Contents:
CAUTION: To prevent damage to the CRT phosphors, immediately turn down the brightness so the line is just barely visible. If the user controls do not have enough range, you will have to locate and adjust the master brightness or screen/G2 pots. Since you have high voltage, the horizontal deflection circuits are almost certainly working (unless there is a separate high voltage power supply - almost unheard of in modern TVs and very uncommon in all but the most expensive monitors). Check for bad solder connections between the main board and the deflection yoke. Could also be a bad horizontal coil in the yoke, linearity coil, etc. There is not that much to go bad based on these symptoms assuming the high voltage and the horizontal deflection use the same flyback. It is almost certainly not an IC or transistor that is bad.
CAUTION: To prevent damage to the CRT phosphors, immediately turn down the brightness so the line is just barely visible. If the user controls do not have enough range, you will have to locate and adjust the master brightness or screen/G2 pots. A single horizontal line means that you have lost vertical deflection. High voltage is most likely fine since there is something on the screen. This could be due to: 1. Dirty service switch contacts. There is often a small switch on the located inside on the main board or perhaps accessible from the back. This is used during setup to set the color background levels. When flipped to the 'service' position, it kills vertical deflection and video to the CRT. If the switch somehow changed position or got dirty or corroded contacts, you will have this symptom. Flip the switch back and forth a couple of times. If there is some change, then replace, clean, resolder, or even bypass it as appropriate. 2. Bad connection to deflection yoke or other parts in vertical output circuit. Bad connections are common in TVs and monitors. Check around the pins of large components like transformers, power transistors and resistors, or connectors for hairline cracks in the solder. Reseat internal connectors. Check particularly around the connector to the deflection yoke on the CRT. 3. Bad vertical deflection IC or transistor. You will probably need the service manual for this and the following. However, if the vertical deflection is done with an IC, the ECG Semiconductor Master Substitution guide may have its pinout which may be enough to test it with a scope. 4. Other bad parts in vertical deflection circuit though there are not that many parts that would kill the deflection entirely. 5. Loss of power to vertical deflection circuits. Check for blown fusable resistors/fuses and bad connections. 6. Loss of vertical oscillator or vertical drive signals. The most likely possibilities are in the deflection output stage or bad connections to the yoke. To locate the vertical output circuitry without a service manual, trace back from the deflection yoke connector. The vertical coils will be the ones with the higher resistance if they are not marked.
This has all the classic symptoms of a loose connection internal to the TV or monitor - probably where the deflection yoke plugs into the main PCB or at the base of the flyback transformer. TVs and monitors are notorious for both poor quality soldering and bad connections near high wattage components which just develop over time from temperature cycling. The following is not very scientific, but it works: Have you tried whacking the TV when this happened and did it have any effect? If yes, this would be further confirmation of loose connections. What you need to do is examine the solder connections on the PCBs in the monitor, particularly in the area of the deflection circuits and power supply. Look for hairline cracks between the solder and the component pins - mostly the fat pins of transformers, connectors, and high wattage resistors. Any that are found will need to be reflowed with a medium wattage (like 40W) or temperature controlled soldering iron. It could also be a component momentarily breaking down in the power supply or deflection circuits. Another possibility is that there is arcing or corona as a result of humid weather. This could trigger the power supply to shut down perhaps with a squeak, but there would probably be additional symptoms including possibly partial loss of brightness or focus before it shut down. You may also hear a sizzling sound accompanied by noise or snow in the picture, static in the sounds, and/or a smell of ozone. If your AC power fluctuates, an inexpensive monitor may not be well enough regulated and may pass the fluctuations on as jitter. The video card is unlikely to be the cause of this jitter unless it correlates with computer (software) activity.
