, at least from some sources! If you get a used one, see that the contact area of the winding is undamaged; you might need to remove a knob and some covers to see it. If the knob is stiff, try some contact/control cleaner/lube; it did wonders for mine!

Learn how to operate a 'scope, and learn why you see what you do. I suspect that some techs are not too well-informed about what goes on inside a 'scope; learn from reliable sources!

Learn to use a digital multimeter, and an analog one as well; the latter is easily damaged if you don't know what you're doing, but it's a great trend indicator.

Learn to use a function generator, and use the triangle output as well! Nothing like a triangle to show a wee bit of clipping or limiting in an amplifier...

Learn how to solder! Solder is not an adhesive; it's a metallurgical bond, according to some sources I trust. It just about has to be with gold, at least! If you *really* want to learn soldering, NASA has developed training courses that will make you a disgustingly good solderer.

(From: Phillip R. Cline (pcline@iquest.net).)

I used to repair consumer electronics from VERY high end stereos down to lowly boom boxes. When repairing stereos there is no substitute for good troubleshooting techniques which come from empirical means. Good knowledge of circuit functions helps a great deal. VCRs are almost always a mechanical problem (70% or more in my experience). Audio stuff can be destroyed by the user and often times the design is just plain crap. All low and mid-fi Japanese stuff made within the last ten years isn't worth a crap from a design standpoint. Even a lot of the high-end stuff is junk. They have 71 volt rated caps running at 69 volts etc.... US and most European stuff is way better designed! There are exceptions. I once saw a Philips amp that had a transformer for the power amp supply that wasn't centertapped yet the supply was bipolar. They just rectified and filtered the AC with series caps and the common was the point they were connected to each other. This is fine if you rate the caps at more voltage than the power supply can deliver but these were rated at just over half the total voltage of the supply from rail to rail. One cap shorted and the other one exploded and launched the can sideways across the component side of the amp PC board. This basically did a nice job of depopulating the board along the ballistic path of the cap's can. I laughed for a good while after seeing this.

I gave up repairing stuff when the customers asked, and rightly so, why it costs $80 to fix something that costs $100 new. The OEM parts cost on some stuff was intended to make the customer go buy a new unit instead of repairing the old one. This basically made most of the stuff disposable.

My background was and still is as an electronic hobbiest so the theory of operation was not a big deal and circuit function wasn't either. I have a brother that was the person from whom I learned a great deal of what I know now about electronics.

Soldering ability cannot be overstressed in importance especially with SMT being very common nowadays. As for the guys that seem to be ripping you off in their pricing, they could be gouging you but most often the overhead in the shop and their cost on parts is the most likely cause of high pricing. While labor might seem high a great deal of repair can be accomplished in an hour by a competent technician and some shops have a flat rate for a given repair. This can work to the benefit of the shop sometimes and to the customer sometimes. Our shop was this way. We had the lowest pricing in town(Indianapolis) and the customers still bitched. Sometimes they would take their units after we gave them the price for labor and a estimate of parts cost. We didn't charge for estimates. They would storm out only to come back with their tail between their legs in a few days after checking around for labor charges elsewhere. Depending on their attitude we might go ahead with the repair. Often times we would decline by telling the customer that the other shops may have done something while checking the unit out.(This depended on the shop that the customer took the unit to.) Some of these places had some real winners for techs!! We really didn't feel like undoing some yoyo's handiwork just to get the unit back to it's original nonworking state!

An EE in electronics is useless by itself and will cause a lot of undue troubleshooting to the beginning tech. They will overlook the obvious easy stuff for some possible but unlikely fault. A few years of repairs under their belt though and they can find the most difficult electronic problems with relative ease.

The best way to become proficient is with hands-on training under an experienced tech. A good overall background in electronics doesn't hurt either.

(From: Michael Black (blackm00@CAM.ORG).)

I think one of the problems of home repair is fear. If you're willing to spend the money to have something repaired, then you may think that if you fiddled with it you may make it worse. On the other hand, if you are about to throw something out because it doesn't work, you have nothing to lose by playing around with it and trying to fix it. Or find some stuff other people have thrown out, and start with that.

You may not fix it, but your willingness to open the cover allows you a familiarity that you won't get from a book. You de-mistify the equipment, and by actually adjusting things and seeing the results, you will learn.

I picked up a VCR for cheap at a garage sale this past summer. I was buying it as a tuner for use with a monitor. The guy said it "must be the power supply because it keeps turning off". Actually, it kept turning off because the mechanics weren't working properly. By moving the parts by hand, I saw how they were supposed to work. With the first hand experience, the S.E.R FAQ made more sense than if I'd just read it first, and so did a book on VCR repair that I took out of the library. I saw that the belts needed replacing because I'd figured out how things were supposed to work, and saw that they weren't working that way.

(From: Malcolm MacArthur (malcolmm@rustic-place.demon.co.uk).)

I have two years' of an Electronic Engineering degree behind me (I gave up on the degree and became a computer programmer. ;) It has been little, if any, help. What you really need is experience... which you'll only gain by fiddling with things. I've been doing repairs since about age 13. After twelve years, I now have a fair success rate, but those first few years were not easy. Best thing to do is get hold of old equipment and just have a go with it (Beware of CRTs, though ;). Be warned, you may break quite a lot of stuff initially! But as the others have said, most of the problems are due to mechanical failures (including dry solder joints).

Tall repair stories time:

• The IDE hard drive, given free, with two broken pins. Fixed with bits of bent paper clip soldered in place of the original connectors.

• The record player which wouldn't play at the right speed, and there wasn't enough adjustment in the pot. Fix: cover belt spindle in Araldite, leave to harden, machine down with scalpel.

• The VCR which wouldn't play. Roughened up drive tire with sandpaper, did same to spindles, worked a treat. Still working fine after 3 years. :)

• Another VCR, rejected by a repair shop as uneconomical to repair, and given back to my friend's aunt. Took it apart. found nothing wrong, cleaned it, reassembled. Still working fine (I have no idea what the "fault" the shop found was).

• A computer monitor with incorrect colour balance. Fixed by adjusting the guns at the back WITHOUT an insulated screwdriver (Hairy!). I didn't have one, so used a normal screwdriver taped with insulating tape to a (dry) 1 ft. wooden spoon. Oh, because of poor design, the CRT kept threatening to impact with the mainboard when I had the back off, so I had to hold it at the front with my other hand. Visions of 25,000 V going straight through my chest made it all a bit scary. :)

• First ever repair (this is true!): taught my mother how to wire up a household (e.g. at power meter) fuse because I'd watched my Dad do it (he's good at this sort of thing as well. :) Age? Five.

• A TV with a broken tuner, had been to a repair shop twice, they'd given up on it. Received it for free, found out the composite video in worked fine, so ran 20 ft. of cable to a spare video downstairs (there was no aerial in my room, see). Must fix that tuner sometime...

• A CDROM drive which couldn't read near the edge of some discs. Disassembled whole unit, (eventually) reassembled it, found out that I was the spring which holds the CD down. Fixed by plugging into computer, running a cdrom and wiggling the spring with my finger until I felt the least vibration. Still works fine, although I am replacing it soon with a faster unit.

• The dropped TV with a crack at the corner of the mainboard. Repaired all the cracks by patching with wire, TV nearly blew up in my face (hence, beware of CRTs). Thank goodness for the fuses... this little incident put me off repairing TVs for over a year. It was eventually chucked, a failure.

Have fun.

THE Question: To Repair or Not to Repair

"Why bother with repair of VCRs (or anything else) when I can buy a new model for $79.95?"

Actually, I've seen prices as low as $39.95 for a promotion (but not requiring the purchase of anything else)!

or:

"This stuff may have been useful 5 years ago but now some/much of the material doesn't apply to newer VCRs."

While both of these deal with VCRs, it should be understood that it applies equally well to much other consumer electronics.

Depending on your background and interests, these statements may have some validity. Thus, the need for some objective (if possible) way of making a decision as to whether to bother at all, and whether to attempt the repair yourself.

So, when does it make sense to attempt *any* repair yourself rather than to toss the item in the trash or take it to a professional? People do this sort of stuff for several reasons:

1. For the challenge and rewards associated with success.

2. To save money.

3. Because they like a particular set of features or the controls or the styling of the equipment and don't want anything new!

4. To avoid cluttering land-fills.

It's quite difficult to suggest an approach in deciding when something is worth repairing. You have to decide how much the equipment is worth to *you* in terms of monetary, sentimental, or other value; how much time you are willing to put into a repair; and whether the failure represents a good excuse to upgrade! To what extent each of the factors is significant will also be determined by how much you enjoy troubleshooting and tinkering. If you'd rather be doing something else or keep thinking about all the time you are spending on this rather than something you can charge for, perhaps you should be doing that something else.

However, it is easier to identify specific situations where equipment probably *isn't* worth attempting to repair on your own (or possibly at all):

1. Serious damage due to water (especially salt water), fire or smoke. Even if the obvious faults can be found and corrected, there are likely to be latent failures just waiting to strike in a few weeks or months.

2. Lightning strike. Lightning is like the 900 pound gorilla - it can go anywhere it wants. Even if you can repair the obvious damage and get the equipment working, there could be hidden problems waiting to appear at a later time due to components that aren't totally fried but just weakened.

3. Extremely high electrical power surge like a 13,000 V feeder line falling across the 115 V wiring to your house. Similar comments as lightning apply.

Where any of these are covered by insurance, that is the best option where the settlement is at all reasonable. If the insurance company allows you to keep the damaged equipment, there is nothing to stop you from attempting repairs as a challenge - you may get lucky. But, it could also be a long drawn out and expensive frustration.

4. Serious physical damage, especially for equipment with mechanical parts like VCRs. It may be impossible to replace broken parts. Twisted metal can be straightened but there is a good chance there will still be erratic misbehavior.

5. Equipment where prior attempts at repair may have resulted in an undetermined number of new unidentified problems. At least when something breaks on its own, your only opposition is the device itself. But, if another person attempts a repair and they are a novice or just plain incompetent, the dumpster may be the best solution.

