Contents:
Notes on the Troubleshooting and Repair of Microwave Ovens
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Radar Range anyone? ------------------ Remember when you actually had to use the real oven to defrost a TV dinner? Think back - way back - before VCRs, before PCs (and yes, before Apple computers as well), almost before dinosaurs, it would seem. There was a time when the term 'nuke' was not used for anything other than bombs and power reactors. For a long time, there was controversy as to whether microwave ovens were safe - in terms of microwave emissions and molecular damage to the food. Whether these issues have been resolved or just brushed aside is not totally clear. Nonetheless, the microwave oven has taken its place in virtually every kitchen on the planet. Connoisseurs of fine dining will turn up their collective noses at the thought of using a microwave oven for much beyond boiling water - if that. However, it is difficult to deny the convenience and cooking speed that is provided by this relatively simple appliance. Microwave ovens are extremely reliable devices. There is a good chance that your oven will operate for 10 years or more without requiring repairs of any kind - and at performance levels indistinguishable from when it was first taken out of the box. Unlike other consumer electronics where a new model is introduced every 20 minutes - some even have useful improvements - the microwave oven has not changed substantially in the last 20 years. Cooking is cooking. Touchpads are now nearly universal because they are cheaper to manufacture than mechanical timers (and also more convenient). However, an old microwave oven will heat foods just as well as a brand new one. This document provides maintenance and repair information applicable to most of the microwave ovens in existence. It will enable you to quickly determine the likely cause and estimate the cost of parts. You will be able to make an informed decision as to whether a new oven is the better alternative. With minor exceptions, specific manufacturers and models will not be covered as there are so many variations that such a treatment would require a huge and very detailed text. Rather, the most common problems will be addressed and enough basic principles of operation will be provided to enable you to narrow the problem down and likely determine a course of action for repair. In many cases, you will be able to do what is required for a fraction of the cost that would be charged by a repair center - or - be able to revive something that would otherwise have gone into the dumpster or continued in its present occupation as a door stop or foot rest. Should you still not be able to find a solution, you will have learned a great deal and be able to ask appropriate questions and supply relevant information if you decide to post to sci.electronics.repair. In any case, you will have the satisfaction of knowing you did as much as you could before taking it in for professional repair. You will be able to decide if it is worth the cost of a repair as well. With your new-found knowledge, you will have the upper hand and will not easily be snowed by a dishonest or incompetent technician.
Microtech maintains a web site with a large amount of information on microwave oven repair including an on-line Tech Tips Database with hundreds of solutions to common problem for many models of microwave ovens. There are also an extensive list of microwave oven related links to other interesting sites (including this document!). The comprehensive Safety Info is a must read as well. Microtech also offers instructional videos and books on microwave oven and VCR repair. It is quite possible your problem is already covered at the Microtech site. 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 Microtech 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 MIDES (Microwave Oven Diagnosis Expert System) site represents an interesting and possibly useful approach for isolating the cause of many common failures. It will take you through a customized step-by-step procedure based on your symptoms (and specific microwave oven model in some cases) and the results of its suggested tests. For the novice, this may be an effective way of obtaining a solution quickly as long as you follow the extremely important safety information provided by MIDES (or this document). You will not be forced to acknowledge that you have read, understood, and followed their safety precautions and warnings before performing each test.
* Bad interlocks switches or door misalignment causing fuses to blow or no operation when the start button is pressed. Locate and replace defective switches and/or realign door. * Arcing in oven chamber: clean oven chamber and waveguide thoroughly. Replace carbonized or damaged waveguide cover. Smooth rough metal edges. Touch up the interior paint. * Blown fuse due to power surge or old age: Replace fuse. On rare occasions, the main fuse may even be intermittent causing very strange symptoms. * Erratic touchpad operation due to spill - let touchpad dry out for a week. * Bugs in the works - the controller circuit board is a nice warm safe cozy place to raise a family..... More detailed explanations are provided elsewhere in this document.
With small to medium size microwave ovens going for $60-100 it hardly makes sense to spend $60 to have one repaired. Even full size microwave ovens with full featured touchpanel can be had for under $200. Thus, replacement should be considered seriously before sinking a large investment into an older oven. However, if you can do the repair yourself, the equation changes dramatically as your parts costs will be 1/2 to 1/4 of what a professional will charge and of course your time is free. The educational aspects may also be appealing. You will learn a lot in the process. Many problems can be solved quickly and inexpensively. Fixing an old microwave for the dorm room may just make sense after all.
To assure safety and convenient, follow these recommendations: * Read your users manual from cover to cover especially if this is your first microwave. What a concept! If nothing else, you may discover that your oven has features you were not aware were even possible. In any case, there may be requirements or suggestions that are specific to your model and will enable you to get the most performance from your new microwave. * Select a stand-alone unit rather than a built-in if possible. It will be cheaper to buy, cheaper and easier to service, and possibly more reliable since ventilation and adjacent heat producing appliances will not be as much of a factor. * Select a convenient location - easy access and not too high or too low. This is particularly important if the door of the oven opens down instead of to the left side (only a few models are built this way, however). * Put the microwave oven on its own dedicated 3 wire grounded circuit. Temporary use of a 3 to 2 prong adapter is acceptable only if the outlet box is properly grounded to begin with (BX, Romex, or conduit with ground). Make sure the outlet is in good condition in either case. Check that the plug (or adapter) fits tightly and that there is no appreciable heating of the outlet during use of the microwave oven. If there is, spread the metal strips of each of the prongs apart if possible and/or replace the outlet. A grounded outlet is essential for safety. Microwave ovens are high power devices and a separate circuit will eliminate nuisance fuse blowing or circuit breaker tripping when multiple appliances are being used at the same time. It will also minimize the possibility of Radio Frequency Interference (RFI) between it and any electronic equipment which might be on the same circuit. A GFCI is not needed as long as the outlet is properly grounded and may result in nuisance tripping with some microwave ovens. Inexpensice outlet testers are available at hardware stores, home centers, and electrical parts distributors, to confirm that the outlet is properly wired and grounded. * Allow adequate ventilation - do not push it up against the wall or wedge it under a tight fitting wall cabinet (or inside one for that matter!). Leave at least 2 inches on all sides and top if possible. * Do not let children use the microwave oven unless properly supervised. It is very easy to cause a fire through the use of excessive times or power settings. Even something as simple as microwave popcorn can explode and/or catch fire if heated for too long - e.g., 5 minutes instead of my precisely determined 3:41 on high :-).
Most people do not do anything to maintain a microwave oven. While not much is needed, regular cleaning at least will avoid potentially expensive repairs in the future: * Clean the interior of the oven chamber after use with a damp cloth and some detergent if necessary. Built up food deposits can eventually carbonize resulting in sparks, arcs, heating, and damage to the mica waveguide cover and interior paint - as well as potentially more serious damage to the magnetron. If there is any chance of food deposits having made their way above the waveguide cover in the roof of the chamber, remove the waveguide cover and thoroughly clean inside the waveguide as well. * Clean the exterior of the cabinet and touchpad in a similar manner. DO NOT use a spray where any can find its way inside through the door latch or ventilation holes, or a dripping wet cloth. Be especially careful around the area of the touchpad since liquid can seep underneath resulting in unresponsive or stuck buttons or erratic operation. Do not use strong solvents (though a bit of isopropyl alcohol is fine if needed to remove sticky residue from unwanted labels, for example). * Inspect the cord and plug for physical damage and to make sure the plug is secure and tight in the outlet - particularly if the unit is installed inside a cabinet (yes, I know it is difficult to get at but I warned you about that!). Heat, especially from a combination microwave/convection oven or from other heat producing appliances can damage the plug and/or cord. If there is evidence of overheating at the outlet itself, the outlet (and possibly the plug as well) should be replaced. * Periodically check for built up dust and dirt around the ventilation holes or grills. Clean them up and use a vacuum cleaner to suck up loose dust. Keeping the ventilation free will minimize the chance of overheating. * Listen for any unusual sounds coming from inside the oven. While these appliances are not exactly quiet, grinding, squealing, scraping, or other noises - especially if they were not there when the oven was new - may indicate the need for some more extensive maintenance like belt replacement or motor lubrication. Attending to these minor problems now may prevent major repairs in the future. * Keep your kitchen clean. Yes, I know, this isn't exactly microwave specific but cockroaches and other uninvited guests might just like to take up residence inside the electronics bay of the oven on the nice warm controller circuit board or its neighborhood and they aren't generally the tidiest folks in the world. If it is too late and you have a recurring problem of cockroaches getting inside the electronics bay, tell them to get lost and then put window screen over the vents (or wherever they are entering). Such an open mesh should not affect the cooling of the electronic components significantly. However, the mesh will likely clog up more quickly than the original louvers so make sure it is cleaned regularly. If possible, clean up whatever is attracting the unwanted tenants (and anything they may have left behind including their eggs!!). WARNING: See the section: "SAFETY" before going inside. CAUTION: Do not spray anything into the holes where the door latch is inserted or anywhere around the touchpad as this can result in internal short circuits and costly damage - or anywhere else inside, for that matter. If you do this by accident, immediately unplug the oven and let it dry out for a day or two.
You have probably been warned by your mother: "Wait a few seconds (or minutes) after the beep for all the microwaves to disappear". There is no scientific basis for such a recommendation. Once the beep has sounded (or the door has opened), it is safe. This is because: 1. There is no such thing as residual microwave radiation from a microwave oven - it is either being produced or is non-existent. 2. There is little energy storage in the microwave generator compared to the amount being used. The typical high voltage capacitor - the only component that can store energy - has a capacity of less than 15 W-s (Watt-seconds) even for the largest ovens. Power consumption is typically 800 to 1500 W depending on oven size. Therefore, the capacitor will be fully drained in much less than .1 second - long before the beep has ended or the door has cleared the front panel. (Based on the numbers, above, for a 1500 W oven with a capacitor storing 15 W-s, it is more like .01 seconds!) WARNING: This only applies to a *working* microwave oven! If there is no heat, the magnetron may not be drawing any current from the HV power supply and the HV capacitor can remain charged for a long time. In this case, there is a very real risk of potentially lethal electrical shock even after several minutes or more of being unplugged! See the section: "SAFETY" if you will be troubleshooting a microwave oven.