Unfortunately, these sorts of problems are often difficult to definitively diagnose and repair and will often involve expensive component swapping. You have just replaced an obviously blown (shorted) horizontal output transistor (HOT) and an hour (or a minute) later the same symptoms appear. Or, you notice that the new HOT is hotter than expected: Would the next logical step be a new flyback (LOPT)? Not necessarily. If the monitor performed normally until it died, there are other possible causes. However, it could be the flyback failing under load or when it warms up. I would expect some warning though - like the picture shrinks for a few seconds before the poof. Other possible causes: 1. Improper drive to horizontal output transistor (HOT). A weak drive might cause the HOT to turn on or (more likely) shut off too slowly (greatly increasing heat dissipation. Check driver and HOT base circuit components. Dried up capacitors, open resistors or chokes, bad connections, or a driver transformer with shorted windings can all affect drive waveforms. 2. Excessive voltage on HOT collector - check LV regulator (and line voltage if this is a field repair), if any. 3. Defective safety capacitors or damper diode around HOT. (Though this usually results in instant destruction with little heating). 4. New transistor not mounted properly to heat sink - probably needs mica washer and heat sink compound. 5. Replacement transistor not correct or inferior cross reference. Sometimes, the horizontal deflection is designed based on the quirks of a particular transistor. Substitutes may not work reliably. The HOT should not run hot if properly mounted to the heat sink (using heatsink compound). It should not be too hot to touch (CAREFUL - don't touch with power on - it is at over a hundred volts with nasty multihundred volt spikes and line connected - discharge power supply filter caps first after unplugging). If it is scorching hot after a few minutes, then you need to check the other possibilities. It is also possible that a defective flyback - perhaps one shorted turn - would not cause an immediate failure and only affect the picture slightly. This would be unusual, however. See the section: "Testing of flyback (LOPT) transformers". Note that running the monitor with a series light bulb may allow the HOT to survive long enough for you to gather some of the information needed to identify the bad component.
The HOT may last a few minutes, days, months or years but then blow again. These are among the hardest problems to locate. It could even be some peculiar combination of user cockpit error - customer abuse - that you will never identify. Yes, this should not happen with a properly designed monitor. However, a combination of mode switching, loss of sync during bootup, running on the edge of acceptable scan rates, and frequent power cycles, could test the monitor in ways never dreamed of by the designers. It may take only one scan line that is too long to blow the HOT.
The picture is squashed vertically and a part of it may be flipped over and distorted. This usually indicates a fault in the vertical output circuit. If it uses an IC for this, then the chip could be bad. It could also be a bad capacitor or other component in this circuit. It is probably caused by a fault in the flyback portion of the vertical deflection circuit - a charge pump that generates a high voltage spike to return the beam to the top of the screen. Test components in the vertical output stage or substitute for good ones.
This would mean that the left and right sides of the picture are 'bowed' and
the screen looks something like the diagram below (or the opposite - barrel
distortion).
However, the obvious symptoms may just be excess width as the curved sides may
be cut off by the CRT bezel.
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In particular, this sounds like a pincushion problem - to correct for
pincushion, a signal from the vertical deflection that looks something
like a rectified sinewave is used to modify width based on vertical position.
There is usually a control to adjust the magnitude of this signal and also
often, its phase. It would seem that this circuit has ceased to function.
If you have the schematics, check them for 'pincushion' adjustments and
check signals and voltages. If not, try to find the 'pincushion' magnitude
and phase adjustments and look for bad parts or bad connections in in the
general area. Even if there are no adjustment pots, there may still be
pincushion correction circuitry.
If the internal controls have absolutely no effect, then the circuit
is faulty. With modern digital setup adjustments, then it is even tougher
to diagnose since these control a D/A somewhere linked via a microprocessor.
Pincushion adjustment adds a signal to the horizontal deflection
to compensate for the geometry of the CRT/deflection yoke. If you have
knobs, then tracing the circuitry may be possible. With luck, you have
a bad part that can be identified with an ohmmeter - shorted or open.
For example, if the pincushion correction driver transistor is shorted,
it will have no effect and the picture will be too wide and distorted as
shown above.
However, without a schematic even this will be difficult. If the adjustments
are digital this is especially difficult to diagnose since you don't even
have any idea of where the circuitry would be located.
Faulty capacitors in the horizontal deflection power supplies often cause
a similar set of symptoms.