6. Equipment with known design or manufacturing problems. When we undertake a repair, one assumption that is usually made is that the equipment originally worked correctly and/or that the fault isn't something that was designed in before the name went on. :) For most things, this is a valid assumption. Even the famous RCA/GE/Proscan and Sony TV solder problems, while no doubt resulting in 100s of thousands of sets ending up in the trash, are repairable with modest effort at low cost. And, the result is a well performing reliable TV. However, some computer monitors may die when fed a particular scan rate or during boot when mated with a particular video card - a design flaw which may not have a (known) solution.

7. Newer throw-aways. I can pretty much guarantee that a $39.95 VCR isn't worth any effort unless the problem is obvious. This junk is built as cheaply as possible with a lot of plastic parts, no thought given to access for testing or repair, and with attention only to the short term bottom line. There has been no miraculous invention to reduce construction cost of the relatively complex VCR mechanism - it comes out of reliability.

8. Equipment like cell phones, pagers, and other modern wireless devices as well as cordless phones; PC mainboards, peripheral boards, and disk drives; TV set-top, cable boxes, satellite receivers, etc. It is essentially impossible to obtain service information on any of these so unless the problem is an obvious broken connector or broken trace on the printed circuit board, or possibly a dead power supply, forget it. You don't have the documentation, test equipment, rework equipment, or any chance of buying many of the repair parts in any case.

9. Any situation where safety would be compromised by your repair. For example, attempting to reconstruct a smashed microwave oven door or jerryrigging a flyback transformer that has serious arcing. Where items (1) to (3) are involved, one must very carefully inspect for any possible safety related damage (like charred insulation in hidden areas) that may not have affected operation.

10. If you really don't know what you are doing, leave it to a professional! Not only is it dangerous to be poking around inside many types of equipment if you don't even know what not to touch, there is a strong likelihood that such attempts will cause additional, possibly fatal damage to the circuitry. Even if the equipment can be repaired, the ultimate cost will end up be much greater than had you not done anything in the first place, both in terms of labor (troubleshooting and repair) and parts. If you can't justify a professional repair, just set it aside until you have gained more experience and can deal with the equipment safely (for you and it).

11. Finally, don't attempt to repair a piece of equipment for which you are not equipped in the tools or test equipment department. Attempting to remove a part from a multilayer printed circuit board without proper desoldering equipment will just make an unsalvageable mess. Guessing at a replacement part ("I heard that the flyback transformer is a likely cause for a dead monitor.") will just end up being frustrating and expensive (unless you've won the Lottery recently in which case maybe your luck is still holding).

In the good old days when life and electronics were simpler and you could count the total number of transistors in a TV on your hands and feet, service information was included with the equipment or was readily available either from the manufacturer or Sams Technical Publishing (formerly Howard Sams) as Sams' Photofacts (no relation to me). There are still Sams' Photofacts for many TVs at least, but for anything else, obtaining schematics may be impossible or even if they are available, the cost may be excessive. Paying $100 for a mediocre copy of a service manual for a computer monitor that can be replaced for $250 may not be justified.

One way to get an idea of your chances of success for popular brands and models is to search the archives of the USENET newsgroup sci.electronics.repair via Google Groups (formerly Deja.com/Dejanews. There are other public USENET archives but even though this archive keeps changing its name, I see little reason to use others which may come and go and provide less reliable coverage.) Where others have experienced - and repaired - similar problems, your chances of success are greatly increased. Then, if you have detailed symptoms, asking for suggestions on that newsgroup may also be beneficial, especially if you have already done some initial testing. If, on the other hand, the consensus from the newsgroup is that your problem is hopeless, then you may be able to save a lot of time and frustration by giving up immediately (or at least postponing your efforts until you have more experience.

What about older equipment?:

The basic technology of TVs and VCRs hasn't changed significantly in 10 or 15 years. Yes, there are convenience features like "auto clock set" which are supposed to make life easier but often don't (if the station transmitting the clock information has their clocks set wrong or uses a feed from a source in a different time zone!). But as far as picture and sound quality, that VCR from 10 years ago will be just as good or better than one purchased today. Any, it will almost certainly be better constructed and more maintainable.

For example, Panasonic VCRs from the mid to late '80s were solid machines that could be kept in shape with a bit of periodic maintenance (cleaning, rubber parts replacement) and repair of known problems (failed electrolytic capacitors in the power supply after 10 years or so). One could not expect that $39.95 special to provide such service. If it lasts through the warranty period, you're probably ahead of the game. I'd still take a middle age Panasonic over any new low to medium priced model. And, even the high-end VCRs may be based on flimsy chassis.

Case studies:

Here are 4 examples of equipment that I did eventually repair but where serious consideration should have been given to the dumpster. The following can be found described in more detail at in the document: Sam's Repair Briefs/

1. GE TV dropped: (From: Repair Brief #69: GE Portable Color TV - Dropped.)

   This TV had taken a nose dive off of a 4 foot shelf onto an unknown surface. And, of course, someone had probably attempted to operate after this with possible additional damage. While the exterior didn't show any major abuse, it was obvious that there was severe trauma as soon as the back was removed. The main circuit board was broken near the (heavy) flyback transformer. Several dozen traces were severed including some to surface mount parts.

   A repair shop would be unlikely to want to tackle this for several reasons: (1) the obvious repairs to circuit board traces would take a couple hours at least, (2) there could be unseen damage to the CRT in form of a distorted shadow mask and this wouldn't be known until the circuit board was fixed, and (3) any repair might not catch everything so future problems could develop.

   As it turned out, the only damage was to the circuit board and after 2 or 3 hours of soldering - and then finding additional traces to solder - the set was fixed, and has continued to operate reliably for several years.

2. GE TV with 'rivlets': (From: Repair Brief #59: GE 13AC1504W Color TV - Dead (with Other Problems)).

   In the early 1980s, some brilliant manufacturing engineer working for GE decided that a good way to save money on circuit boards would be to use what were dubbed 'rivlets' instead of actual plated through holes to connect top and bottom. A rivlet is basically a rivet which, the theory goes, is then soldered to the copper traces. That's the theory. In practice, due to the thermal mass of the rivet, soldering was never reliable. And, as a result of thermal cycling, cracks developed between the rivet and traces over time. Problems ranged from a dead set to loss of color depending on which rivlet happened to be unhappy on any given day.

   Attempting to repair just the problem rivlets was impossible because as soon as you found a bad one and soldered it, another in its vicinity would decide to fail. The only approach that worked was to reheat every one that could be located using a soldering gun. Since there were many dozens of these on the circuit board, this took quite awhile and it was easy to miss some. In fact, the only truly reliable repair would be to remove the solder from each rivlet, snake a bare wire through it, and solder the wire directly to the traces top and bottom. This repair would also take a couple hours and likely be too expensive for a small TV, though if the same chassis were used on a 27 incher, might be worth it.

3. CD player restoration: (From: Repair Brief #10: Pioneer PD5100 CD Player Trashed).

   Here is a case of a piece of equipment being partially destroyed by previous repair attempts. The Pioneer PD5100 is a basic solid CD player but this one had broken parts in the loading mechanism and was in unknown operational condition. If it were taken to a repair shop, the response would probably be something along the lines of: "Well, that certainly looks like a CD player.". It simply wouldn't be worth the time and effort to repair what was obviously broken with the possibility of finding more serious electronic problems after that.

   I had nothing better to do (!!) so decided to attempt to restore it to something usable. After repairing the mechanical damage, there was indeed a servo problem which ultimate required the replacement of a motor driver chip - for which I got lucky. The player would read the disc directory but was unable to seek to any track, even #1. One of the chips was getting hot. So, I replaced it and after servo alignment, the play problems were cured. If that hadn't worked, there was probably little more I could have done. Very likely, the servo chip was the original problem and the previous repair attempt created the mechanical mess.

4. Sony TV with bad butchered soldering: (From: Repair Brief #81: Sony KV-19TR20 Color TV - No Reception).

   The final example is of a Sony TV that had the infamous tuner/IF box solder problems. This is normally a fairly easy repair, especially for this particular model where the IF box (which was faulty in this case) is readily accessible without taking the whole thing to bits. Once repaired, like the RCA/GE/Proscan TVs with similar solder problems, the result is a solid reliable TV. However, the friend of a friend who had attempted to replace it, apparently used a Weller soldering gun to do the fine soldering, leaving nearly every pad detached or missing. Fortunately, only the pads appeared to have suffered and after 20 minutes and several jumper wires, this one was healthy again.

Repairs for the novice:

It would be way too easy to poison your future outlook on servicing by attempting to repairs multiple times and failing or making things worse.

Equipment that is good to learn on because there will likely be immediate or at least ultimate gratification might include: small appliances, power tools, remote controls, and basic audio equipment like tape decks and low power amplifiers (not big power amps!). And, while electronic troubleshooting of CD players and VCRs is definitely for the advanced course, they often have problems that can be easily remedied by a proper cleaning and/or general maintenance. Electronic problems are tough to diagnose but most are mechanical. Microwave ovens are generally easy to repair but due to the very serious safety issues, I'd suggest holding off on these unless you are experienced in dealing with high voltage high power equipment.

With reasonable care, PC troubleshooting involving basic swapping of components, can also be rewarding. But, don't expect to repair a mainboard with a peculiar failure of IRQ2 (unless you find a lockwasher that ate through to some PCB traces!).

Intermediate level troubleshooting and repair would add TVs since service information in the form of Sams' Photofacts is available for the majority of popular models. Video (not computer) monitors are also straightforward to deal with. And perhaps, audio amplifiers and receivers.

For those just starting out, there are some types of equipment to avoid (beyond those mentioned above). One in particular is modern computer monitors. With their wide scan rate range, microprocessor control, need for decent test equipment, dangerous voltages, and the general difficulty in obtaining service information, even professionals will stay away from many of these - particularly no-name or non-major brand models. Except for obvious problems like bad solder connections, a blown fuse (replace ONCE only, might have been a power surge), or the need for degaussing, they may not be worth the frustration, certainly not as your first project. TVs are not only much simpler than computer monitors, but as noted, complete service information is usually available.