WARNING! WARNING! WARNING! WARNING! WARNING! WARNING! WARNING! WARNING! Microwave ovens are probably the most dangerous of consumer appliances to service. Very high voltages (up to 5000 V) at potentially very high currents (AMPs) are present when operating - deadly combination. These dangers do not go away even when unplugged as there is an energy storage device - a high voltage capacitor - that can retain a dangerous charge for a long time. If you have the slightest doubts about your knowledge and abilities to deal with these hazards, replace the oven or have it professionally repaired. Careless troubleshooting of a microwave oven can not only can fry you from high voltages at relatively high currents but can microwave irradiate you as well. When you remove the metal cover of the microwave oven you expose yourself to dangerous - potentially lethal - electrical connections. You may also be exposed to potentially harmful levels of microwave emissions if you run the oven with the cover off and there is damage or misalignment to the waveguide to the oven chamber. There is a high voltage capacitor in the microwave generator. Always ensure that it is totally discharged before even thinking about touching or probing anything in the high voltage power circuits. See the troubleshooting sections later in this document. To prevent the possibility of extremely dangerous electric shock, do not operate the oven with the cover off if at all possible. If you must probe live, remove the connections to the magnetron (see below) to prevent the inadvertent generation of microwaves except when this is absolutely needed during troubleshooting. Discharge the high voltage capacitor and then use clip leads to make any connections before you apply power to the oven. The microwave oven circuitry is especially hazardous because the return for the high voltage is the chassis - it is not isolated. In addition, the HV may exceed 5000 V peak with a continuous current rating of over .25 AMP at 50/60 Hz - the continuous power rating of the HV transformer may exceed 1500 W with short term availability of much greater power. Always observe high voltage protocol.
These guidelines are to protect you from potentially deadly electrical shock hazards as well as the equipment from accidental damage. Note that the danger to you is not only in your body providing a conducting path, particularly through your heart. Any involuntary muscle contractions caused by a shock, while perhaps harmless in themselves, may cause collateral damage - there are many sharp edges inside this type of equipment as well as other electrically live parts you may contact accidentally. The purpose of this set of guidelines is not to frighten you but rather to make you aware of the appropriate precautions. Repair of TVs, monitors, microwave ovens, and other consumer and industrial equipment can be both rewarding and economical. Just be sure that it is also safe! * Don't work alone - in the event of an emergency another person's presence may be essential. * Always keep one hand in your pocket when anywhere around a powered line-connected or high voltage system. * Wear rubber bottom shoes or sneakers. * Don't wear any jewelry or other articles that could accidentally contact circuitry and conduct current, or get caught in moving parts. * Set up your work area away from possible grounds that you may accidentally contact. * Know your equipment: TVs and monitors may use parts of the metal chassis as ground return yet the chassis may be electrically live with respect to the earth ground of the AC line. Microwave ovens use the chassis as ground return for the high voltage. In addition, do not assume that the chassis is a suitable ground for your test equipment! * If circuit boards need to be removed from their mountings, put insulating material between the boards and anything they may short to. Hold them in place with string or electrical tape. Prop them up with insulation sticks - plastic or wood. * If you need to probe, solder, or otherwise touch circuits with power off, discharge (across) large power supply filter capacitors with a 25 W or greater resistor of 5 to 50 ohms/V approximate value. For the microwave oven in particular, use a 25K to 100K 25 W resistor with a secure clip lead to the chassis. Mount the resistor on the end of a well insulated stick. Touch each of the capacitor terminals to the non-grounded end of the resistor for several seconds. Then, to be doubly sure that the capacitor if fully discharged, short across its terminals with the blade of a well insulated screwdriver. I also recommend leaving a clip lead shorting across the capacitor terminals while working as added insurance. At most, you will blow a fuse if you should forget to remove it when powering up the microwave. * Connect/disconnect any test leads with the equipment unpowered and unplugged. Use clip leads or solder temporary wires to reach cramped locations or difficult to access locations. * If you must probe live, put electrical tape over all but the last 1/16" of the test probes to avoid the possibility of an accidental short which could cause damage to various components. Clip the reference end of the meter or scope to the appropriate ground return so that you need to only probe with one hand. * Perform as many tests as possible with power off and the equipment unplugged. For example, the semiconductors in the power supply section of a TV or monitor can be tested for short circuits with an ohmmeter. * Use an isolation transformer if there is any chance of contacting line connected circuits. A Variac(tm) is not an isolation transformer! The use of a GFCI (Ground Fault Circuit Interrupter) protected outlet is a good idea but will not protect you from shock from many points in a line connected TV or monitor, or the high voltage side of a microwave oven, for example. A circuit breaker is too slow and insensitive to provide any protection for you or in many cases, your equipment. A GFCI may, however prevent your scope probe ground from smoking should you accidentally connect an earth grounded scope to a live chassis. * Don't attempt repair work when you are tired. Not only will you be more careless, but your primary diagnostic tool - deductive reasoning - will not be operating at full capacity. * Finally, never assume anything without checking it out for yourself! Don't take shortcuts! As noted, a GFCI (Ground Fault Circuit Interrupter) will NOT protect you from the high voltage since the secondary of the HV transformer is providing this current and any current drawn off of the secondary to ground will not be detected by the GFCI. However, use of a GFCI is desirable to minimize the risk of a shock from the line portions of the circuitry if you don't have an isolation transformer. An isolation transformer is even limited value as well since the chassis IS the HV return and is a large very tempting place to touch, lean on, or brush up against. And, of course, none of these devices will protect fools from themselves! Take extreme care whenever working with the cover off of a microwave oven.
Many problems have simple solutions. Don't immediately assume that your problem is some combination of esoteric complex convoluted failures. For a microwave oven, there may be a defective door interlock switch or just a tired fuse. 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 (particularly with microwave ovens) and mostly non-productive (or possibly destructive - very destructive). If you need to remove the cover or other disassembly, make notes of which screw went where - they may not all be identical. More notes is better than less. Pill bottles, film canisters, and plastic ice cube trays come in handy for sorting and storing screws and other small parts after disassembly. Select a work area which is well lighted and where dropped parts can be located - not on a deep pile shag rug. Something like a large plastic tray with a slight lip may come in handy as it prevents small parts from rolling off of the work table. 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. A basic set of high quality hand tools will be all you need to work on a microwave oven. These do not need to be really expensive but poor quality tools are worse than useless and can cause damage. Stanley or Craftsman are fine. Needed tools include a selection of Philips and straight blade screwdrivers, needlenose pliers, wire cutters and wire strippers. A medium power soldering iron and rosin core solder (never never use acid core solder or the stuff for sweating copper pipes on electronic equipment) will be needed if you should need to disconnect any soldered wires (on purpose or by accident) or replace soldered components. However, most of the power components in microwave ovens use solderless connectors (lugs) and replacements usually come with these as well. See the document: "Troubleshooting and Repair of Consumer Electronics Equipment" for additional info on soldering and rework techniques and other general information. An assortment of solderless connectors (lugs and wirenuts) is handy when repairing the internal wiring. A crimping tool will be needed as well but the $4 variety is fine for occasional use. Old dead microwaves can often be valuable source of hardware and sometimes even components like interlock switches and magnetrons as these components are often interchangeable. While not advocating being a pack rat, this does have its advantages at times.
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 microwave oven problems are easily solved with at most a multimeter (DMM or VOM). You do not need an oscilloscope for microwave oven repair unless you end up trying to fix the logic in the controller - extremely unlikely. A DMM or VOM is necessary for checking of power supply voltages (NOT the high voltage, however) and testing of interlock switches, fuses, wiring, and most of the components of the microwave generator. This does not need to be expensive but since you will be depending on its readings, reliability is important. Even a relatively inexpensive DMM from Radio Shack will be fine for most repair work. You will wonder how you ever lived without one! Cost: $25-50. Other useful pieces of 'test equipment': * A microwave leakage detector. Inexpensive types are readily available at home centers or by mail order. These are not super accurate or sensitive but are better than nothing. Also see the sections: "Microwave leakage meters" and "Simple microwave leak detectors". * A microwave power detector. These can be purchased or you can make one from a small neon (NE2) or incandescent bulb with its lead wires twisted together. Sometimes these homemade solutions do not survive for long but will definitely confirm that microwave power is present inside the oven chamber. Note: always have a load inside the oven when testing - a cup of water is adequate. * A thermometer (glass not metal) to monitor water temperature during power tests. * High voltage probe (professional, not homemade!). However, this is only rarely actually required. Low voltage, resistance, or continuity checks will identify most problems. WARNING: the high voltage in a microwave oven is NEGATIVE (-) with respect to the chassis. Should you accidentally use the wrong test probe polarity with your meter, don't just interchange the probes = it may be last thing you ever do. Unplug the oven, discharge the HV capacitor, and only then change the connections. There are special magnetron and microwave test instruments but unless you are in the business, these are unnecessary extravagances.
It is essential - for your safety and to prevent damage to the device under test as well as your test equipment - that the large high voltage capacitor in the microwave generator be fully discharged before touching anything or making measurements. While these are supposed to include internal bleeder resistors, these can fail. In any case, several minutes may be required for the voltage to drop to negligible levels. The technique I recommend is to use a high wattage resistor of about 5 to 50 ohms/V of the working voltage of the capacitor. This will prevent the arc-welding associated with screwdriver discharge but will have a short enough time constant so that the capacitor will drop to a low voltage in at most a few seconds (dependent of course on the RC time constant and its original voltage). * For the high voltage capacitor in a microwave oven, use a 25 W or larger 100 K ohm resistor for your discharge widget with a clip lead to the chassis. The reason to use a large (high wattage) resistor is again not so much power dissipation as voltage holdoff. You don't want the HV zapping across the terminals terminals of the resistor. * Clip the ground wire to an unpainted spot on the chassis. Use the discharge probe on each side of the capacitor in turn for a second or two. Since the time constant RC is about .1 second, this should drain the charge quickly and safely. * Then, confirm with a WELL INSULATED screwdriver across the capacitor terminals. If there is a big spark, you will know that somehow, your original attempt was less than entirely successful. There is a very slight chance the capacitor could be damaged by the uncontrolled discharge but at least there will be no danger. * Finally, it is a good idea to put a clip lead across the capacitor terminals just to be sure it stays fully discharged while you are working in the area. Yes, capacitors have been known to spontaneously regain some charge. At worst, you will blow the fuse upon powering up if you forget to remove it. WARNING: DO NOT use a DMM for checking voltage on the capacitor unless you have a proper high voltage probe. If your discharging did not work, you may blow everything - including yourself. A suitable discharge tool can be made as follows: * Solder one end of the appropriate size resistor (100K ohms, 25W in this case) to a well insulated clip lead about 2 to 3 feet long. Don't just wrap it around - this connection must be secure for safety reasons. * Solder the other end of the resistor to a well insulated contact point such as a 2 inch length of bare #14 copper wire mounted on the end of a 2 foot piece of PVC or Plexiglas rod which will act as an extension handle. * Secure the resistor to the insulating rod with some plastic electrical tape. This discharge tool will keep you safely clear of the danger area. The capacitor discharge indicator circuit described in the document: "Capacitor Testing and Safe Discharging" can be built into the discharge tool if desired. Again, always double check with a reliable high voltage meter or by shorting with an insulated screwdriver! Reasons to use a resistor and not a screwdriver to discharge capacitors: 1. It will not destroy screwdrivers and capacitor terminals. 2. It will not damage the capacitor (due to the current pulse). 3. It will reduce your spouse's stress level in not having to hear those scary snaps and crackles.