"I just bought a new Sony 200SX 17" monitor and I just can't get the pin-cushion control to work right. If I get the outer edges straight then any window an inch or so from the edge will curve like crazy. The only way around this is to shrink my screen size so I'll have 3/4 in or so of black space. This is very irritating since I am not getting the 15.9" viewable size as advertised. Is this normal?" (From: Jeroen H. Stessen (Jeroen.Stessen@ehv.ce.philips.com)). The distortion that you describe is called 'inside pincushion'. Normally it can be corrected by a dynamic S-correction circuit. Maybe Sony didn't do a too good job on this, or none at all. It may also be that the correction is optimized for certain horizontal scan frequencies only, as dynamic S-correction is a resonant circuit. You might want to test at another frequency. (From: markmtf@earthlink.net)). You may have a monitor that is at the edge of the acceptance tolerance, (which is a defined acceptable variation for cost and production yield reasons). A typical worse case tolerance may be up to 3mm of a deviation from a straight line for the edges. This applies for all monitors and all manufacturers. Of course some companies actually control the variation better than others, (and some just say they do). For reference; try using the "Recall" function which will set the adjustments to the original factory settings. (This assumes that your video timing matches the preset timing used in the factory). Check the infamous user manual.
A faulty deflection yoke can affect the geometry (size and shape) of the
raster, result in insufficient high voltage and/or other auxiliary power
problems, and blow various components in the low voltage power supply or
elsewhere.
* A simple test to determine if the yoke is at fault for a major geometry
problem (e.g., a keystone shaped picture) is to interchange the connections
to the yoke for the axis that is not affected (i.e., the vertical coils if
the width is varying from top to bottom). If the raster/picture flips
(indicating that you swapped the proper connections) but the shape of the
raster remains the same - the geometry is unchanged, the problem is almost
certainly in the deflection yoke.
* Where high voltage (and other flyback derived voltages) are reduced and
other problems have been ruled out, unplugging the deflection yoke (assuming
no interlock) may reveal whether it is likely at fault. If this results in
high voltage and a relatively clean deflection waveform or returns the power
supply or deflection chip load to something reasonable, a defective yoke is
quite possible.
CAUTION: powering a TV or monitor with a disconnected yoke must be done with
care for several reasons:
- The CRT electron beam(s) will not be deflected. If it turns out that the
yoke is the problem, this may result in a very bright spot in the center
of the screen (which will turn into a very dark permanent spot quite
quickly) :-(. Disconnecting only the winding that is suspect is better.
Then, the other direction will still scan resulting in a very bright line
instead of a super bright spot. In any case, make sure the brightness is
turned all the way down (using the screen/G2 control on the flyback if
necessary). Keep an eye on the front of the screen ready to kill power at
the first sign of a spot or line. Disconnecting the CRT heater as an
added precaution would be even better unless you need to determine if
there is a beam.
- Removing the yoke (which is effectively in parallel with the flyback)
increases the inductance and the peak flyback voltage on the HOT. In the
extreme, this may blow the HOT if run at full line voltage/normal B+. It
is better to perform these tests using a Variac at reduced line voltage if
possible.
- The deflection system will be detuned since the yoke inductance plays a
very significant role in setting the resonance point in most designs.
Don't expect to see totally normal behavior with respect to high voltage.
However, it should be much better than with the faulty yoke.
* If possible, compare all measurements with a known good identical deflection
yoke. Of course, if you have one, swapping is the fastest surest test of
all! In many cases, even a not quite identical yoke will be close enough to
provide useful information for testing. However, it must be from a similar
piece of equipment with similar specifications - size and scan range. Don't
expect a color TV yoke to work in a high performance SVGA monitor!
Note: the substitute yoke doesn't have to be mounted on the CRT which would
disturb purity and convergence adjustments but see the caution above about
drilling holes in the CRT face plate!
The deflection yoke consists of the horizontal coils and vertical coils (wound
on a ferrite core), and mounting structure. Little magnets or rubber/ferrite
strips may be glued in strategic locations. DO NOT disturb them! In rare
instances, there may be additional coils or other components mounted on the
same assembly. The following deals only with the actual deflection coils
themselves - the other components (if any) can be tested in a similar manner.
Where the test procedure below requires removal of the yoke, see the section:
"Removing and replacing the deflection yoke" first.
* Horizontal - the horizontal section consists of an even number of windings
hooked up in parallel/interleaved with half of the windings on each of the
two ferrite core pieces.
The horizontal windings will be oriented with the coil's axis vertical and
mounted on the inside of the yoke (against the CRT neck/funnel). It may be
wound with thicker wire than that used for the vertical windings.
- Resistance check - This may be possible without removing the yoke from
the CRT if the terminal block is accessible. Disconnect the individual
windings from each another and determine if the resistances are nearly
equal. Check for shorts between windings and between the horizontal and
vertical windings as well.