If You Decide That You Don't Want to Bother Repairing Something

Don't just toss it in the trash. See if a local charity like the Salvation Army or Goodwill accepts broken appliances and electronics. They may have someone on staff who can perform at least simple repairs and then resell the item. Not only will this reduce clutter in the land fill, you may benefit on your taxes (and in the good deeds department). However, it really isn't proper to do this if you have already worked on the item and given up or reduced it to a pile of slag!

Smoking Around Electronic Equipment

If you still doubt the harmful effects of the chemical compounds in tobacco smoke on your health and that of others around you, whatever I say below probably won't matter and you may want to skip it since it may upset you. However, perhaps, you worry more about your fancy, costly, finely tuned electronic entertainment and computer equipment. In that case, read on.

The several hundred chemical compounds found in tobacco smoke have the following effects on electronic equipment. What isn't trapped in your lungs or in the lungs of those around you:

• coats the precision optics of CD and DVD players, CDROM and DVDROM drives, and other optical disc/k equipment AND the media they use.

• coats the read/write heads of floppy disk drives, Zip drives, tape drives, AND the media they use.

• coats the tape path of VCRs and audio decks including the audio, video, and control heads AND the cassettes and tape inside.

• coats mechanical parts and promotes the loss of lubrication in all equipment.

• may contribute to deterioration of plastic and rubber parts.

• coats the screens of TVs and monitors, display windows of VCRs and other devices, and the outside and inside of everything eventually resulting in ugly brown discoloration and a horrible stench.

The resulting film WILL eventually cause problems and is very difficult to remove. Damage done due to chemical action may require the replacement of costly parts. Increased maintenance will be needed or the equipment may simply fail before its time and not be worth fixing. Contamination will often find its way into critical places that are not accessible and to media which is irreplaceable.

When someone trys to get me to look at something that has been in a smoker's residence (I know because it will reek of stale tobacco smoke essence), my first inclination is to put it in a sealed bag to go out with the garbage. (I have been known to drop portable TVs directly into the nearest trash can under these circumstances.) If this isn't an option, my next objective is to get it evaluated and repaired or refused as quickly as possible. However, my concentration may not be at its peak for such equipment! It is a good thing that I don't need to do this for a living - I would have to refuse service to a good portion of the world's population :-(.

So, now you have a few more reasons to give up the stupid, disgusting, filthy, obnoxious, inconsiderate of others, costly, dangerous, killer habit!

Sorry, end of editorial. :-)

General Safety Considerations

• Electrical shock hazard from TV, computer and other video monitors, microwave ovens, the switchmode power supplies in some VCRs and computer peripherals, electronic flash units, some parts of audio equipment, hand and stationary power tools, large appliances, and even many small line powered appliances.

• Mechanical hazards from the moving parts of various appliances, computer peripherals, hand and stationary power tools, and especially gasoline powered yard equipment.

• Risk of CRT implosion from equipment using large CRTs.

• Vision hazards from the lasers in CD players and CDROM drives, DVD players and DVDROM drives, other optical data storage devices, and laser disc players.

See the document: Safety Guidelines for High Voltage and/or Line Powered Equipment for general safety information.

See the SAFETY sections of the documents dealing with your equipment for additional safety information for your equipment.

Basic Troubleshooting

Some of My Rules of Troubleshooting

1. Safety first - know the hazards associated with the equipment you are troubleshooting. Take all safety precautions. Expect the unexpected. Take your time.

2. Always think 'what if'. This applies both to the analytic procedures as well as to precautions with respect to probing the equipment. When probing, insulate all but the last 1/8" of the probe tip to prevent costly shorts. (If I had a nickel for every time I have been screwed not following this advice...)

3. Learn from your mistakes. We all make mistakes - some of them can be quite costly. A simple problem can turn into an expensive one due to a slip of the probe or being over eager to try something before thinking it through. While stating that your experience in these endeavors is measured by the number of scars you have may be stretching the point, expect to screwup - we all can point to that disaster due to inexperience or carelessness. Just make it a point not to make the same mistake again.

4. Don't start with the electronic test equipment, start with some analytical thinking. Many problems associated with consumer electronic equipment do not require a schematic (though one may be useful). The majority of problems with VCRs, CD players, tape decks, and answering machines, are mechanical and can be dealt with using nothing more than a good set of precision hand tools; some alcohol, degreaser, contact cleaner, light oil and grease; and your powers of observation (and a little experience). Your built in senses and that stuff between your ears represents the most important test equipment you have.

5. If you get stuck, sleep on it. Sometimes, just letting the problem bounce around in your head will lead to a different more successful approach or solution. Don't work when you are really tired - it is both dangerous and mostly non-productive (or possibly destructive).

6. Many problems have simple solutions. Don't immediately assume that your problem is some combination of esoteric complex convoluted failures. For a TV, it may just be a bad connection or failed diode. For a VCR, it may just be a bad belt or idler tire - or an experiment in rock placement by your 3-year old. For a CD player, a dirty lens or need for lubrication. Try to remember that the problems with the most catastrophic impact on operation - a dead TV or a VCR that eats tapes - usually have the simplest solutions. The kind of problems we would like to avoid at all costs are the ones that are intermittent or difficult to reproduce: subtle color noise, the occasional interference, or the dreaded horizontal output transistor blowing out every 3 months syndrome.

7. Whenever possible, try to substitute a working unit. With modular systems like component stereos and computers, narrowing down a problem to a single unit should be the first priority. This is usually safe to do in such cases and will quickly identify which unit needs work. This same principle applies at the electronic or mechanical parts level. Note that there is the possibility of damaging the known good part by putting it into a non-working device or vice versa. This risk is most likely with the power circuitry in amplifiers, TVs and monitors, power supplies, etc. With appropriate precautions (like the series light bulb) the risk can be minimized.

8. Don't blindly trust your instruments. If your get readings that don't make sense, you may be using your equipment in a way which is confusing it. DMMs are not good at checking semiconductors in-circuit or the power transistor you are testing may have a built in damper diode and/or base resistor. Your scope may be picking up interference which is swamping the low level signal you are searching for (TVs and Monitors, or low level circuits in VCRs and CD players). Your frequency counter may be double triggering due to noise or imperfect signal shape.

9. Realize that coincidences do happen but are relatively rare. Usually, there is a common cause. For example, if a TV has no vertical deflection and no picture, it is much more likely that a common power supply output has failed than for parts in both the deflection and video subsystems to be bad. In other words, first look for a common root cause rather than trying to locate bad parts in separate circuits.

   Exceptions include lightning, power surge, dropped, water, or previous repair person damaged equipment. However, multiple electrolytic capacitors in older equipment may be degrading resulting in failures of unrelated circuits. Determine if all the problems you are troubleshooting have just appeared - see below. It is very common to be given a device to repair which has now died totally but prior to this had some behavior which you consider marginal but that was not noticed by the owner.

10. Confirm the problem before diving into the repair. It is amazing how many complaints turn out to be impossible to reproduce or are simple cockpit error. It also makes sense to identify exactly what is and is not working so that you will know whether some fault that just appeared was actually a preexisting problem or was caused by your poking. Try to get as much information as possible about the problem from the owner. If you are the owner, try to reconstruct the exact sequence of events that led to the failure. For example, did the TV just not work when turned on or were there some preliminary symptoms like a jittery or squished picture prior to total failure? Did the problem come and go before finally staying bad for good?

11. Get used to the idea of working without a schematic. While service info for TVs is nearly always available in the form of Sams' Photofacts, this is hardly ever true of other types of equipment. Sams VCRfacts exist for less than 10 percent of VCR models and only the older ones include anything beyond (obvious) mechanical information. While a service manual may be available from the manufacturer of your equipment or another Sams-like source, it may not include the information you really need. Furthermore, there may be no way to justify the cost for a one time repair. With a basic understanding of how the equipment works, many problems can be dealt with without a schematic. Not every one but quite a few.

12. Whenever working on precision equipment, make copious notes and diagrams. You will be eternally grateful when the time comes to reassemble the unit. Most connectors are keyed against incorrect insertion or interchange of cables, but not always. Apparently identical screws may be of differing lengths or have slightly different thread types. Little parts may fit in more than one place or orientation. Etc. Etc.

13. Pill bottles, film canisters, and plastic ice cube trays come in handy for sorting and storing screws and other small parts after disassembly. This is particularly true if you have repairs on multiple pieces of equipment under way simultaneously.

14. Select a work area which is wide open, well lighted, and where dropped parts can be located - not on a deep pile shag rug. The best location will also be relatively dust free and allow you to suspend your troubleshooting to eat or sleep or think without having to pile everything into a cardboard box for storage.

15. Understand the risk of ESD - Electro-Static Discharge. Some components (like ICs) in solid state electronic devices are vulnerable to ESD. There is no need to go overboard but taking reasonable precautions such as getting into the habit of touching a **safe** ground point first.

    WARNING: even with an isolation transformer, a live chassis should **not** be considered a safe ground point. This applies mostly to TVs, computer and video monitors, some AC operated strobe lights, and other line connected devices. You shouldn't be touching components with the device powered and plugged in (at least, not until you really know what you are doing!). Once unplugged, sheet metal shields or other ground points should be safe and effective.

Some Quick Tips or Rules of Thumb

• Problems that are erratic or intermittent - that come and go suddenly - are almost always due to bad connections - cold solder joints or internal or external connectors that need to be cleaned and reseated. It is amazing what a large percentage of common problems fall into the category.

  Pay particular attention to areas of the circuit board where there are large and/or high power components, connectors, or evidence of discoloration or actual charring due to excessive heat. Your eyeballs, a bright light, and magnifier will be the most useful test equipment for this purpose!

• Problems that change gradually - usually they decrease or disappear - as the equipment warms up are often due to dried up electrolytic capacitors.