You will void the warranty - at least in principle. There are usually no warranty seals on a microwave so unless you cause visible damage or mangle the screws or plastic, it is unlikely that this would be detected. You need to decide. A microwave still under warranty should probably be returned for warranty service for any covered problems except those with the most obvious and easy solutions. Unplug the unit! Usually, the sheet metal cover over the top and sides is easily removed after unscrewing 8-16 philips head sheet metal screws. Most of these are on the back but a few may screw into the sides. They are not usually all the same! At least one of these includes a lockwasher to securely ground the cover to the case. Make note of any differences in screw types so they can be put back in the same place. The cover will then lift up and off. Note how fingers on the cover interlock with the main cabinet - these are critical to ensure prevention of microwave leakage after reassembly. Discharge the high voltage capacitor as described in the section: "Safe discharging of the high voltage capacitor" before even thinking about touching anything. A schematic showing all of the power generation components is usually glued to the inside of the cover. How much of the controller is included varies but is usually minimal. Fortunately, all the parts in a microwave can be easily replaced and most of the parts for the microwave generator are readily available from places like MCM Electronics, Dalbani, and Premium Parts. Reassemble in reverse order. Take particular care to avoid pinching any wires when reinstalling the cover. Fortunately, the inside of a microwave is wide open and this is not difficult. Make sure ALL of the metal fingers around the front edge engage properly with the front panel lip. This is critical to avoid microwave emissions should the waveguide or magnetron become physically damaged in any way. Confirm that the screws you removed go back in the proper locations, particularly the one that grounds the cover to the chassis.
A typical microwave oven uses between 500 and 1000 W of microwave energy at 2.45 GHz to heat the food. This heating is caused mainly by the vibration of the water molecules. Thus plastic, glass, or even paper containers will heat only through conduction from the hot food. There is little transfer of energy directly to these materials. This also means that the food does not need to be a conductor of electricity (try heating a cup of distilled water) and that electromagnetic induction (used elsewhere for high frequency non-contact heating) is not involved. What is significant about 2.45 GHz? Not that much. Water molecules are not resonant at this frequency. A wide range of frequencies will work to heat water efficiently. 2.45 GHz was probably chosen for a number of other reasons including not interfering with existing EM spectrum assignments and convenience in implementation. In addition, the wavelength (about 5 inches) results in reasonable penetration of the microwave energy into the food. The 3 dB (half power) point is about 1 inch for liquid water - half the power is absorbed in the outer 1 inch of depth, another 1/4 of the power in the next inch, and so forth. Since the oven chamber cavity is a good reflector of microwaves, nearly all the energy generated by the oven is available to heat the food and heating speed is thus only dependent on the available power and how much food is being cooked. Ignoring losses through convection, the time to heat food is roughly proportional to its weight. Thus two cups of water will take around twice as long to bring to a boil as one. Heating is not (as popularly assumed) from the inside out. The penetration depth of the microwave energy is a few cm so that the outside is cooked faster than the inside. However, unlike a conventional oven, the microwave energy does penetrate these few cm rather than being totally applied to the exterior of the food. The misconception may arise when sampling something like a pie filling just out of the microwave (or conventional oven for that matter). Since the pie can only cool from the outside, the interior filling will appear to be much hotter than the crust and will remain that way for a long time. One very real effect that may occur with liquids is superheating. It is possible to heat a pure liquid like water to above its boiling point if there are no centers for bubbles to form such as dust specks or container imperfections. Such a superheated liquid may boil suddenly and violently upon removal from the oven with dangerous consequences. This can take place in a microwave since the heating is relatively uniform throughout the liquid. With a stovetop, heating is via conduction from the burner or coil and there will be ample opportunity for small bubbles to form on the bottom long before the entire volume has reached the boiling point. Most metal objects should be excluded from a microwave oven as any sharp edges (areas of high electric field gradient) may create sparking or arcing which at the very least is a fire hazard. Microwave safe metal shelves will have nicely rounded corners. A microwave oven should never be operated without anything inside as the microwave generator then has no load - all the energy bounces around inside an a great deal is reflected back to the source. This may cause expensive damage to the magnetron and other components.
"I am trying to find out what the glass on a microwave consists of exactly. i have not been able to get a better answer than 'a wire mesh'. if you can help, i would greatly appreciate it." There *is* a wire mesh embedded in the glass panel. Since the holes in the mesh are much much smaller than the wavelength of the 2.45 GHz microwaves (about 5 inches or 12.5 cm), it is essentially opaque to microwaves and essentially all the energy is reflected back into the oven cavity. (From: Filip (I'll buy a vowel) Gieszczykiewicz (filipg@repairfaq.org)). Greetings. Did you ever see a "mesh" satellite disk up close? You will note that it looks much like it's made out of simple wire mesh that you can get in a hardware store (in the USA, it's called "chicken fence" :-). The reason this works is that the wave that the dish picks up is longer than the hole in the mesh. Consider bouncing a tennis ball on the "wire mesh" in the microwave - it WOULD work because the ball is bigger than the holes. The wave in the microwave is about 2.5cm "long" ... as long as the holes are smaller than that (actually, you want them as small as possible - without affecting the "watching the food" - to minimize any stray and harmonic waves from escaping... like bouncing tennis and golf and ping-pong balls and marbles off the mesh - you want to catch all the possible sizes - yet still be able to see through it) they will not let anything out of the oven. BTW, it's not really "glass" but rather a 'sandwich' of glass, from the outside, wire mesh (usually a sheet of metal which is either stamped or drilled with a hole pattern - like a color TV CRT mask!), and followed by a sheet of glass or plastic to make sure that food splatters and vapor condensation are easy to clean - imagine scraping the mesh!
The operation of a microwave oven is really very simple. It consists of two parts: the controller and the microwave generator. A schematic diagram of the microwave generating circuitry and portions of the controller is usually glued to the inside of the cover. The controller is what times the cooking by turning the microwave energy on and off. Power level is determined by the ratio of on time to off time in a 10-30 second cycle. The microwave generator takes AC line power. steps it up to a high voltage, and applies this to a special type of vacuum tube called a magnetron - little changed from its invention during World War II (for Radar).
The controller usually includes a microcomputer, though very inexpensive units may simply have a mechanical timer (which ironically, is probably more expensive to manufacture!). The controller runs the digital clock and cook timer; sets microwave power levels; runs the display; and in high performance ovens, monitors the moisture or temperature sensors. Power level is set by pulse width control of the microwave generator usually with a cycle that lasts 10-30 seconds. For example, HIGH will be continuous on, MEDIUM may be 10 seconds on, 10 seconds off, and LOW may be 5 seconds on, 15 seconds off. The power ratios are not quite linear as there is a 1 to 3 second warmup period after microwave power is switched on. The operating voltages for the controller usually are derived from a stepdown transformer. The controller activates the microwave generating circuitry using either a relay or triac.
More sophisticated ovens may include various sensors. Most common are probes for temperature and moisture. A convection oven will include a temperature sensor above the oven chamber. Since these sensors are exposed to the food or its vapors, failures of the sensor probes themselves are common.
Since 30 to 50 percent of the power into a microwave oven is dissipated as heat in the Magnetron, cooling is extremely important. Always inspect the cooling fan/motor for dust and dirt and lubricate if necessary. A couple of drops of electric motor oil or 3-in-One will go a long way. If there are any belts, inspect for deterioration and replace if necessary. An oven that shuts off after a few minutes of operation could have a cooling problem, a defective overtemperature thermostat, a bad magnetron, or is being operated from very high AC line voltage increasing power to the oven. One interesting note: Since 30 to 50 percent of the power goes out the vents in the back as heat, a microwave oven is really only more efficient than conventional means such as a stovetop or gas or electric oven for heating small quantities of anything. With a normal oven or stovetop, wasted energy goes into heating the pot or oven, the air, and so on. However, this is relatively independent of the quantity of food and may be considered to be a fixed overhead. Therefore, there is a crossover point beyond which it is more efficient to use conventional heat than high tech microwaves.
This is the subsystem that converts AC line power into microwave energy. It consists of 5 parts: high voltage transformer, rectifier diode, capacitor, magnetron, waveguide to oven chamber. * High Voltage Transformer. Typically has a secondary of around 2,000 VRMS at .25 amp - more or less depending on the power rating of the oven. There will also be a low voltage winding for the Magnetron filament (3.3 V at 10 A is typical). You cannot miss this as it is the largest and heaviest component visible once the cover is removed. There will be a pair of quick-connect terminals for the AC input, a pair of leads for the Magnetron filament. and a single connection for the HV output. The HV return will be fastened directly to the transformer frame and thus the chassis. * Rectifier - usually rated 12,000 to 15,000 PRV at around .5 amp. Most commonly, this will be rectangular or cylindrical, about .5 inch long with wire leads. Sometimes, it is a box bolted to the chassis. One end will be electrically connected to the chassis. * Capacitor - .65 to 1.2 uF at a working voltage of around 2,000 VAC. Note that this use of 'working voltage' may be deceiving as the actual voltage on the capacitor may exceed this value during operation. The capacitor is metal cased with quick-connect terminals on top (one end). Always discharge the capacitor as described below before touching anything inside once the cover is removed. * Magnetron - the microwave producing tube includes a heated filament cathode, multiple resonant cavities with a pair of permanent ceramic ring magnets to force the electron beams into helical orbits, and output antenna. The magnetron is most often box shaped with cooling fins in its midsection, the filament/HV connections on the bottom section, and the antenna (hidden by the waveguide) on top. Sometimes, it is cylindrical in shape but this is less common. The frequency of the microwaves is usually 2.45 GHz.