Typical resistance of the intact windings (at the yoke connector assuming
no other components): TV or NTSC/PAL monitor - a few ohms (3 ohms typical),
SVGA monitor - less than an ohm (.5 ohms typical).
- Inspection - Look for charring or other evidence of insulation breakdown
due to arcing or overheating. For the horizontal windings, this will
require removing the yoke from the CRT since little if any of the windings
are visible from the outside. However, even then, most of the windings
are hidden under layers of wire or behind the ferrite core.
- Ring test. See the document "Testing of flyback (LOPT) transformers".
This deals with flyback transformers but the principles are the same.
Disconnecting the windings may help isolate the location of a fault.
However, for windings wound on the same core, the inductive coupling
will result in a short anywhere on that core reducing the Q.
* Vertical - The vertical section is usually manufactured as a pair of windings
wired in parallel (or maybe in series) though for high vertical scan rate
monitors, multiple parallel/interleaved windings are also possible.
The vertical windings will be oriented with the coil's axis horizontal and
wound on the outside of the yoke. The wire used for the vertical winding
may be thinner than that used for the horizontal windings.
- Resistance check - This may be possible without removing the yoke from
the CRT if the terminal block is accessible. Disconnect the individual
windings from each other and determine if the resistances are nearly
equal. Check for shorts between windings and between the horizontal
and vertical windings as well.
Typical resistance of the intact windings (at the yoke connector assuming
no other components): TV or NTSC/PAL monitor - more than 10 ohms (15 ohms
typical), SVGA monitor - at least a few ohms (5 ohms typical).
- Inspection - Look for charring or other evidence of insulation breakdown
due to arcing or overheating. The accessible portions of the vertical
windings are mostly visible without removing the yoke from the CRT.
However, most of the windings are hidden under layers of wire or behind
the ferrite core.
- Ring test - Since the vertical windings have significant resistance and
very low Q, a ring test may be of limited value.
So you found a big black charred area in/on one of the yoke windings. What can be done? Is it possible to repair it? What about using it for testing to confirm that there are no other problems before ordering a new yoke? If the damage is minor - only a few wires are involved, it may be possible to separate them from each other and the rest of the winding, thoroughly clean the area, and then insulate the wires with high temperature varnish. Then, check the resistances of each of the parallel/interleaved windings to make sure that you caught all the damage. Simple plastic electrical tape can probably be used for as insulation for testing purposes - it has worked for me - but would not likely survive very long as a permanent repair due to the possible high temperatures involved. A new yoke will almost certainly be needed.
How and why do flyback transformers fail? Flybacks fail in several ways: 1. Overheating leading to cracks in the plastic and external arcing. These can often be fixed by cleaning and coating with multiple layers of high voltage sealer, corona dope, or even plastic electrical tape (as a temporary repair in a pinch). 2. Cracked or otherwise damaged core will effect the flyback characteristics to the point where it may not work correctly or even blow the horizontal output transistor. 3. Internal shorts in the FOCUS/SCREEN divider network, if present. One sign of this may be arcover of the FOCUS or SCREEN sparkgaps on the PCB on the neck of the CRT. 4. Internal short circuits in the windings. 5. Open windings. More than one of these may apply in any given case. First, perform a careful visual inspection with power off. Look for cracks, bulging or melted plastic, and discoloration, Look for bad solder connections at the pins of the flyback as well. If the TV or monitor can be powered safely, check for arcing or corona around the flyback and in its vicinity, Next, perform ohmmeter tests for obvious short circuits between windings, much reduced winding resistances, and open windings. For the low voltage windings, service manuals may provide the expected DC resistance (SAMs PhotoFact, for example). Sometimes, this will change enough to be detected - if you have an ohmmeter with a low enough scale. These are usually a fraction of an ohm. It is difficult or impossible to measure the DC resistance of the HV winding since the rectifiers are usually built in. The value is not published either. Caution: make sure you have the TV or monitor unplugged and confirm that the main filter capacitor is discharged before touching anything! If you are going to remove or touch the CRT HV, focus, or screen wires, discharge the HV first using a well insulated high value resistor (e.g., several M ohms, 5 W) to the CRT ground strap (NOT signal ground. See the section: "Safe discharging of capacitors in TVs and video monitors". Partially short circuited windings (perhaps, just a couple of turns) and sometimes shorts in the focus/screen divider will drastically lower the Q and increase the load the flyback puts on its driving source with no outputs connected. Commercial flyback testers measure the Q by monitoring the decay time of a resonant circuit formed by a capacitor and a winding on the flyback under test after it is excited by a pulse waveform. It is possible to easily construct testers that perform a well. See the companion document "Testing of flyback (LOPT) transformers" for further information.