  While capacitors will occasionally leak making diagnosis easy, in most cases, there are no obvious signs of failure. (Note: Don't be misled into thinking that the adhesive often used to anchor large capacitors and other components to the circuit board is leakage.) The most useful testing device for electrolytic capacitors is an ESR meter. However, heating suspect caps with a hair dryer may get the equipment going for the purposes of making a diagnosis. See the document: Capacitor Testing, Safe Discharging and Other Related Information.

• Problems that result in a totally dead unit or affect multiple functions are generally power supply related. These are usually easy to fix.

  Common failure items are the large hybrid power regulator ICs used in many VCRs and TVs, diodes and transistors, and remarkably - high value resistors that open up.

• Catastrophic failures often result in burnt, scorched, cracked, exploded, or melted components, or similar catastrophic consequences. However, some components run hot by design and slight discoloration on the circuit board in their vicinity, while not desirable, may be normal.

  Use your senses of sight and smell for the preliminary search for such evidence.

• Listen for signs of arcing or corona - snapping or sizzling sounds. A component on the brink of failing due to overheating may provide similar audible clues.

  Some discharge sounds are normal for a TV or monitor when powered on or off and occasional sounds of thermal expansion are nothing to worry about. The flyback, yoke, or other (usually) magnetic component may also emit a buzz or while constantly or intermittently without any other symptoms or implication of impending doom. However, repeated loud snaps or a sizzling sounds accompanied by the smell of ozone should be dealt with immediately since they can lead to more serious and expensive consequences.

• Most VCR problems are mechanical in nature. Worn or deteriorated rubber parts, gummed up lubrication, or abuse (bad tapes or toy or penut butter and jelly sandwich storage.).

  For any problem but a totally dead VCR, a check should be made for dirty or worn mechanical parts before even thinking about electronic problems or trying to locate a schematic - especially if the unit hasn't been cleaned in a few years.

• Many CD player problems are mechanical - dirty lens, worn or oily drawer belts, dirt/gummed up grease on sled tracks/gears, bad/partially shorted spindle or sled motor. Power problems with portables seem to be common as well. No matter what the symptoms, always make it a habit to clean the lens first - many peculiar failure modes are simply due to a dirty lens. Actual laser failure is relatively uncommon despite what the typical service shop may claim. CD players are also remarkably robust. Optical alignment should never be needed under normal conditions of operation.

• TV and monitor problems are very often power supply or deflection related. These tend to have obvious causes - blown posistor, rectifier diodes, filter capacitor, HOT, or chopper. Flyback with shorted windings or shorts between windings or in the voltage multiplier (if used) or screen/focus divider network are also common. Where the HOT or chopper is involved, operation should be observed after the repair as components in the vicinity may cause the new parts to fail. HOTs should generally not run hot. If they do, check for weak drive, excess B+, etc.

• Microwave oven problems are almost always power related. Faulty components in the microwave generator - magnetron, HV diode, HV capacitor, HV transformer - are relatively easy to identify. Sometimes, components on the primary side can cause baffling symptoms like the misaligned interlock switches that blow fuses or the weak triac that causes the oven to blow the main fuse only when the cycle *ends*. Control problems may be due to a spill in the touchpad, dried up electrolytic capacitors in the low voltage power supply, or failure confused state due to a power surge.

• Ink-jet printers are extremely reliable electrically. Look for simple problems such as caked ink in the 'service station' area, misaligned print-head contacts, or a nearly empty cartridge when erratic printing problems develop.

• Laser printers tend to develop problems in the fuser, scanner, or power control modules. These are often simple like a burned out lamp, bad motor, or bad connections.

• Turntables or record changer problems are very likely to be due to gummed up grease.

• Problems with audio tape decks like VCRs are mostly mechanical. Similar solutions apply. Where one channel is out, suspect a broken wire at the tape head before a bad chip.

• Telephone line connected equipment like modems and phones are susceptible to phone like surges. Where a device seems to respond to user commands but does not dial or pickup, suspect a blown part near the phone line connector.

• Sam's Magic Spit(tm). This approach - using a moistened finger to probe LOW VOLTAGE CIRCUITS has come to the rescue many times. Touching various parts of a circuit from the solder side of the board in an attempt to evoke some sort of response can work wonders. Once an suspect area has been identified, use a metal probe or nail to narrow it down to a specific pin.

  The reason this works is that the reduced resistance of your moist skin and your body capacitance will change the signal shape and/or introduce some slight signal of its own.

  • Logic circuits - marginal timing or signal levels will result in a dramatic change in behavior with a slight 'body' load. It has been possible to locate a race condition or glitchy signal on a 305 pin PGA chip using this approach in less time than it would have taken to roll the logic analyzer over to the system under test. Signals which have proper levels and timing are gnerally remarkably immune to this sort of torture.

  • Analog circuits - behavior can again be altered. In the case of audio amps, probing with a finger is just as effective as the use of a signal injector - which is what you are doing - and the equipment is always handy. By evoking hum, buzz, clicks, and pops, locating the live or dead parts of a circuit is rapid and effective.

  • Unknown circuits - where no schematics are available, it may be possible to get the device to do something or locate an area that is sensitive to probing. The function of a section of circuitry can often be identified by observing the effects of touching the components in that area.

    For example, I was able to quickly identify the trigger transistor of in a wireless door bell by using my finger to locate the point that caused the chimes to sound. This quickly confirmed that the problem was in the RF front end or decoder and not the audio circuitry.

  • Bad bypass capacitors - touching the power/signal side of a good bypass cap should result in little or no effect. However, a cap with high ESR and/or reduced uF will not be doing its job bypassing the pickup from your finger to ground - there will be a dramatic effect in audio or video systems.

  Don't get carried away - too much moisture may have unforeseen consequences.

  Depending on the condition of your skin, a tingle may be felt even on low voltage circuits under the right conditions. However, this is pretty safe for most battery operated devices, TTL/CMOS logic, audio equipment (not high power amps), CD players, VCRs (not switching power supply), etc.

  WARNING: Make sure you do this only with LOW VOLTAGE circuitry. You can easily fry yourself if you attempt to troubleshoot your TV, computer monitor, photoflash, or microwave oven in this manner!

On-Line Tech-Tips Databases

A tech-tips database is a collection of problems and solutions accumulated by the organization providing the information or other sources based on actual repair experiences and case histories. Since the identical failures often occur at some point in a large percentage of a given model or product line, checking out a tech-tips database may quickly identify your problem and solution.

In that case, you can greatly simplify your troubleshooting or at least confirm a diagnosis before ordering parts. My only reservation with respect to tech-tips databases in general - this has nothing to do with any one in particular - is that symptoms can sometimes be deceiving and a solution that works in one instance may not apply to your specific problem. Therefore, an understanding of the hows and whys of the equipment along with some good old fashioned testing is highly desirable to minimize the risk of replacing parts that turn out not to be bad.

The other disadvantage - at least from one point of view - is that you do not learn much by just following a procedure developed by others. There is no explanation of how the original diagnosis was determined or what may have caused the failure in the first place. Nor is there likely to be any list of other components that may have been affected by overstress and may fail in the future. Replacing Q701 and C725 may get your equipment going again but this will not help you to repair a different model in the future.

One alternative to tech-tips databases is to search via Google Groups (formerly Deja.com/Dejanews) for postings with keywords matching your model and problem and the newsgroup sci.electronics.repair. See the section: Searching for Information from USENET Newsgroups.

Please see the document: On-Line Tech-Tips Databases for the most up to date compilation of these resources for TVs, VCRs, computer monitors, and other consumer electronic equipment.

Getting Inside Consumer Electronic Equipment

Yes, you will void the warranty, but you knew this already.

Hint: The crowbar and 12 pound hammer are *laset* resorts! Really :-).

Manufacturers seem to take great pride in being very mysterious as to how to open their equipment. Not always, but this is too common to just be a coincidence. Opening the equipment non-destructively may be the most difficult and challenging part of many repairs!

A variety of techniques are used to secure the covers on consumer electronic equipment:

1. Screws. Yes, many still use this somewhat antiquated technique. Sometimes, there are even embossed arrows on the case indicating which screws need to be removed to get at the guts. In addition to obvious screw holes, there may be some that are only accessible when a battery or cassette compartment is opened or a trim panel is popped off.

   These will often be of the Philips variety. (Strictly speaking, many of these are not actual Philips head screws but a slight variation. Nonetheless, a Philips screwdriver of suitable size will work on them.) A precision jeweler's screwdriver set including miniature Philips head drivers is a must for repair of miniature portable devices.

   Sometimes, you will find Torx or a variety of security type fasteners. Suitable driver bits are available. Sometimes, you can improvise using regular tools. In the case of security Torx, the center post can usually be broken off with a pair of needlenose pliers allowing a normal Torx driver to be used. In a pinch, a suitable size hex wrench can substitute for a Torx driver. Places like MCM Electronics carry a variety of security bits.

2. Hidden screws. These will require prying up a plug or peeling off a decorative decal. It will be obvious that you were tinkering - it is virtually impossible to put a decal back in an undetectable way. Sometimes the rubber feet can be pryed out revealing screw holes. For a stick-on label, rubbing your finger over it may permit you to locate a hidden screw hole. Just puncture the label to access the screw as this may be less messy then attempting to peel it off.

3. Snaps. Look around the seam between the two halves. You may (if you are lucky) see points at which gently (or forcibly) pressing with a screwdriver will unlock the covers. Sometimes, just going around the seam with a butter knife will pop the cover at one location which will then reveal the locations of the other snaps.

4. Glue. Or more likely, the plastic is fused together. This is particularly common with AC adapters (wall warts). In this case, I usually carefully go around the seam with a hacksaw blade taking extreme care not to go through and damage internal components. Reassemble with plastic electrical tape.

5. It isn't designed for repair. Don't laugh. I feel we will see more and more of this in our disposable society. Some devices are totally potted in Epoxy and are throwaways. With others, the only way to open them non-destructively is from the inside.