The cavity magnetron was invented by the British before World War II. It is considered by many to be the invention most critical to the Allied victory in Europe. The story goes that shortly after the War, a researcher at the Raytheon Corporation, Dr. Percy Spencer, was standing near one of the high power radar units and noticed that a candy bar in his shirt pocket had softened. In the typical 'I have to know why this happened' mentality of a true scientist, he decided to investigate further. The Amana Radarange and the entire future microwave oven industry were the result. Here are two descriptions of magnetron construction. The first is what you will likely find if you go to a library and read about radar. (Some really old microwave ovens may use the classic design as well.) This is followed by my autopsy of a dead magnetron of the type that is probably in the microwave oven in your kitchen. (Items (1) to (6) in the following sections apply to each type while items (7) to (9) apply to both types.) For more detailed information with some nice diagrams, see the articles at the Microtech Web Site. Topics include basic microwave theory as well as a complete discussion of microwave oven magnetron construction and principles of operation.
This is the description you will find in any textbook on radar or microwave engineering. The original Amana Radarange and other early microwave ovens likely used this design as well. 1. A centrally located cylindrical electron emitting cathode. This is supplied with pulsed or continuous power of many thousands of volts (negative with respect to the anode. 2. A cylindrical anode block surrounding but separate and well insulated from the cathode. 3. Multiple cylindrical resonator cavities at a fixed radius from the cathode bored in the anode block. Channels link the cavities to the central area in which the cathode is located. The wavelength of the microwave energy is approximately 7.94 times the diameter of the cavities. (For the frequency of 2.45 GHz (12.4 cm) used in a microwave oven this would result in a cavity diameter of approximately .62" (15.7 mm). 4. An antenna pickup in one of the cylindrical cavities which couples the microwave energy to the waveguide. 5. The entire assembly is placed in a powerful magnetic field (several thousand Gauss compared to the Earth's magnetic field of about .5 Gauss). This is usually supplied by a permanent magnet though electromagnets have been also used. The original designs used huge somewhat horseshoe shaped permanent magnets which were among the most powerful of the day. 6. Cooling of the anode block must be provided by forced air, water, or oil since the microwave generation process is only about 60 to 75 percent efficient and these are often high power tubes (many kilowatts).
This description is specifically for the 2M214 (which I disassembled) or similar types used in the majority of medium-to-high power units. However, nearly all other magnetrons used in modern domestic microwave ovens should be very similar. The item numbers are referenced to the diagram in the section: "Cross section diagram of typical magnetron". Also see this photo of the Typical Magnetron Anode and Resonant Structure. This is a view looking up through the anode cylinder from the filament end of the tube. See the text below for parts names and dimensions. 1. The filament and cathode are one in the same and made of solid tungsten wire, about .020" (.5 mm) diameter, formed in a helix with about 8 to 12 turns, 5/32" (4 mm) diameter and just over 3/8" (9.5 mm) in length. The cathode is coated with a material which is good for electron emission. Note: this coating is the only material contained in the microwave oven magnetron that might be at all hazardous. Beryllium, a toxic metal, may be used in large radar magnetrons but should not be present in the types found in domestic microwave ovens. The filament gets its power via a pair of high current RF chokes - a dozen or so turns of heavy wire on a ferrite core - to prevent microwave leakage back into the filament circuit and electronics bay of the oven. Typical filament power is 3.3 VAC at 10 A. The cathode is supplied with a pulsating negative voltage with a peak value of up to 5,000 V. 2. The anode is a cylinder made from .062" (1.5 mm) thick copper with an inside diameter of 1-3/8" (35 mm) and a length of about 1" (25.4 mm). Steel plates (which probably help to shape the magnetic field, see below) and thin steel covers (to which the filament and antenna insulators are sealed) are welded to the ends of the cylinder. The filament leads/supports enter through a cylindrical ceramic insulator sealed to the bottom cover and then pass through a hole in the bottom end plate. 3. Rather than cylindrical cavities (as you would find in most descriptions of radar magnetrons), there are a set of 10 copper vanes .062" (1.5 mm) thick and approximately 1/2" (12.7 mm) long by 3/8" (9.5 mm) wide. These are brazed or silver soldered to the inside wall of the cylinder facing inward leaving a 5/16" (8 mm) central area clear for the filament/cathode. Surrounding this space are the .062" (1.5 mm) thick edges of the 10 vanes with gaps of approximately .04" (1 mm) between them. Copper shorting rings at both ends near the center join alternating vanes. Thus, all the even numbered vanes are shorted to each other and all the odd numbered vanes are shorted to each other. Of course, all the rings are also all shorted at the outside where they are joined to the inner wall of the cylinder. This structure results in multiple resonant cavities which behave like sets of very high quality low loss L-C tuned circuits with a sharp peak at 2.45 GHz. At this high frequency, individual inductors and capacitors are not used. The inductance and capacitance are provided by the precise configuration and spacing of the copper vanes, shorting rings, and anode cylinder. 4. A connection is made near the middle of a single vane to act as the output power takeoff. It passes through a hole in the top end plate, exits the tube via a cylindrical ceramic insulator sealed to the top cover, and attaches to the pressed-on bull-nose antenna cap. 5. The entire assembly is placed in a powerful magnetic field (several thousand Gauss compared to the Earth's magnetic field of about .5 Gauss). This is provided by a pair of ceramic ring magnets placed against the top and bottom covers of the anode cylinder. For the 2M214, these are about 2-1/8" (54 mm) OD, 1-13/16" (46 mm) ID, 1/2" (12.7 mm) thick. 6. A set of thin aluminum fins act as a heat sink for removing the significant amount of wasted heat produced by the microwave generation process since it is only about 60 to 75 percent efficient. These are press fit on the magnetron anode and also in contact with the magnetron case. There will always be a cooling fan to blow air through this assembly. The anode and magnetron case are at ground potential and connected to the chassis.
The following items apply to all types of magnetrons. 7. The gap between the cathode and anode, and the resonant cavities, are all in a vacuum. 8. When powered, electrons stream from the cathode to the anode. The magnetic field forces them to travel in curved paths in bunches like the spokes of a wheel. The simplest way to describe what happens is that the electron bunches brush against the openings of the resonating cavities in the anode and excite microwave production in a way analogous to what happens when you blow across the top of a Coke bottle or through a whistle. 9. The frequency/wavelength of the microwaves is mostly determined by the size and shape of the resonating cavities - not by the magnetic field as is popularly thought. However, the strength of the magnetic field does affect the threshold voltage (the minimum anode voltage required for the magnetron to generate any microwaves), power output, and efficiency.
The really extraordinary ASCII art below represents (or is supposed to
represent) a cross section of the 2M214 type magnetron (not to scale) through
the center as viewed from the side.
________
| ____ |
|_| |_| Antenna cap
/ |____| \
| | || | | Antenna insulator
| | || | |
xxxxxxxx|__| || |__|xxxxxxxx RF sealing gasket
____________________| || |____________________
| | (5)|| || || (5)| |
| | Top || || || Top | |
| | Magnet || || || Magnet | | Outer case
| |__________|| || ||__________| |
| ______| \\ |______ |
| /____ (7) \\ ____\ |
|____________|| \__ ______ \\ / ||____________|
| ||_______ |__ __| _\\ ___|| |
|____________|| | o || o | ||(4)||____________|
| || | o || o | || (6) | Heat sink fins
|____________|| Vane | o || o | Vane ||____________|
| || (3) | o || o | (3) || |
|____________|| | o || o | ||____________| o: Filament
| ||_______|(1)|| o |_______|| | helix
|____________|| __ |_||||_| __ ||____________|
| ||____/ || || \____||<-- (2) |
| \______ \\ \\ ______/ |
| __________ | || || | __________ |
| | (5)|| || || || (5)| |
| | Bottom || || || || Bottom | |
| | Magnet || || || || Magnet | |
|________|__________|| || || ||__________|________|
| |__||__||__| |
| | || || | Filament |
| | || || | insulator |
| (RF chokes |_||__||_| |
| not shown) || || Filament/cathode |
| || || connections |
|____________________________________________|
Nearly all microwave ovens use basically the same design for the microwave
generator. This has resulted in a relatively simple system manufactured at
low cost.
The typical circuit is shown below. This is the sort of diagram you are
likely to find pasted inside the metal cover. Only the power circuits
are likely included (not the controller unless it is a simple motor driven
timer) but since most problems will be in the microwave generator, this
schematic may be all you need.
|| +------------------------+
||( 3.3 VAC, 10 A, typical |
TP Relay or || +------------+------+ | Magnetron
_ Fuse I __ Triac || | +-|----|-+
o------- _---+---/ -- ----/ ----+ || +------||----+ | |_ _| |
| )||( HV Cap | | \/ |
AC I \ I=Interlock )||( __|__ | ___ |
Line | TP=Thermal Prot. )||( 2,000 VAC _\_/_ +----|:--+
o------------+-------------------+ ||( .25 A | HV |'--> Micro-
||( typical | Diode | waves
(Controller not shown) || +------------+---------+
_|_
- Chassis ground
Note the unusual circuit configuration - the magnetron is across the diode,
not the capacitor as in a 'normal' power supply. What this means is that the
peak voltage across the magnetron is the transformer secondary + the voltage
across the capacitor, so the peaks will approach the peak-peak value of the
transformer or nearly 5000 V in the example above. This is a half wave voltage
doubler. The output waveform looks like a sinusoid with a p-p voltage equal to
the p-p voltage of the transformer secondary with its positive peaks at chassis
ground (no load). The peaks are negative with respect to the chassis. The
negative peaks will get squashed somewhat under load. Take extreme care - up
to 5000 V at AMPs available! WARNING: Never attempt to view this waveform on
an oscilloscope unless you have a commercial high voltage probe and know how
to use it safely!
The easiest way to analyze the half wave doubler operation is with the
magnetron (temporarily) removed from the circuit. Then, it becomes a simple
half wave rectifier/filter so far as the voltage acrtoss the capacitor is
concerned - which will be approximately V(peak) = V(RMS) * 1.414 where V(RMS)
is the output of the high voltage transformer. The voltage across the HV
rectifier will then be: V(peak) + V where V is the waveform out of the
transformer. The magnetron load, being across the HV diode, reduces the peak
value of this somewhat - where most of its conduction takes place.
WARNING: What this implies is that if the magnetron is not present or is not
drawing power for some reason - like an open filament - up to V(peak) will
still be present across the capacitor when power is removed. At the end of
normal operation, some of this will likely be discharged immediately but will
not likely go below about 2,000 V due to the load since the magnetron does not
conduct at low voltages.
Other types of power supplies have been used in a few models - including high
frequency inverters - but it is hard to beat the simplicity, low cost, and
reliability of the half wave doubler configuration. See the section: "High frequency inverter type HV power supplies".
There is also usually a bleeder resistor as part of the capacitor, not shown.
HOWEVER: DO NOT ASSUME THAT THIS IS SUFFICIENT TO DISCHARGE THE CAPACITOR -
ALWAYS DO THIS IF YOU NEED TO TOUCH ANYTHING IN THE MICROWAVE GENERATOR AFTER
THE OVEN HAS BEEN POWERED. The bleeder may be defective and open as this does
not effect operation of oven and/or the time constant may be long - minutes.
Some ovens may not have a bleeder at all.
In addition, there will likely be an over-temperature thermostat - thermal
protector - somewhere in the primary circuit, often bolted to the magnetron
case. There may also be a thermal fuse or other protector physically
elsewhere but in series with the primary to the high voltage transformer.
Other parts of the switched primary circuit include the oven interlock
switches, cooling fan, turntable motor (if any), oven light, etc.
Various door interlock switches prevent inadvertent generation of microwaves unless the door is closed completely. At least one of these will be directly in series with the transformer primary so that a short in the relay or triac cannot accidentally turn on the microwaves with the door open. The interlocks must be activated in the correct sequence when the door is closed or opened. Interestingly, another interlock is set up to directly short the power line if it is activated in an incorrect sequence. The interlocks are designed so that if the door is correctly aligned, they will sequence correctly. Otherwise, a short will be put across the power line causing the fuse to blow forcing the oven to be serviced. At least that is the most likely rational for putting a switch across the power line. Failed door interlocks account for the majority of microwave oven problems - perhaps as high as 75 percent.
The following chart lists a variety of common problems and nearly all possible causes. Diagnostic procedures will then be needed to determine which actually apply. The 'possible causes' are listed in *approximate* order of likelihood. Most of these problems are covered in more detail elsewhere in this document. While this chart lists many problems, it is does not cover everything that can go wrong. However, it can be a starting point for guiding your thinking in the proper direction. Even if not listed here, your particular problem may still be dealt with elsewhere in this document. Problem: Totally dead oven. Possible causes: 1. No power to outlet (overload or fault in microwave or other appliance). 2. Blown main fuse - likely due to other problems. 3. Open thermal protector or thermal fuse. 4. Defective controller or its power supply. 5. Clock needs to be set before other functions will operate (some models). Problem: No response to any buttons on touchpad. Possible causes: 1. Door is not closed (some models). 2. You waited to long (open and close door to wake it up). 3. Controller is confused (pull plug for a minute or two to reset). 4. Defective interlock switches. 5. Faulty controller or its power supply. 6. Touchpad or controller board contaminated by overenthusiastic cleaning. 7. Defective/damaged touchpad. Problem: Oven runs when door is still open. Possible causes: 1. Damaged interlock assembly. 2. Cooling fans (only) running due to bad sensor or still warm. Problem: Oven starts on its own as soon as door is closed. Possible causes: 1. Defective triac or relay. 2. Controller is confused (pull plug for a minute or two to reset). 3. Defective controller or its power supply. 4. Touchpad or controller board contaminated by overenthusiastic cleaning. 5. Defective/damaged touchpad. Problem: Oven works but display is blank. Possible causes: 1. Defective controller or its power supply. 2. Broken display panel. 3. Oven needs to be reset (pull plug for a minute or two to reset). Problem: Whacked out controller or incorrect operation. Possible causes: 1. Previous or multipart cook cycle not complete. 2. Controller is confused (pull plug for a minute or two to reset). 3. Defective controller or its power supply. 4. Touchpad or controller board contaminated by overenthusiastic cleaning. 5. Defective/damaged touchpad. 6. Defective sensor (particulalry covection/mirowave combos). Problem: Erratic behavior. Possible causes: 1. Previous or multipart cook cycle not complete. 2. Bad connections in controller or microwave generator. 3. Faulty relay - primary (or HV side, much less commonly used). 4. Defective controller or its power supply. 5. Bad contacts/connections on mechanical timers. Intermittent fuse. 6. Power surge at start of cook cycle confusing controller. 7. Microwave (RF) leakage into electronics bay. Problem: Some keys on the touchpad do not function or perform the wrong action. Possible causes: 1. Touchpad or controller board contaminated by overenthusiastic cleaning. 2. Defective/damaged touchpad. 3. Controller is confused (pull plug for a minute or two to reset). 4. Faulty controller. Problem: Microwave oven does not respond to START button. Possible causes: 1. Defective START button. 2. Faulty interlock switches. 3. Door is not securely closed. 4. Faulty controller. 5. You waited too long - open and close door to wake it up! Problem: No heat but otherwise normal operation. Possible causes: 1. Blown fuse in HV transformer primary circuit or HV fuse (if used). 2. Bad connections (particularly to magnetron filament). 3. Open thermal protector or thermal fuse. 4. Open HV capacitor, HV diode, HV transformer, or magnetron filament. 5. Shorted HV diode, HV capacitor (will blow a fuse), or magnetron. 6. Defective HV relay (not commonly used). Problem: Fuse blows when closing or opening door: Possible causes: 1. Defective door interlock switchs. 2. Misaligned door. Problem: Loud hum and/or burning smell when attempting to cook. Possible causes: 1. Shorted HV diode, magnetron. 2. Burnt carbonized food in or above oven chamber. 3. Shorted winding in HV transformer. 4. Frayed insulation on HV wiring. Problem: Arcing in or above oven chamber. Possible causes: 1. Burnt carbonized food deposits. 2. Exposed sharp metal edges. Problem: Fuse blows when initiating cook cycle. Possible causes: 1. Defective interlock switches or misaligned door. 2. Shorted HV capacitor. 3. Shorted HV diode. 4. Shorted magnetron (probably won't blow main fuse but HV fuse if used). 5. Defective triac. 6. Old age or power surges. 7. Defective HV transformer. 8. Short in wiring due to vibration or poor manufacturing. Problem: Fuse blows when microwave shuts off (during or at end of cook cycle). Possible causes: 1. Defective triac (doesn't turn off properly). 2. Defective relay. 3. Shorting wires. Problem: Oven heats on high setting regardless of power setting. Possible causes: 1. Faulty primary relay or triac or HV relay (not commonly used). 2. Faulty controller. Problem: Oven immediately starts to cook when door is closed. Possible causes: 1. Shorted relay or triac. 2. Faulty controller. Problem: Oven heats but power seems low or erratic. Possible causes: 1. Low line voltage. 2. Magnetron with low emission. 3. Faulty controller or set for wrong mode. 4. Stirrer (or turntable) not working. 5. Intermittent connections to magnetron filament or elsewhere. 6. Faulty primary relay or triac or HV relay (not commonly used). Problem: Oven heats but shuts off randomly. Possible causes: 1. Overheating due to blocked air vents or inoperative cooling fan. 2. Overheating due to bad magnetron. 3. Bad connections in controller or microwave generator. 4. Faulty interlock switch or marginal door alignment. 5. Faulty controller. 6. Overheating due to extremely high line voltage. Problem: Oven makes (possibly erratic) buzzing noise when heating. Possible causes: 1. Fan blades hitting support or shroud. 2. Vibrating sheet metal. 3. Vibrating transformer laminations. 4. Turntable or stirrer hitting some debris. Problem: Oven light does not work. Possible causes: 1. Burnt out bulb :-). 2. Bad connections. Problem: Fans or turntables that do not work. Possible causes: 1. Gummed up lubrication or bad motor bearing(s). 2. Loose or broken belt. 3. Bad motor. 4. Bad thermostat. 5. Bad connections.
The most common problems occur in the microwave generating portion of the system, though the controller can be blown by a lightning strike or other power surge. Bad interlock switches probably account for the majority of microwave oven problems. Also, since the touchpad is exposed, there is a chance that it can get wet or damaged. If wet, a week or so of non-use may cure keys that don't work. If damaged, it will probably need to be replaced - this is straightforward if the part can be obtained, usually direct from the manufacturer. Unfortunately, it is an expensive part ($20-50 typical). The interlock switches, being electromechanical can fail to complete the primary circuit on an oven which appears to operate normally with no blown fuses but no heat as well. Faulty interlocks or a misaligned door may result in the fuse blowing as described above due to the incorrect sequencing of the door interlock switches. Failed interlocks are considered to be the most common problems with microwave ovens, perhaps as high as 75% of all failures. See the section: "Testing and replacing of interlock switches". No adjustments should ever be required for a microwave oven and there are no screws to turn so don't look for any!
The following problems are likely power or controller related and not in
the microwave generator unless due to a blown fuse or bad/intermittent
connections:
* Totally dead oven.
* No response to any buttons on touchpad
* Oven runs when door is still open.
* Oven starts on its own as soon as door is closed.
* Oven works but display is blank.
* Whacked out controller or incorrect operation.
* Erratic behavior.
* Some keys on the touchpad do not function or perform the wrong action.
* Microwave oven does not respond to START button.
First, unplug the microwave oven for a couple of minutes. Sometimes, the
microcontroller will get into a whacko mode for some unknown reason - perhaps
a power surge - and simply needs to be reset. The problem may never reoccur.
Note: when working on controller related problems, unplug the connection
to the microwave generator (HV transformer primary) from the power relay
or triac - it is often a separate connector. This will prevent any possible
accidental generation of microwave energy as well as eliminating the high
voltage (but not the AC line) shock hazard during servicing.
If this does not help, there is likely a problem with the controller circuitry
or its power and you will have to get inside the oven.
Some cockroaches (or other lower life forms) may have taken up residence on the controller circuit board. It is warm, cozy, safe, and from their point of view makes an ideal habitat. If you got the microwave oven from a flea market, garage sale, the curb, a relative, or friend, or if your kitchen isn't the cleanest in the world, such visitors are quite possible. Creatures with six or more legs (well, some two legged varieties as well) are not known for their skills in the areas of housekeeping and personal hygiene. Clean the circuit board and connectors thoroughly with water and then isopropyl alcohol. Dry completely. Inspect the circuit traces for corrosion or other damage. If there are any actual breaks, these will have be be jumpered with fine wire and then soldered. Hopefully, no electronic components were affected though there is always a slight possibility of other problems.