The most annoying (to be polite) situation is when after removing the 18 screws holding the case together (losing 3 of them entirely and mangling the heads on 2 others), removing three subassemblies, and two other circuit boards, you find that the adjustment you wanted was accessible through a hole in the case just by partially peeling back a rubber hand grip! Been there, done that. :(

And on the still lighter side, from an IBM maintenance manual, circa 1925 (displayed in the Chicago Museum of Science & Industry):

"All parts should go together without forcing. You must remember that all the parts you are reassembling were disassembled by you. Therefore, if you can't get them together again, there must be a reason. By all means, do not use a hammer."

When reassembling the equipment make sure to route cables and other wiring such that they will not get pinched or snagged and possibly broken or have their insulation nicked or pierced and that they will not get caught in moving parts. Replace any cable ties that were cut or removed during disassembly and add additional ones of your own if needed. Some electrical tape may sometimes come in handy to provide insulation insurance as well.

For those hard-to-open LCD panels:

(From: Onat Ahmet (onat@turbine.kuee.kyoto-u.ac.jp))

The LCD display housings are usually secured by plastic catches built into the case. They still may have a couple of screws that are positioned in the most innovative places! Obvious places are sides of the display, and under stickers (rub your finger over a sticker and see if you can feel the hole for a screw). Also, try to look around the hinge connecting the LCD to the main housing. Look with the LCD closed, and also open; rotating open the housing might hide some screws from view. Expect it to be awkward! BTW, do not forget small hatches, that do not look like one!

After that, it is patience, and knowing the right place to twist the case to pop it open. Try not to use screwdrivers; they leave unsightly marks along the seam.

Also, if it is your own unit, and you break a few of the catches along the way, do not worry; you can put the housing back together with a few spots of adhesive.

How to Build Obsolescence In Before the Name Goes On

These are in no particular order.

(Portions from various people including Alan Liefting (aliefting@ihug.co.nz), Heath Young (heathryoung@hotmail.com), Craig Osborn (eelcr@worldnet.att.net), Phil Allison (bilup@bigpond.com), Franc Zabkar (franczabkar@dingoblue.net.au), and Sam.)

1. Place metal shields around the power supplies without holes for air circulation. This will ensure power supply capacitors will dry out and make the power supply fail prematurely. And make sure not to use high temperature capacitors. Keeping the electros hot makes the ESR lower so cheaper ones will do.

2. Push as many capacitors as possible into the sides of heatsinks, power transistors, regulators, and power resistors - anything hot!

3. Do not bother with putting thermal conducting paste between heat sink and transistor. It only oozes and dries out anyhow.

4. Do not bother to solder the heavy components or joints subject to thermal cycling. Touch up selected connections by hand? You have to be joking.

5. The following components can be left out during manufacture: protection zener diodes, VDR's, spark gaps, decoupling capacitors, ferrite beads etc. If it works most of the time without them, the expense can't be justified.

6. Keep all factory tests as short as possible. If there isn't a dirt cheap solution to any problems found, don't test for them.

7. Buy the cheapest components from manufacturers who cannot guarantee the longevity of their product. It is easy to have a CRT fail within three years by using impure materials. This will make the cathodes oxide and reduce the emissions giving a dull, hard to read picture. The customer won't realize their TV or monitor has deteriorated but will see newer models with bright vibrant colors and just have to buy one.

8. Fit the so-called head cleaning device to VCR's. This will ensure numerous service calls increasing the customer frustration which will lead them to buy a new product. There is one born every minute.

9. Dispense with EMI/RFI components in the power supplies. As well as producing unacceptable interference, this will also mean that the power supply is more prone to damage from voltage spikes. These problems are by definition the responsibility of the customer anyhow so the result will only be a positive effect on the bottom line.

10. Use the softer plastics rather than the more wear resistant phenolic for kettles and toasters. This will guarantee the consumer will purchase a new product to replace the shabby appliance.

11. Make the covers of heat generating appliances out of painted instead of chrome-plated sheet metal. The paint will darken unevenly with use resulting in an old looking appliance in need or replacement.

12. Do not use spot welding on your heating elements. Spot welding makes a reliable connection. Crimped connections will fail within a short period. This is good (for the manufacturer) since spare parts are not available. The consumer will have to buy a new product and have been conditioned to not expect appliances to be repairable. People like to buy new things anyhow.

13. Install an Appliance Leakage Circuit Interrupter (ALCI) on the cord of the appliance. This is a one time use GFCI (or what I call a GFCK - Ground Fault Circuit Killer). When constructed with suitably cheap parts, it, rather than the appliance will fail but the customer will be informed that they must have gotten the appliance wet.

14. Use the cheaper carbon film resistors instead of the more reliable metal film resistor with better voltage ratings. What the customers can't see won't bother them.

15. Fasten heavy (or just random) parts to the PCB with that white/beige glue that decays after awhile to brown and conductive causing all sorts of nasty problems.

16. Use a design style which will rapidly look dated. This is another ploy in order to have the consumer purchase the latest style regardless of whether the item is working or not. People are slaves to fashion - and they love slaves.

17. Reduce the number of service agencies and the number of products the service agencies will repair for you. This will increase the sale of new products by frustrating the consumers efforts in order to have their item repaired.

18. Fit fast blow fuses inside sealed double insulated appliances that will fail from metal fatigue. (Why did they do this??? - it was sealed with heatshrink onto one of the wires?!)

19. Manufacture equipment as a collection of modules - example. Optics train in certain models of projectors - instead of replacing just a hot mirror ($15 or so) you have to replace the lot! (All optics and LCD screen etc as a module) Now, try getting the poor serviceman to explain why the projector that cost the customer $5,000 costs $3,000 to fix.

20. Encourage consumers to purchase the cheapest items so when someone does make a good product, no one will buy it, and the company will go out of business.

21. Make your spare parts very expensive or do not bother with having spare parts available.

22. When the prototype has been built and working correctly, start removing components until it fails to work properly. Then manufacture it with the minimum parts required to make it work. In mid production run, change selected over designed components to their border line equivalents.

23. If a government regulating agency forces a major recall due to a severe fire hazard, close the current brand name, and start a new brand with a redesigned outside shell that emulates an ergonomic look but with the same internals as before. Add words like "New, Improved, Upgraded, and Leading Edge" to the new marketing slogan.

24. Don't make service manuals or circuit diagrams available. If you do, ensure that only your service agents get one, or price them so highly that the job becomes uneconomical. Think of it this way: Instead of buying the equipment which was the usual way of doing business in the past, the purchase price is now actually only a rental fee for the duration of the warranty. After that, the manufacturer may let the smoke out at any time without notice and thus there is no reason to support repairs. :)

25. Don't identify yourself as the manufacturer. Hide behind your OEM resellers who of course will have no facilities to repair anything except by total replacement.

26. Scrape the numbers off your IC's, or have them stamped with obscure in-house part numbers. You don't want to make repairs too easy for the end user or for a third party repairer who would otherwise be able to source these $1 garden variety parts from any electronics store.

27. Don't stock parts that could reasonably be expected to fail or wear out like battery contacts, flex PCBs, remote control keypads, curly cords. If you must supply them, then do so as part of a complete, uneconomically priced assembly.

28. Use flimsy non-standard (and hence unavailable) connectors for attaching expensive proprietary AC adaptors, chargers, and accessories.

29. Use expensive sealed units which cannot be opened for service without being damaged, and which are powered by an inexpensive internal battery.

30. Have a single solid wire between circuit boards and anything that moves, e.g. sockets at the back of VCRs, or almost any part of a remote control, especially the battery connection. This means that the soldered connection will give up.

31. Have flimsy hinged flaps on a remote control that break after a short time.

32. On a remote control, have rows and rows of identical buttons that have functions that are almost never needed. Also omit functions that would be very handy.

33. Make sure the coax to the tuner of the VCR or TV has no proper mechanical anchorage to the chassis so it takes little more than the house cat to trip on the cable and yank the active innards from the tuner.

34. Make sure all large metal components (particularly the audio in/out sockets at the rear of amplifiers) are only soldered to the same degree as the flimsy IC pins - just soldered sufficiently to pass the production final test.

35. Don't use fiber reinforced plastic gears if you can use plain plastic types.

36. Never use keys to lock shafts to plastic pulleys.

37. Always use etched copper film thin ribbon board interconnects wherever there is maximum flexing such as in laptop screen hinges.

38. Never make rubber drive belts accessible by just removal of the back plate.

39. Use as many non-standard components as possible.

40. Don't use 2 boards if you can mount components on both sides of one board.

41. Always use the smallest possible wattage for a resistor: if the dissipation is 2 watts, then use a 2 watt resistor.

42. Take advantage of exciting new legislation in creative ways to make it illegal for others to make compatible user-replacable parts. No more third party batteries, inkjet cartridges, etc.

43. Add useless complexity to accessories like batteries and inkjet cartridges to make it more difficult for third parties to manufacture replacements. Include warnings about the risks of using third party accessories.

44. Shorten product introduction cycles so that by the time the end-user requires a repair to the item, it's already obsolete.

45. Set the cost of the repair to greatly exceed the cost of a new model of the same type of equipment, which, you just happen (surprise surprise) to have in stock and for sale.

Tools, Test Equipment, and Other Stuff

Hand Tools

Some basic hand tools.

• Screwdrivers of all types and sizes including straight, Philips, Torx. Security bits for some video games, PS2s, etc. Notched straight blade for VCR mechanical tracking adjustment - make or buy.

• Jewelers screwdrivers - both straight and philips. These are generally inexpensive but quality is also quite variable.

• Small socket driver set.

• Hex key wrenches or hex drivers. Miniature metric sizes for VCRs.

• Pliers - long nose, round nose, curved. Both smooth and serrated types are useful.

• Adjustable wrench (small).

• Cutters - diagonal and flush. Linesman's pliers.

• Wire strippers - fixed and adjustable. Crimp tool.

• Alignment tools - (at least a standard RCA type for coils).

• Files - small set of assorted types including flat, round, square, and triangular.

• Dental picks - maybe a reason to go to the dentist? :-) These are useful for poking and prodding in restricted areas (but you knew that).

• Locking clamps - hemostats - for securing small parts while soldering etc.

• Magnetic pickup tool - you can never tell when you will drop something deep inside a VCR. If you keep a strong magnet stuck to your workbench, you can use it to magnetize most steel tools such as screwdrivers. Just keep anything magnetized away from the tape path and magnetic heads.

• Hand drill, electric drill, drill press - one or all. A small benchtop drill press (e.g., 8") is invaluable for many tasks. A good set of high speed bits (not the 1000 bits for $9.95 variety). Also, miniature bits for PCB and small plastic repairs.

• Soldering and desoldering equipment. An entire chapter is devoted to this topic with the name, you guessed it: "Soldering and Desoldering Equipment and Techniques".

Emergency Screw Removal

It may be possible to remove such screws even if nothing in your driver assortment quite fits (short of buying the proper tool, that is - what a concept!). There is also the situation (very common) where someone (we won't say who) has pre-mangled the screw head! Here are a few approaches to try when you are stuck at 2:00 AM on a Sunday morning with an uncooperative screw:

1. Select a driver type (usually Philips) and size that privides the best grip. Then apply as much pressure as is safely possible without destroying anything and attempt to turn the screw. What you want to avoid is slippage - once the blade slips, the head will be quickly destroyed and then you are left with options (2) or (3), below. For a jeweler's type screwdriver, clamping something larger to its shaft can provide valuable additional leverage.

2. Use a hand grinder (e.g., Dremel tool) or thin file to create a slot in the screw head which one of your straight-blade screwdrivers will fit. Obviously, take care to avoid damage to adjacent parts and dam off the area to prevent grinding chips from getting over everything.

3. Grab the center and edge of the screw with a pair of sharp diagonal cutters and turn it. This, of course, also damages the screw head and if you are too forceful, will break your cutters as well.

4. Drill out the screw using a bit just large enough to sever the head from the shank of the screw. Then, use a pair of needlenose pliers to unscrew what remains. For large screws, drill only part way and then use a screw extractor like Easy-Out(tm).

5. Superglue the screwdriver or some other suitable tool to what's left of the screw. Wait till it hardens and turn.

6. There are also products available for the purpose of improving the grip of screwdrivers. For example: ScrewGrab.

7. For screws into plastic holding metal covers, remove all other screws so the cover can be used to pull on the screw and heat the screw with a soldering iron. With care, it will melt its way out but the plastic will solidify to a smaller hole that can be used to install a new screw. Where the cover is also plastic, it may melt first so probably not a good idea.

Note: some of these screws have had some material like Lock-Tight(tm) (which looks like colored nail polish) applied to the top to prevent the screw from loosening on its own. This also prevents the blade of a screwdriver from properly seating, so removal is essential before attempting removal.

Plastic Screw Thread Repair

1. Install a larger screw in original worn hole. :) For repairs where the appearance isn't important, this may be the best solution.

2. Use a soldering iron from the hidden side to "adjust" the hole. Doing this with the screw in place will result in proper threads being preformed.

3. Fill the hole with a mixture of the same type of powdered plastic and solvent or other similar material. When dry, drill a pilot hole and then install the original screw.

4. Glue a metal nut to the underside of plastic and use a machine screw.

There are many other possibilities.

To avoid this problem in the future, realize that plastic is very soft and it is essential to gently start the screw into the hole to get a feel for it properly mating with the existing threads. The use of an undersized screwdriver to get the screw started may be helpful in that it won't accidentally apply too much torque and strip the threads. Something that is less obvious is that screws for plastic are often made with a wide thread and a narrow thread wound that alternate, sort of like a deformed hunk of DNA. :) With these, there is only one proper way for them to mate with an existing hole and forcing them is asking for stripped threads and a fine strand of plastic being pulled out along with the loose screw.

About Those Other Funny-Headed Screws

As well as Phillips, there are Pozidriv and JIS:

Pozidriv screws can be recognized by the 'starburst' - the little lines on the head between the main slots. These are very common (certainly in Europe) in all sorts of equipment.

• There are also JIS (Japanese Industry Standard) heads. These _look_ just like Philips, but the screwdriver is a little flatter on the end. Not surprisingly these do turn up in Japanese stuff.

It's not uncommon for all 3 to be used in the same equipment, especially if subassemblies were made by different companies.

(From: Robert McPherson (rm502@bellsouth.net).)

There is a type of screwdriver called a "Reed & Prince" which fits these screws which are similar in appearance to Phillips screws. Cooper tools makes them.

Workbench and AC Power

A size of 3 x 6 feet should be adequate, longer is better if you have the space. Workbench height is typically 36 inches. Make sure the legs are sturdy and rigid - some equipment can be quite heavy. Get yourself a comfortable stool to sit on for those marathon troubleshooting sessions.

The surface can be laminate, particle board, plywood, butcher block, or some other insulator. It shouldn't have a dramatic pattern though since small parts will be hard to find. Wood products should have multiple coats of varnish or polyurethane. Using a cheap material that can be replaced will enable the surface to be rejuvenated after it gets pitted and burnt - as it invariably will after awhile. An antistatic surface is desirable but probably expensive to put on the entire workbench so just get an antistatic matt for use when needed. (An antistatic surface isn't quite a perfect insulator but has just enough conductivity to minimize the buildup of static electricity, essential for any work with devices like CMOS ICs and laser diodes that can be destroyed by even a small static discharge.)

Install a shelf or shelves along the back that are about half the depth of the workbench surface to hold smaller pieces of test equipment, power supplies, parts cabinets, and other odds and ends. Add a shelf or shelves underneath for storage.

Install AC outlets along the rear edge, vertically so debris can't fall into the holes. How many? The more the merrier - they will all get filled no matter how many are there! At a minimum, one every 6 inches or a duplex every foot, double this won't hurt. Power the workbench from two branch circuits fed from opposite sides of the 115-0-115 VAC (in the U.S.A.) Consider including at least one 230 VAC outlet (in the U.S.A.). Providing some outlets that are switched with power indicator lamps and protected by fuses or circuit breakers. Most outlets, particularly those used to plug in equipment being worked on, should be GFCI (Ground Fault Circuit Interrupter) protected for safety. But a few - clearly marked "NOT GFCI PROTECTED" - should be available for equipment that will not function reliably on a GFCI with the understanding that these lack such protection. Most test equipment and power supplies with properly wired grounded power cords do not need to be GFCI protected but won't complain if they are. However, some equipment may nuisance trip (immediately or at random) GFCIs even if functioning properly.

The total cost can be well under $100 for all of this even if the materials and parts are purchased new. With some reasonable scrounging abilities, it can be a lot less.

Basic Test Equipment

• DMM and/or VOM. I prefer to have both. A good old Simpson 260 is better in many ways than a cheap digital multimeter. For most measurements, I still use a 25 year old Lafayette (remember them?) VOM. I only go for the DMM when I need to measure really low ohms or where better accuracy is needed (though this can be deceptive - just because a DMM has 3-1/2 digits does not mean it is that accurate - check you manual, it may prove enlightening). The Simpson 260 also has a nice 5000 V AC/DC scale which the others lack.

  A fancy expensive multimeter is not needed, at least not while you are just starting out (and likely to make some occasional mistakes like attempting to measure line voltage on the ohms scale.) However, if someone offers to give you a nice Fluke DMM, don't turn it down :-).

  Scales for transistor, capacitor, frequency counter, etc. are not really essential. A diode test function on a DMM is needed, however, to properly bias semiconductor junctions. Even this is not useful for in-circuit tests or for some power transistors or transistors with built in damper diodes and/or base resistors.

  Make sure you have a good well insulated set of test probes. This is for your own safety as you may be measuring relatively high voltages. Periodically inspect for damage and repair or replace as needed. If the ones that came with your multimeter are substandard - flimsy connectors or very thin insulation, replace them as well.

  A high impedance high voltage probe is sometimes useful for TVs and monitors. You can build one of these which will suffice for most consumer electronics work.

• AC clamp-on ammeter. This permits the measurement of currents in appliances or electrical wiring without having to cut any wires. At most, you will need an easily constructed adapter to permit access to a single conductor of a line cord. This may be an option for your multimeter.

• Oscilloscope - dual trace, 10 to 20 MHz minimum vertical bandwidth, delayed sweep desirable but not essential. A good set of proper 10x/1x probes. High vertical bandwidth is desirable but most consumer electronics work can be done with a 10 MHz scope. If you get into digital debugging, that is another story - 100 MHz and up will be required. If money is no object, get a good digital storage scope. You can even get relatively inexpensive scope cards for PCs, but unless you are into PC controlled instrumentation, a stand-alone scope is much more useful.

  I would recommend a good used Tektronix (Tek) or Hewlett Packard (HP) scope over a new scope of almost any other brand. You will usually get more scope for your money and these things last almost forever. Until recently, my 'good' scope was the militarized version (AN/USM-281A) of the HP180 lab scope. It has a dual channel 50 MHz vertical plugin and a delayed sweep horizontal plugin. I have seen these going for under $300 from surplus outfits. For a little more money, you can get a Tek 465 or 465B (slightly newer but mostly similar specifications) 100 Mhz scope ($200 to 600) which is what I use now. The HP-180 is still fine but I couldn't pass up a really good deal. :) The Tek 465/B or other similar model will suffice for all but the most demanding (read: RF or high speed digital) repairs. (See the additional comments below on the Tek 465 as well.) From my experience with this scope many years ago and now as well, I really do agree with some who say that this is the best scope Tektronix ever designed.