First, check power to the outlet using a lamp or radio you know works. The fuse or circuit breaker at your service panel may have blown/tripped due to an overload or fault in the microwave oven or some other appliance. You may just have too many appliances plugged into this circuit - microwave ovens are high current appliances and should be on a dedicated circuit if possible. If you attempt to run a heating appliance like a toaster or fryer at the same time, you *will* blow the fuse or trip the circuit breaker. A refrigerator should never be plugged into the same circuit for this reason as well - you really don't want it to be without power because of your popcorn! If you find the fuse blown or circuit breaker tripped, unplug everything from the circuit to which the microwave is connected (keep in mind that other outlets may be fed from the same circuit). Replace the fuse or reset the circuit breaker. If the same thing happens again, you have a problem with the outlet or other wiring on the same branch circuit. If plugging in the microwave causes the fuse to blow or circuit breaker to trip immediately, there is a short circuit in the power cord or elsewhere. Next, try to set the clock. With some ovens the screen will be totally blank following a power outage - there may be nothing wrong with it. Furthermore, some ovens will not allow you perform any cooking related actions until the clock is set to a valid time. Assuming these are not your problems, a fuse has probably blown although a dead controller is a possibility. If the main fuse is upstream of the controller, then any short circuit in the microwave generator will also disable the controller and display. If this is the case, then putting in a new fuse will enable the touchpad/display to function but may blow again as soon as a cook cycle is initiated if there is an actual fault in the microwave circuits. Therefore, try a new fuse. If this blows immediately, there may be a short very near the line cord, in the controller, or a defective triac (if your oven uses a triac). If it does not blow, initiate a cook cycle (with a cup of water inside). If the oven now works, the fuse may simply have been tired of living. This is common. If the fuse still blows immediately, confirm that the controller is operational by unplugging the microwave generator, power relay, and/or triac from the controller. If a new fuse does not now blow when a cook cycle is initiated - and it appears to operate normally - then one of the components in the microwave generator is defective (shorted). See the section: "Microwave generator problems". Some models have a thermal fuse as well and this may have failed for no reason or a cooling fan may not be working and the oven overheated (in which case it probably would have died while you were cooking something for an important guest - assuming you would use a microwave oven for such a thing!). Other possible causes: bad controller power supply or bad controller chip. The most common way that the controller circuitry can be harmed is by a power surge such as from a lightning strike. Hopefully, only components on the primary side of the power transformer will be affected. In some cases, circuit board traces may have been vaporized (but repair may still be possible by simply jumpering across the crater). Assuming that the main fuse checks out, then check the power supply for the controller next. Also check for bad solder connections.
There can be many causes for this behavior (or lack of behavior): * Door is not closed - on many ovens, there will be no response to any buttons - even setting the clock - unless the door is securely closed. * You waited too long - some models (like Sharp) have a timeout. If you close the door but don't proceed to activate any functions with a couple of minutes, they will require you to open and close the door to reset their pathetic brains. * Controller is confused - a power surge or random non-reproducible action of the universe may have resulted in the controller's program ending up in an infinite loop. Pull the plug for a minute or two to reset it. * Defective interlock switches - this can result in the controller thinking the door is open and ignoring you. * Faulty controller or its power supply - a power surge may have damaged the electronics. Other than checking for bad connections and obviously bad power supply components, diagnosing this will be tough without a schematic (and possibly much more). * Touchpad or controller board contaminated by overenthusiastic cleaning - if you recently power washed the oven (or even if you only use some spray cleaner), some may have gotten inside and shorted out the touchpad or controller. * Defective or damage touchpad - physical abuse is not a recommended technique for getting a microwave oven to cooperate. If there is any visible damage to the touchpad - the outer film is broken - it will probably need to be replaced. Also see the section: "Some of the keys on the touchpad do not function or perform the wrong action".
WARNING: Needless to say, DO NOT operate the oven with the door open! While extremely unlikely, the microwave be generator could be running! For microwaves to actually be generated with the door still open would require the failure of all 3 interlock switches. The only way this could really happen would be for the 'fingers' from the door that engage the interlocks to break off inside the oven keeping the interlocks engaged. In this case, the controller would think the door was always closed. Where no such damage is evident, a failure of this type is extremely unlikely since power to the microwave generator passes through 2 of the 3 interlock switches. If both of these failed in the closed position, the third switch would have blown the fuse the last time the door was opened. Another more benign possibility is that one or more fans are running as a result of either a defective sensor or normal operation to maintain air flow until all parts have cooled off.
If the oven starts up as soon as the door is closed - regardless of whether a cook cycle has been selected, the cause could be a shorted triac or relay or a problem with the controller or touchpad. First, unplug the oven for a couple of minutes to try to reset the controller. If this doesn't help, put a cup of water into the oven and let it run for a minute to check for heating. (You could also note the normal sound change or slight dimming of lights that accompanies operation of the magnetron.) Much more must be enabled to actually power the magnetron so this might point more to the controller as being faulty but not always. Also see the section: "Whacked out controller or incorrect operation".
If all functions work normally including heating but the display is blank (assuming you can issue them without being able to see the display), the problem is almost certainly in the controller or its power supply. Try pulling the plug for a minute or two - for some reason the display portion of the controller may have been sent out to lunch by a power surge or alpha particle. It woudn't be the first time. Check for bad connections between the display panel and the power supply and solder joints on the controller board. With everything else operational, a bad microcontroller chip is not that likely but is still a possibility. If the oven was physically abused, the display panel may have fractured though it would take quite a bit of violence. In this case, more serious damage to the door seals may have resulted as well which would be a definite hazard.
The following are some of the possible symptoms: * All the display digits may have come on, EEEE or FFFF, or be displaying in Greek. * The end-of-cooking cycle or keypress tone may be wailing away continuously. (By 'tone' I mean from the controller (not a low buzzing or humming when attempting to cook which would indicate a microwave generator power problem like a shorted magnetron). * Pressing a button on the touchpad may result in a totally incorrect action such as entering the time resulting in the oven starting to cook. However, for the special case where pressing START results in erratic behavios, see the section: "Erratic behavior". * The oven may start cooking (or at least appear to) as soon as the door is closed. Pressing buttons on the touchpad may or may not have any effect. (This could also be a shorted triac or power relay). First, try unplugging the oven for a couple of minutes - perhaps the controller is just confused due to a power surge, lightning strike or the EMP from a nearby nuclear detonation because it wanted attention. If you recently cleaned the oven, some liquid may have accidentally gotten inside the touchpad or even the controller circuitry (though this is less likely). See the section: "Some of the keys on the touchpad do not function or perform the wrong action". If the oven seems to have a mind of its own - running a cycle you didn't think you programmed, are you sure a previous cook cycle was not interrupted and forgotten? Try to recreate the problem using a cup of water as a load. Assuming this does not apply, it sounds like a controller problem - possibly just a power supply but could also be the controller chip. My guess is that unless you were to find some simple bad connections or an obvious problem with the controller's power supply, the cost to repair would be very high as the custom parts are likely only available from the manufacturer. The controller's program may be corrupted (unlikely) but we have no real way of diagnosing this except by exclusion of all other possibilities. Depending on the model, some or all operations - even setting the clock - may be conditional on the door interlocks being closed, so these should be checked. Some ovens will not allow any actions to be performed if the door has been closed for more than a few minutes - open and close the door to reset. A controller failure does little to predict the reliability of the rest of the oven. The microwave generator circuits could last a long time or fail tomorrow. The output of the magnetron tube may decrease slightly with use but there is no particular reason to expect it to fail any time soon. This and the other parts are easily replaceable. However, unless this oven has a lot of fancy features, you can buy a replacement (depending on size) for $100-200 so it is probably not worth fixing unless it is something relatively simple and inexpensive.
There are three different situation: * Whenever the oven performs unexpectedly both during setup and the cook cycle, suspect the controller power supply or bad connections. * Where problems only occur when entering or during the cook cycle, suspect a power relay or mechanical timer (if used) with dirty or worn contacts, or (less likely) the power surge from energizing the microwave generator or microwave (RF) leakage into the electronics bay affecting the controller. * However, if erratic simply means that it doesn't heat consistently, see the section: "Oven heats but power seems low or erratic". The filter capacitor(s) in the controller's power supply may be dried up or faulty. Check with a capacitor meter or substitute known good ones. Prod the logic board to see if the problem comes and goes. Reseat the flex cable connector to the touchpad. For mechanical timers, the timing motor could be defective or require lubrication. The contacts could be dirty or worn. There may be bad connections or loose lugs. The primary relay may have dirty or burnt contacts resulting in erratic operation. If the oven uses a HV relay for power control, this may be defective. If the times and power levels appear on the display reliably but then become scrambled when entering the cook cycle or the oven behaves strangely in some other way when entering the cook cycle, there are several possibilies: * The power surge caused by the cook cycle starting is resulting in changes to the settings or else the microcontroller is not interpreting them properly. This may be due to a faulty part of bad connections in the controller or elsewhere. As with intermittent problems, a thorough search for loose ground and other connections and bad solder joints may locate the source of the difficulty. * Microwave (RF) leakage into the electronics bay due to an faulty joint between the magnetron and the waveguide or structure failure of the magnetron may be interfering with the operation of the microcontroller. Unless the oven was dropped or 'repaired' by an butcher, this sort of failure is unlikely. If you suspect either of these, inspect the integrety of the magnetron-waveguide joint and make sure the RF gasket is in place. Unfortunately, this is sometimes difficult to pinpoint because unless there is obvious mechanical damage, the 'problem' may disappear once the cover is removed for testing. See the section: "Problems with internal microwave leakage". * On rare occasions, the main fuse may become intermittent rather than failing completely. The surge or vibration of starting can jiggle the element open or closed. It is easy to try replacing it!
(From: Charles Godard (cgodard@iamerica.net)). I only service Amana's, but have serviced lot's of them over the years. I've only found a few that leaked with my expensive leak detector. The most memorable was the one with the leak that was due to the copper gasket that's between the magnetron tube and the cavity. I just reformed the gasket and reseated the magnetron and that fixed the leak. The symptom was that the Touch Pad timer lights and indicators would change while the unit was cooking. I thought I had a timer problem. I took it apart and checked for loose solder joints and even cleaned the glass touch pad contacts. For some reason that I don't remember now, I checked for radiation with the cover off the unit and found it extremely high. It turned out that the radiation was affecting the controller. From the outside, with the cover on, the unit didn't leak. Long ago, I tried one of the cheapie detectors because one of my parts supply houses suggested it, and it detected leaks on everything. After that I shelled out the bucks and bought a real detector. (From: Matthew Sekulic (goatboy@telusplanet.net)). I have had a similar experience with a Sanyo, similar symptoms, but with the leakage from the spot welded waveguide inside the unit. Our calibration meter showed a two watt leakage, with none escaping the outer case when attached. (My worst case of actual external leakage was from a misaligned door at .75 watts with the probe's styrofoam spacer placed against the door, of course dropping off to near zero a few inches away. My clue in was a spark between the waveguide and the case, when I was messing with the Controller PCB.)