  Auctions like eBay can sometimes be a source of good used Tek and other scopes at reasonable prices though sometimes the bid price goes way beyond what is reasonable. :) A search for "oscilloscope" will typically turn up several hundred hits. However, to have any confidence in the operational condition of a scope, the seller must be reputable and know something about testing them. A warranty may be of limited value since a major part of the cost of a used scope is likely to be the shipping and you'd end up having to pay that both ways. Check out Phil's Tek Scope Prices on eBay List as well as catalog pages of surplus test equipment dealers. A Web search (e.g., Google) will usually turn up enough sites for any specific model to provide both specifications and typical prices from surplus equipment dealers (which are usually high!).

  My instant checklist for a used scope:

  • Bright, sharp trace, no screen burns, short warmup time.
  • Vertical amp(s) operational on all ranges.
  • Horizontal timebase(s) operational on all ranges.
  • Stable triggering at low and high end of frequency range.
  • Switches and controls reasonably noise-free (may need cleaning though).
  • Decent cosmetic condition, all knobs and buttons present, no signs of major trauma.

  You don't absolutely need an oscilloscope when you are just starting out in electronics but it would help a great deal. It need not be a fancy one at first especially if you are not sure if electronics is for you. However, being able to see what is going on can make all the difference in your early understanding of much of what is being discussed in the textbooks and the newsgroups. You can probably find something used that will get you through a couple of years for less than $100. An oldie but goodie is much better than nothing at all even if it isn't dual channel or high bandwidth!

  And a note about digital versus analog scopes: Analog scopes are what we used to think of as an oscilloscope: The CRT is the place where the waveform is generated. Digital scopes use a fast A/D converter to capture data in memory in the form of 1s and 0s and then display this on a raster-scan CRT (like a computer monitor screen). Digital scopes are automatically storage scopes and are great for analyzing waveforms. However, most older digital scopes are really poor at real time display and in addition, appear to have been designed by computer programmers, not test equipment engineers. Ever try to play a menu-driven piano? :) For general electronics and troubleshooting, I'd rather have a 20 year old Tek analog scope than a 5 year old digital scope costing 25 times as much. The inherent real-time presentation of an analog scope can be invaluable when attempting to observe the subtle characteristics of a waveform. Those who go through school never having touched a true analog scope have missed out on a great experience.

• Logic probe - for quick checks of digital circuitry for activity. A logic pulsar can be used to force a momentary 1 or 0. Some people swear by these. I consider them of marginal value at best.

• TV set (color is desirable) and/or video monitor for testing of video equipment like VCRs, camcorders, laserdisc players, etc. I have an old CGA monitor which includes an NTSC input as well.

  A great deal of information can be gathered more quickly by examining the picture on a TV or monitor than can be learned from the video waveform on displayed on a scope.

• VCR or other video signal source for testing of video monitors and TVs. These will have both RF (F connector) and baseband (RCA jacks) outputs.

• Stereo tuner or other audio signal source for testing of audio equipment.

• Audio signal generator. A function generator (sine, square, triangle) is nice as well. The usual audio generator will output from a few Hz to about 1 MHz.

• Audio amp connected to a loudspeaker. The input should be selectable between line level and mic level and be brought out through a shielded cable to a test probe and ground clip. This is useful for tracing an audio circuit to determine where a signal is getting lost.

• Signal injector. A readily accessible portable source of a test tone or other signal (depending on application) that can be introduced into the intermediate or early stages of a multistage electronic system.

  For audio, a simple transistor or 555 timer based battery powered oscillator can be built into a hand held probe. Similar (but generally more specialized) devices can be constructed for RF or video testing.

• RF signal generator. For serious debugging of radio and tuner front-ends. These can get quite sophisticated (and expensive) with various modulation/sweep functions. For most work, such extravagance is unnecessary.

• LCR meter - a capacitor tester is desirable but I prefer to substitute a known good capacitor rather than trusting a meter which will not test under the same conditions as exist in-circuit.

• Adjustable power supplies. At least one of these should bo of the totally indestructable variety - one you can accidentally short out without fear of damage. Mine is a simple 1 amp 0-40 V transformer and rectifier/filter cap affair with a little Variac for adjustment.

• The following book has a number of simple test equipment projects you can build with readily available parts:
  • Test Equipment Projects You Can Build
    Delton T. Horn
    Tab Books, a division of McGraw-Hill, Inc., 1992
    Blue Ridge Summit, PA 17214
    ISBN 0-8306-4154-8 (hardcover), 0-8306-4155-6 (paperback)
    

Jerry's Comments on Used Scopes and the Tek 465

If you are buying a used 465, look for the 465B. It is a better unit, and is the same price most of the time. Take care that this scope is about 20 years old, and there is no support from Tek on it. The replacement parts are not available if something blows. I used to have a few of them. One needed a CRT, and the other I sold while it was still working. For consumer electronics, you will get by with a 100 MHz unit, but it is preferable to have over 200 MHz bandwidth if you want to do front end service on consumer FM radio receivers. Read up on Nyquist and you will see the answer.

If you also call Tektronix technical services, tell them that you are looking for a used Tek scope to be used for hobby purposes. They will be very helpful in giving you any information you require. They will even recommend models and what to look for. If you talk to their sales people, they will sometimes even give you their authorized dealers who handle used Tek equipment so that you can shop around.

If you go a bit more for your used scope you can get a 200 or 300 MHz unit that is a newer version of an analog scope. It will have improvements over the 465 series. Look at the 2000 analog series scopes. These have a lot of enhancements like on the screen display. This will be very handy for precise work. When buying any type of scope, I would stress that the Tektronix is the best. If you find a good working used one, you will have a very high quality product, and it should give you years of service. Most of the analog scope that they made include the TV sync options.

Even if you buy a used one, and the parts are not available, it pays then to buy a second used one and you will have spare parts. These scopes used to cost in the many thousands of dollars when new, and you are probably paying between eight hundred to fifteen hundred for a used one (somewhat cheaper now, even from surplus companies. --- Sam). These scopes will be far superior to even the newer ones from the consumer level scopes. In 1978 I believe my company paid over $8,000 for the 465B scope new. A new Chevy fully loaded was less!

Repairing Tectronix 400 Series Scopes

The most likely causes are shorted tantalum "dipped" capacitors dragging down one or more power supply rails. Apparently, Tek used a batch of unreliable caps on the some of the 400 Series scopes and while aluminum electrolytics usually just dry out with decreased capacitance and increased ESR, these dipped tantalums go short circuit. Fortunately, the design of the switching power supplies in these scopes is such that the controller shuts down from a serious overload or short rather than letting its smoke out. If the overload is on only one voltage rail and not severe (e.g., through a resistor), only that voltage may be low or absent resulting in loss of functionality but not a totally dead scope.

So, the first step is (WITH POWER OFF) to check the resistance of each voltage test point to ground with a multimeter. While the expected resistances may not be known except from a service manual (if that), anything very low (e.g., 10 ohms) is suspect. Here are typical values measured on a Tek 485 using a Fluke 87 DMM with the black lead on ground: +50 V, 2.1K ohms; +15 V, 89 ohms; +5 V, 70 ohms; -5 V, 222 ohms; -15 V, 152 ohms. The resistance for +5 V changes significantly depending on front panel settings and which incandescent indicator lamps should be lit and may go below 35 ohms. On this scope, the -15 V rail originally measured about 10 ohms due to a bad cap. Where one of these is found, attempt to determine the location of the short to a specific circuit board. Then, trace the wiring on that board to locate the possible bad caps. A good DMM or milliohmmeter can help to track down the cap since PCB foil resistance is high enough to be measured and the resistance to ground will be lowest at the location at the bad cap. At this point, unsoldering one lead of each cap and checking its resistance is the safest approach. With care, this can be done from the component side of the board which is fortunate since removing some of these large PCBs can be a royal pain. Heat the lead with a soldering iron and pull it free. Then, use a vacuum desoldering tool ("SoldaPullet") to clear the hole. Check the resistance of the cap and/or across the supply rail to determine if you found the correct one. The bad cap mentioned above was found in about 5 minutes in this manner. There are typically only a few of these caps on each board but it's possible for the bad one to be on a board that isn't easily accessible.

Where this approach doesn't work or for the lazy but daring among us, the alternative is to apply voltage from an external adjustable current limited supply to the bad power rail. If the bad part isn't a perfect short circuit, it will dissipate heat and let its smoke out or explode. Wear safety glasses! If this doesn't happen, it may actually be possible to power up the scope with the external voltage applied to determine functionality. In either case, I won't be responsible for any destroyed equipment should this be done.

So You Can't Afford a $20,000 Transient Event Recorder?

However, there may be no need for such extravagance. If you have an oscilloscope and camcorder or video camera/VCR, you probably have all that is needed.

For a TV or monitor, point the camera at the CRT and the scope screen so that they are both in the picture and record on a 6 hour tape. Then, when your event takes place, you have a permanent record!

That old video camera will be perfectly adequate. It doesn't need a 100X digitally stabilized enhanced reprocessed zoom or 1/10,000th second shutter. It doesn't even need to be color!

Sure, this won't capture the 1 ns glitch. But, for the occasional flash in the picture, it is more than adequate to eliminate a video signal line as the source of the problem.

Extensions to more convoluted problems are left as an exercise for the student!

Transformers - Isolation and Variable

Isolation Transformers

Since Earth Ground and the Neutral of the power line are connected together at your service panel (fuse or circuit breaker box), grounds like cold water pipes, test equipment chassis, and even a damp concrete floor make suitable returns for the line voltage (Hot or live wire). Since this is just as true with the conductor being being a wire or your body, such a situation is very dangerous.

An isolation transformer as its name implies provides a barrier such that accidental contact with an earth ground results in negligible current flow (only due to the parasitic capacitance and inductance of the transformer) - a slight tingle at worst. This also protects your test equipment as well as the device you are troubleshooting since a similar accidental contact can result in a short circuit, sparks, smoke, and many destroyed parts.