Look carefully for any visible signs of damage or spills. The touchpads often use pressure sensitive resistive elements which are supposed to be sealed. However, any damage or just old age may permit spilled liquid to enter and short the sensors. A week or so of drying may cure these problems. If there is actual visible damage, it may be necessary to replace the touchpad unit, usually only available from the original manufacturer. Also, check the snap type connector where the touchpad flex-cable plugs into the controller board. Reseating this cable may cur a some keys dead problem. With a little bit of effort (or perhaps a lot of effort), the internal circuitry of the touchpad can be determined. This may require peeling it off of the front panel). Then, use resistors to jumper the proper contacts on the flex cable connector to simulate key presses. This should permit the functions to be verified before a new touchpad is ordered. Caution: unplug the microwave generator from the controller when doing this sort of experiment! If the problem was the result of a spill into the touchpad, replacement will probably be needed. However, if you have nothing to lose, and would dump it otherwise, remove the touchpad entirely and wash it in clean water in an effort to clear out any contamination, then do the same using high purity alcohol to drive out the water, and then dry it out thoroughly. This is a long shot but might work.
While all other functions operate normally including clock, cook time, and power setting, pressing START does nothing, including no relay action and the timer digits do not count down. It is as though the START button is being totally ignored. (However, if there is a momentary response but then the oven shuts off, see the section: "Erratic behavior". If there is an alternate way of activating the cook cycle, try it. For example, Sharp Carousel IIs have a 'Minute Plus' button which will cook for one minute on HIGH. Use this to confirm the basic controller logic and interlock circuitry. If it works, then the problem may indeed be a faulty START button. If it is also ignored, then there may be a bad interlock or some other problem with the controller. Check for bad interlocks or interlocks that are not being properly activated. Next confirm if possible that the START touch pad button is not itself faulty. If you can locate the matrix connections for this button, the resistance should go down dramatically (similar to the other buttons). See the section: "Some of the keys on the touchpad do not function or perform the wrong action" The START button does, after all, sees quite a lot of action! Assuming it is not the touch pad, it sounds like the controller is either not sensing the start command or refusing to cooperate for some reason - perhaps it thinks an interlock is open. Otherwise, the timer would start counting. Testing the relay or triac control signal will likely show that it is not there. Check that there are no missing power supply voltages for the controller and bad connection.
Failures in the microwave generator can cause various symptoms including: * No heat but otherwise normal operations. * Fuse blows when closing or opening door. * Loud hum and/or burning smell when attempting to cook. * Arcing in or above oven chamber. * Fuse blows when initiating cook cycle. * Fuse blows when microwave shuts off (during or at end of cook cycle). * Oven heats on high setting regardless of power setting. * Oven immediately starts to cook when door is closed. * Oven heats but power seems low or erratic. * Oven heats but shuts off randomly. Most of these are easy to diagnose and the required parts are readily available at reasonable prices.
If the main power fuse is located in the primary of the high voltage transformer rather then at the line input, the clock and touchpad will work but the fuse will blow upon initiating a cook cycle. Or, if the fuse has already blown there will simply be no heating action once the cook cycle is started. There are other variations depending on whether the cooling fan, oven light, and so forth are located down stream of the fuse. Some models may have a separate high voltage fuse. If this is blown, there will be no heating but no other symptoms. However, high voltage fuses are somewhat rare on domestic ovens. A number of failures can result in the fuse NOT blowing but still no heat: * Bad connections - these may be almost anywhere in the microwave generator or the primary circuit of the HV transformer. A common location is at the crimp connections to the magnetron filament as they are high current and can overheat and result in no or intermittent contact. See the section: See the section: "Testing the magnetron". * Open thermal protector - usually located on magnetron case. Test for continuity. It should read as a dead short - near zero ohms. See the section: "Testing thermal protectors and thermal fuses". * Open thermal fuse - some ovens have one of these in the primary circuit. It may be in either connection to the HV transformer or elsewhere. Test for continuity. It should read as a dead short - near zero ohms. * Open HV capacitor - see the section: "Testing the high voltage capacitor". A shorted HV capacitor would likely immediately blow the fuse. * Open HV diode - see the section: "Testing the high voltage diode". * Open magnetron filament - This failure may also be due to loose, burnt, or deteriorated press (Fast-on) lugs for the filament connections and not an actual magnetron problem. See the section: "Testing the magnetron". * Open winding in HV transformer. See the section: "Testing the high voltage transformer". * Defective HV relay. A few models use a relay in the actual high voltage circuitry (rather than the primary) to regulate cooking power. This may have dirty or burnt contacts, a defective coil, or bad connections * Shorted HV diode - see the section: "Testing the high voltage diode". * Short or other fault in the magnetron - see the section: "Testing the magnetron". * Short in certain portions of the HV wiring. See the section: "Testing and repairing the wiring and connections". A shorted HV diode, magnetron, or certain parts of the HV wiring would probably result in a loud hum from the HV transformer but will likely not blow the main fuse. (However, the HV fuse - not present on most domestic ovens - might blow.) Depending on design, a number of other component failures could result in no heat as well including a defective relay or triac, interlock switch(s), and controller.
This means that the main fuse in the microwave (or less commonly, the fuse or circuit breaker for the power outlet) pops when the microwave oven door is closed or opened. This may be erratic, occurring only 1 out of 10 times, for example. The cause is almost certainly related to either the door interlock switches or the door itself. Marginal door alignment, broken 'fingers' which operate the switches, dislocated parts in the interlock mechanism, or a defective interlock switch may result in either consistent or erratic behavior of this type. On some ovens, this can happen at any time regardless of the control panel settings or whether the oven is in the cook cycle or not. On others, it can only happen when interrupting the cook cycle by opening the door or when initiating the cook cycle from the front panel (if the switches are in the wrong state). The rational for this basic design - some form of which is used in virtually all microwave ovens - is that a defect in the interlock switches or door alignment, which might result in dangerous microwave radiation leakage, will produce a hard permanent failure. This will prevent the oven from being used until it is inspected and repaired. * As noted, one of the interlock switches is actually across the power line. If the switches are activated in the wrong sequence due to a misaligned door, that switch will not turn off before the other switches turn on shorting the power line. Similarly, if its contacts are welded closed, the power line will be shorted when the other switches close. See the section: "Testing and replacing of interlock switches". * Inspect the door, its mounting, and the plastic 'fingers' which operate the interlock switches as well. Again, if the sequence is not correct, the power line will be shorted blowing the fuse. If the oven was dropped, then such damage is quite likely. Look for broken or dislocated parts, warpage, and other indications of problems with the door and interlock mechanism Of course, if the oven was dropped, there could be much more extensive internal damage as well.
A loud abnormal hum is an indication of a short somewhere. The sound may
originate from the HV transformer vibrating and/or from within the magnetron
depending on cause. There may be a burnt odor associated with this behavior:
* Shorted HV diode - see the section: "Testing the high voltage diode".
* Shorted magnetron (filament to anode) or other internal fault in the
magnetron - see the section: "Testing the magnetron". Arcing within
the Magnetron case (visible through ventilation holes in the bottom
section) is usually an indication of a bad magnetron.
* Other short resulting from frayed insulation or wires touching in the
microwave generator.
* Shorted HV transformer - see the section: "Testing the high voltage transformer".
* Short resulting from burnt on food (usually) in or around the waveguide.
If the odor is coming from the oven chamber, see the section: "Arcing in or above oven chamber".
The following procedure will quickly identify the most likely component if
the problem is not food/spills/carbon related:
(Usually a loud hum is caused by a short in the HV transformer, HV diode, or
magnetron. The other items listed below would likely blow the main fuse but
possibly not always.)
(Portions from: Tony (tonyb@ramhb.co.nz)).
1. Discharge HV capacitor! (If there is a short it is doubtful if it has any
charge but never hurts to be safe).
2. Remove one end of the lead from the HV capacitor to the transformer.
3. Start the oven.
* Hum gone? If so, it is the HV circuitry, go to step 4.
* If it still hums you probably have a faulty HV Transformer. (Not
uncommon.)
4. Discharge the HV capacitor again, reconnect wire and disconnect the 2 wires
to the magnetron.
5. Restart oven.
* Hum Gone? If so, magnetron is shorted. Replace or get a new oven.
* Hum still there? If so, go to step 6.
6. You have either
* Shorted HV capacitor,
* Shorted HV Diode,
* Shorted clamp diode across the HV Cap terminals (if one is present, about
30% of microwave ovens use these). (The oven will run 100% without this
protection for the HV capacitor but it should be replaced if possible.)
* Some older Panasonic ovens have a HV reed switch and these can also short
but these are rare now because of the age.
There is often a simple cause: * Arcing in the oven chamber with a normal load (a cup of water, for example), often just indicates that a thorough cleaning of the oven chamber is needed, particularly around and inside/above the waveguide cover. Any food that gets trapped here will eventually burn and carbonize resulting in a focal point for further arcing. Usually, the waveguide cover is designed to be removable without taking the cover off of the oven. However, burnt food and carbon often make this difficult so that some disassembly will be required. Clean the waveguide cover and clean inside the waveguide as well. If the waveguide cover is broken or damaged seriously, replacement material is available. The oven will work fine without it but replacement will prevent contamination of the waveguide with food vapors or splatters which can lead to more expensive damage. Take extra care to cover all food (which you should do anyhow) until the waveguide cover is replaced. * Any sharp metal edges may also result in arcing or sparking. However, the only way such damage could occur as part of the oven (not added knives or forks!) would be through physical abuse. * If your oven uses a stirrer above the oven chamber (no turntable), it may be stuck. The result will be an uneven distribution of microwave energy and localized heating, arcing, and possibly melting plastic or metal.
The fuse may only blow when actually attempting to cook but depending on design, triacs and/or door switches may always be live and may result in a blown fuse at any time when plugged in or when the door is opened or closed. The following can cause the fuse to blow (in approximate order of likelihood): * Defective interlock switches or misaligned door. At least one of the interlock switches is across the power line and will blow the fuse if not activated in the correct sequence. See the sections: "Fuse blows when closing or opening door" and "Testing and replacing of interlock switches". * Shorted HV capacitor. See the section: "Testing the high voltage capacitor". * Shorted HV diode (see note below). See the section: "Testing the high voltage diode". * Shorted magnetron (filament to anode - see note below). See the section: "Testing the magnetron". * Defective triac (shorted or partially shorted). See the section: "Testing and replacing the triac". * Old age or power surge. Fuses sometimes blow for no apparent reason. * Defective HV transformer (shorted windings. See the section: "Testing the high voltage transformer". * Shorted wiring due to vibration or poor manufacturing quality. See the section: "Testing and repairing the wiring and connections". Note that a shorted magnetron or shorted HV diode - which you would think should blow the fuse - probably will not do so because current will be limited by the impedance of the HV capacitor (assuming it is not shorted as well). However, there will likely be a loud hum from the HV transformer as it strains under the excess load. Such a sound in conjunction with no heat is a likely symptom of a shorted magnetron or HV diode. If your oven has a separate high voltage fuse - somewhat rare in domestic ovens - it may certainly blow due to a fault in any of the HV components. Fuses also die of old age. The types of fuses used in microwave ovens are subjected to a heavy load and you may find that all that is needed is to replace the fuse with one with equivalent ratings. (but check for shorts first). There could be an intermittent problem as well which will only show up at some random time in the future. A poorly timed power surge (as opposed to the well timed variety) could also weaken the fuse element resulting in eventual failure. The fuses used in microwave ovens are usually ceramic 1-1/4" x 1/4" 15 or 20 A 250 V fast blow type. Replace with exactly the same type and rating. Another possible cause of a blown fuse is a partially bad triac. Some ovens use a triac rather than a relay to control the main power to the high voltage transformer. One type of failure of a triac is for it to be totally shorted causing the oven to come on whenever the door is closed. Alternatively, the gate may be defective preventing the triac from ever turning on. A third, and most interesting possibility, is that one half of the triac is bad - shorted or open, or doesn't turn on or turn off reliably. Recall that a triac is in effect a pair of SCRs in parallel in opposite directions. If one side is defective, the main fuse will blow due to transformer core saturation since the triac will act as a rectifier and transformers really do not like DC. See the chapter: "Testing and Replacement of Components" for more information on this and similar problems.
This could be due to a number of faults including shorting wires or defective relay. However, a common cause that might not be obvious is that the triac used to switch power to the high voltage transformer is faulty. What is probably happening is that only one half of the triac (recall that a triac is controlled for both polarities of the line voltage/current) is turning off completely resulting in DC to the HV transformer, core saturation, and excessive current which blows the fuse. Drive to the triac could also be marginal but the bad triac is more likely. The following description applies directly to some GE and Hotpoint models. Modify it accordingly for your oven. Depending on model, the triac may be located on the control board or mounted directly on the chassis. (From: John Gallawa (mtek@pen.net)). I have seen exactly this problem; and I've seen it baffle many a repair shop. It is likely that the triac on the 'Power Control Board' is breaking down. This is a fairly common problem in GE and Hotpoint models that use this board. You can usually confirm the problem by setting the oven to a lower power level, say "medium," and heat a cup of water. You will probably hear a 'thump!' each time the magnetron cycles on. This is an indication of a weakened triac. Replace the triac (Q1) with either of the following: ECG 56010, or SK 10265. Finally, replace the line fuse, install the outer cover, and test the oven for proper operation. The only other alternative is to replace the board. The cost used to be pretty reasonable, but now it's gotten expensive - probably about $80.00. The triac is probably located beneath a red plastic guard on the power control board. Its designation is usually Q1. (From: John Montalbano (jrmont@iquest.net)). The microwave oven in my General Electric JHP65G002AD cooking center blew its 15 AMP fuse each time the timing cycle expired. Replacing the triac GE Part number WB27X5085 ($65.00 from GE) with a new NTE56014 ($13.00) solved the problem. (From: Les Bartel lbartel@veribest.com)). I had the exact same symptoms on my GE microwave. I replaced the triac with a $3 15 amp off-the-shelf triac and it has been working for several years since. See the chapter: "Testing and Replacement of Components" for more information on triac testing though replacement is probably the only sure test.
Power levels in a microwave oven are controlled by cycling the microwave generator on and off with a variable duty cycle - kind of like slow pulse width modulation. For 'HIGH', it runs continuously; for low, it may run 10% on and 90% off; other settings are in between. When the oven always seems to be stuck at high power, it is likely to be due to one of two possible causes - a faulty relay or Triac, or controller. The relay or triac may have failed in the on state. This will probably show up with ohmmeter tests (with the oven unplugged!) but not always. Replacements should be readily available. If the problem is is the controller, it will be more difficult to diagnose as schematics for the controller are usually not readily available. However, it could be something simple like a bad connection or dirty connector.
This means that the relay or triac which controls power to the microwave generator, or the controller or timer has failed in the on state. If the problem is the relay or triac, it is simple to diagnose and repair since the component can be easily identified and tested. If the problem is is the controller, it will be more difficult as schematics are usually not readily available. Check the relay or triac with an ohmmeter. Disconnect the output of the timer or controller and see if it still come on immediately. With a mechanical timer, repair may be possible.
Some considerations are how old the oven is and did the problem happen suddenly or did it just gradually weaken over the years. First, are you sure the problem is real? Perhaps you are just a little less patient than you used to be. Perform a water heating test or try to pop a bag of popcorn using you usual time setting. See the section: "Testing the oven - the water heating test". * If you are subject to brownouts or are running on your own generator, the line voltage may be low. Power output is quite sensitive to the AC input - there is no regulation. A 10% drop in line voltage is likely to reduce microwave power output by more than 20%. * Magnetrons, like other vacuum tubes, can weaken with age and use. An oven that sees daily use may indeed weaken over the course of several years. It is unlikely that any other electronic components could change value in such a way as to significantly affect power output. However, a failure of the controller or sensor (if you have one) could result in short cycling. Testing on HIGH will eliminate this possibility. Make sure the magnetron is powered continuously and it is not cycling. You can often tell by listening for the relay clicks and/or by observing the oven light/other lights dimming as the magnetron kicks in. 50% power should result in approximately equal on and off times. * If you run the oven on HIGH, can you tell if it is actually heating continuously or rather it thinks you want LOW? Many microwave ovens make a clicking sound as they use a relay to switch microwave power on and off - check if you can hear this. Alternatively, lights on the same circuit or the oven light may dim slightly when the magnetron kicks in. There should not be any cycling on HIGH - the microwave power should stay on continuously while it is cooking. If it is cycling, there may be a problem with the controller or you may unknowingly be in a low power mode - check it. * Mechanical problems are also possible. Where a spinning paddle wheel is used to 'stir' the microwave energy (often where there is no turntable), its failure to rotate can result in hot and cold spots. Thus, you may see an unexplained variation in cooking times. The paddle is often accessible by unclipping a plastic cover above the oven cavity. Check for bearing failure, binding, broken or lose belt if direct driven, etc. Note that some are rotated by air flow from the cooling fan and require that cover to be in place to rotate. Therefore, it is not really possible to inspect for correct operation with the cover removed. However, you can put a microwave power indicator (NE2 neon light bulb with its leads twisted together) in the oven (with a cup of water for a load) and observe it through the window. You should see a periodic variation in intensity as the paddles do their job. * There could be intermittent connections to the magnetron filament, thermal protector, or elsewhere. But, these would likely show up as erratic operation - no heat at all sometimes - not just a weak oven. Inspect and clean and tighten (if necessary) all connections in the microwave generator including the magnetron filament, HV transformer, HV Diode, HV capacitor, and thermal protector. Be sure to unplug the unit first and discharge the HV capacitor before touching anything! * The thermal protector may be intermittent. Test by clipping a light bulb across it or monitoring with a multimeter on AC voltage. See the section: "Testing thermal protectors and thermal fuses".
Everything operates normally, but the oven shuts off after varying amounts of time. This could be a faulty magnetron, bad cooling fan (or just built up dust and grime block ventilation grilles), bad thermal protector, faulty controller, some other intermittent component, or bad connections. * If resetting it allows cooking to resume immediately, if even for a few seconds, I would not suspect the magnetron or thermal problem as no cool down time is required. It could be bad connections in the controller or elasewhere, a marginal door interlock switch, or a controller problem. Jiggle the door to see if this will cause it to shut off. * If the magnetron was overheating, you would not be able to resume cooking until it cooled and the thermal protector reset. If it just stopped working (i.e., the filament opened), everything would appear normal but there would be no heating. If the magnetron were shorting, there would likely be a loud hum associated with the periods where there was no heat. * If it is not possible to resume cooking for a few minutes indicating that something needs time to cool off, then the magnetron could be faulty but check for the obvious cooling problems first: blocked or dirty ventilation grill. Determine if the magnetron cooling fan is operating by listening for its sound or looking through the ventilation opening in the back of the oven. If it is not, there could be a broken or weak belt, gummed up or lack of lubrication, other mechanical problems, a bad motor, or bad connections. * Extremely high power line voltage may also result in overheating on a poorly designed or oven where the components are marginal.
Assuming operation is normal otherwise, this is most likely either a fan or other motor vibrating on its mounts, fan blades hitting something, or some sheet metal or the high voltage power transformer laminations vibrating. There may be something stuck under the turntable or above the waveguide cover interfering with the stirrer. Something may have loosened up with age and use. If the noise is caused be simple vibrations, no damage is likely to result. However, if the main cooling fan is on its way out and it stops or gets stuck, parts will overheat quite quickly at which point the oven will shut down (hopefully) and there could be damage to the magnetron or other components. Therefore, at least identifying the cause is probably a good idea. The solution may be as simple as tightening a screw or weging a shim between two pieces of vibrating sheet metal.
If the oven light no longer works, believe it or not, a burned out light bulb is likely. You would think that something like replacing a light bulb would be trivial and self evident. Unfortunately, not always so with microwave ovens. Light bulbs may be typically located in any of 3 places: 1. Oven chamber - it may be behind a mesh grill requiring a screw or snap to be removed. This is the easiest. 2. Rear - the bulb may be in a recessed compartment accessible by removing a screw or two on the back of the oven. 3. Inside - it may be behind a non-removable grille requiring the removal of the cover