The schematic for a typical isolation transformer is shown below:

                               _        1:1
                H o-----/ ----- _------+    +-----------o 115 V
                       Power  Fuse      )||(
                       Switch           )|| +-----------o 105 V
                                        )||(
                                        )||(
                                Primary )||( Secondary
            Tied together at            )||(
            service panel               )||(
            |                           )||(
            |                           )||(
            +-> N o----------+---------+ |  +---+--------o Return
            |                |  4.7 M*   |      |
            |                +---/\/\----|------+
            |                            |
            +-> G o----------------------+--------------o Ground

The resistor (*) is desirable to permit any static charge to leak off to ground. Since it is quite large - 2 M ohms - no perceptible current will flow between the secondary and primary sides but this value is low enough to dissipate any static charge. CAUTION: The resistor must be a high voltage rated type (as in 4,200 V isolation, large size light blue color to assure that arc over will not result due to voltage differences that may be present when the isolation transformer is being used in its normal manner.

Although the power line Neutral and Ground wires are tied together at the main service panel (fuse or circuit breaker box), the transformer prevents any significant current flow between any of its outputs and earth ground should a fault occur.

Even if you were standing with bare feet in a puddle of salt water on a concrete floor (noting that this is definitely NOT recommended) and were to touch something connected to the secondary of the isolation transformer or its return, or equipment circuitry attached to these, there is no direct return path for current to flow through you.

However, this shouldn't encourage a false sense of security. If you were to touch two points at different potentials on the secondary side, you could still be fried! And some equipment like microwave ovens use their chassis, and thus ground, as the high voltage return so an isolation transformer is of limited value for these whether it passes ground through or not.

Isolation transformers can be purchased or constructed from a pair of similar power transformers connected back-to-back. I built mine from a couple of old tube-type TV power transformers mounted on a board with an outlet box including a fuse. Their high voltage secondary windings were connected together. The unused low voltage secondary windings can be put in series with the primary or output windings to adjust voltage. See the section: Typical Homemade Isolation Transformer.

For super critical applications like in hospitals where every microamp of leakage counts, special isolation transformers are available (no doubt at equally super cost) which have shielding between the primary and secondary to minimize the inter-winding capacitance and inductance as well. This should not really be necessary for general servicing.

Note: Not all definitions of the term 'isolation transformer' are created equal! For some purposes, this may mean just preventing line born electrical noise from passing to the equipment. So, if you acquire something called an 'isolation transformer' on its nameplate, confirm that the primary and secondary are indeed not tied together by a low resistance. If they are, it can probably be modified for service needs by disconnecting a jumper but it may not have the insulation ratings desirable for high voltage isolation.

(From: Filip "I'll buy a vowel" Gieszczykiewicz (filipg@repairfaq.org).)

Ever wonder how those guys repair HV transformers running 200 kV without shutting off the power lines feeding the city? They use *very well isolated* cherry pickers! The guy on that platform is working on ONE wire which - since he's not connected to the ground - is at ZERO potential! That wire has no reference at all so no current flows. And he prays each morning that it stays that way or he goes off with a flash! [ugh!].

You're doing something like that on a much safer level. :)

Typical Homemade Isolation Transformer

                                       +-------------------------o 109 V
                                       |
                                       |     +-------------------o 121 V
                                       |     |
                                       | +---------------+
                                       | |   |           |
                          || +--o NC   | |   +---+ ||    |
                          ||(          | |        )||    |
                          ||(          | |  6.3 V )||    |
                          || +--o NC   | +-------+ ||    |
                   _      ||(          |          )||    |
       H o--/ ----- _---+ ||(          |    6.3 V )|| +--+-------o 115 V
           Power  Fuse   )|| +--o NC   +---------+ ||(
           Switch        )|| +-------------------+ ||(
                   115 V )||(                     )||( 115 V
                         )||(                     )||(
                         )||( 350 V         350 V )||(
       N o---------+----+ ||(                     )|| +----+-------o Return
                   |      || +--o NC       NC o--+ ||      |
                   |      ||(                     )||      |
                   |      ||(                     )||      |
                   |      ||( 350 V         350 V )||      |
                   |      ||(                     )||      |
                   |      |  +-------------------+  |      |
                   | Pri1 |  Sec1              Sec2 | Pri2 |
       G o----------------+-------------------------+-------------o Ground
                   | Transformer 1    2M*    Transformer 2 |
                   +------------------/\/\-----------------+

The 2 M resistor (*) is to bleed away any static charge as described above.

The power/VA ratings of the transformers you use need to be greater than your expected load. And, since some equipment like TVs and computer monitors draw a lot of current at power-on (from the degauss circuit), the isolation transformer will limit the peak current and may cause problems during startup (though overall, the limited current may prevent some types of disasters!). In any case, don't expect a pair of 6.3 VAC, 1 A transformers wired back-to-back to be useful for testing much of anything!

Also see the section: Isolation Transformers from Dead Microwave Ovens.

(From: David Moisan (dmoisan@shore.net).)

It's not as hard as you think to find inexpensive isolation transformers. At the next hamfest, look for someone selling dead UPS's (Uninterruptible Power Sources) or other power conditioning equipment. Isolation transformers are often sold for use in the computer industry; that's how I got mine. 250 VA for $20, and I could have gotten 1000 VA for $50 if I wanted. Definitely increases my safety *and* confidence level!

Isolation Transformers from Dead Microwave Ovens

However, note that microwave oven transformers are usually designed with as little copper as possible in the primary winding and do go into core saturation at normal line voltage with no load. For example, measurements using a clamp-on AC ammeter of a transformer from a mid-size microwave oven shows:

            Input VAC   Input Amps
           ------------------------
                80           .3
                90           .6
               100          1.1
               110          2.0
               115          3.0
               120         >4.0

Thus, this sort of approach isn't recommended unless you really need the high capacity - testing of other microwave ovens or ion laser power supplies, for example!

A pair of these trnasformers can be connected in a similar manner to the tube-type TV power transformers described in the section: Typical Homemade Isolation Transformer, there are a few more things to keep in mind:

• These transformers are DANGEROUS. Their high voltage output is between 1,500 and 3,000 VRMS at AMPS - an instantly deadly combination. Therefore, thoroughly insulate the connections between the HV secondaries.

• The high voltage returns are connected to the cores so these must be tied together AND to earth ground for safety.

• These transformers may not be rated for continuous duty operation. So, they should probably not be left plugged in when not in use.

• The more limited capacity of a small isolation transformer can sometimes protect you from yourself - preventing the burnout of a horizontal output transistor due to excessive load or carelessness. You will have no such guardian looking over your shoulder with a microwave oven monster!

How Safe is a Homemade Isolation Transformer?

Keep in mind that I am not talking about using something that has been rusting away in a damp basement for 20 years. The power transformers from tube-type TVs or audio amplifiers must have been designed with isolation requirements in mind to obtain regulatory approval in the first place since they are used in equipment where the user may come in contact with metal parts.

Also, the use of an isolation transformer is no excuse to ignore the other aspects of safe troubleshooting.

It is easy to test for AC and DC leakage - and this should be done - to be sure that your transformers are in good condition. With two transformers, the probability of a failure is even smaller - 1/(P*P). Personally, I would trust the homemade transformer over a cheap import any day!

Variable Autotransformers

The internal wiring of a typical Variac is shown below:

                 _                1 
          H o---- _-----/ ------>o--+     Tap 1: 0 to 115 VAC
               Fuse 1  Power      2  )||
              (Input)  Switch    o--+ ||  Tap 2: 0 to 140 VAC
                                     )||
                                     )||      _
      Tied together at               )<------- _--------o Adjustable output
      service panel           Power  )||     Fuse 2
      |              220       LED   )||    (Output)
      |          +--/\/\--|>|--|>|--+ ||
      |          |                   )||
      +-> N o----+------------------+-|-----------------o Return
      |                               |
      +-> G o-------------------------+-----------------o Ground

CAUTION: Keep any large transformer of this type well away from your monitor or TV. The magnetic field it produces may cause the picture to wiggle or the colors to become messed up - and you to think there is an additional problem!

Note: the 'Power LED' circuit is soldered directly to a winding location determined to produce about 6 VAC.

• Sam's Rule #61453: Your lab or shop can never have too many Variacs!
• Sam's Rule #61454: If you only have a single Variac, it should be LARGE!

Wiring a Variable Autotransformer

Wiring is straightforward if you have acquired a bare unit (the following assumes a 115 VAC line, the extension to 230 VAC should be obvious):

• Decide on the output voltage range and direction of rotation (clockwise should always be used to increase output as far as the user is concerned but depending on mounting, actual direction of the shaft with respect to the body of the unit may be either way):
  • For 0 to 115 VAC output, Hot and Neutral go between the ends of the winding with Neutral at the terminal where the wiper will be when you want the output to be 0 VAC; The output goes between Neutral and the wiper.

  • For 0 to 140 VAC output, Hot is moved to the tap about 20 percent away from the terminal where the wiper will be when the output is at 140 VAC; The output goes between Neutral and the wiper as above.

  • For some (mostly small) Variacs that do not have intermediate taps, it may still be possible to add a tap to permit 140 VAC operation. However, this will reduce the number of turns on the primary and could lead to overheating from core saturation if the design is marginal. Most of these are overdesigned and this shouldn't be an issue. But, nonetheless, if you try this, monitor the temperature of the unloaded Variac for, say, an hour to make sure it doesn't get excessively hot.

• Include a primary-side power switch and power-on indicator lamp. In the old days, a neon lamp would be used (e.g., NE2H with a 47K ohm resistor). Nowadays, neon lamps may be hard to find. An alternative as suggested above is to add a tap on the Variac winding at about 6 to 10 VAC and use an LED and current limiting resistor.

• Provide fuses for both input and output. They should be the same rating as the Variac. The fuse for the input protects against primary side shorts. The one for the output protects the Variac winding from excessive load. Commercial units may only have one fuse but fuses are inexpensive and the added protection won't hurt.

• Use an adequately rated grounded cordset and mount everything in a well insulated box. The Variac frame and box (if made of metal) should be grounded.

Variable Isolation Transformers

Variac/Isolation Transformer with Current Limiting

Constant Voltage Ferroresonant Transformer