You are playing your favorite game (read: addiction) and suddenly, the picture size increases by 20% and the brightness may have changed as well. What part should I replace? I only used my phasers on the #3 setting! Unfortunately, I do not have a crystal ball. There are a number of parts that could be faulty and no way of know for your monitor and your symptoms which it is. Sorry, you will almost certainly have to have it professionally repaired or replaced. What it sounds like is happening is that the circuitry that selects internal components depending on scan rate have failed in some way. They could be making an incorrect selection or the power supply could be faulty and applying an incorrect voltage to the horizontal and vertical deflection circuits. The brightness changes since it is not compensated for properly.
Check the capacitors that couple the yoke to to ground. If they become reduced in value or develop a high ESR, the current will be diverted to other components with unfortunate and rapid consequences.
The first thing to determine is if this is a position or phase problem:
* A fault with horizontal position means that the entire raster is shifted
left or right. This is almost certainly a monitor problem. If you turn
up the brightness control, the edges of the scan lines will probably be
visible on one side.
- Assuming the position or centering controls do not work at or or have
insufficient range, check for a defective centering pot and bad centering
diodes and other components in their vicinity. If digitally controlled,
you will probably need a schematic to find the cause.
- If the monitor was dropped, the yoke or other assembly on the CRT neck
may have shifted (though there would probably be other symptoms as well).
- Monochrome monitors have centering rings on the CRT neck which may have
be knocked out of adjustment. Color monitors adjust the centering
electronically since magnetic rings would mess up the purity and/or
convergence.
* A fault with horizontal phase means that the raster is still centered on
the screen but the picture itself is shifted (and may have some wrap-around)
within the raster. This could be a fault in the monitor or video card or
incorrect settings in the software setup for the video card.
- If this happened while trying out this monitor on a different or modified
computer, just after you have done a software upgrade, or just after
something strange happened (like your PC's CMOS settings got corrupted -
monitor settings are generally not in the CMOS setup but may have been
affected at the same time), reset the monitor's controls to their default
or middle position and then use the software setup or install program that
came with your video card to set scan rates, size, position, and sync
polarity.
- Some monitors have a user accessible horizontal phase control in addition
to horizontal position. This adjusts the delay in the sync circuits so
check that area of the electronics if the control doesn't work or have
enough range.
* There could also be a problem with base drive to the HOT. This may result
in position, phase, size, and linearity errors as the scan being initiated
too soon or too late.
- Weak drive to the HOT due to faulty components in the base circuit or
driver stage might result in the HOT coming out of saturation early. The
picture would be shifted to the right and the HOT might run excessively
hot and blow.
WARNING: The case of the HOT has >1,000 V spikes and B+ when off - don't
touch with power on or until you confirm no voltage is present after
pulling plug.
- If marginal, a drift of position, phase, size, and linearity with warmup
is also likely. Check for dried up electrolytic capacitors and use cold
spray to isolate other bad components. If the drive becomes too weak,
the HOT may blow after after being on for a while.
Most, monitors derive the high voltage for the CRT second anode (THE high voltage, focus, and (sometimes) screen (G2) from the horizontal deflection system. This technique was developed quite early in the history of commercial TV and has stuck for a very simple reason - it is very cost effective. A side effect is that if the horizontal deflection fails and threatens to burn a (vertical) line into the CRT phosphors, the high voltage dies as well. Of course, if the vertical deflection dies.... Some high end monitors utilize a separate high voltage supply. One reason for this approach is to decouple the horizontal deflection from the HV in auto-scan monitors thus simplifying the design. Usually it is a self contained inverter module. It if can be opened, then repair may be possible. With a separate HV supply, there is no need for a HV flyback transformer on the mainboard. Some designs may use a separate HV supply including a flyback which is part of the mainboard but is self contained and independent of the horizontal deflection system. Most TV and monitor (flyback) high voltage supplies operate as follows: 1. Horizontal output transistor (HOT) turns on during scan. Current increases linearly in primary of flyback transformer since it appears as an inductor. Magnetic field also increases linearly. Note: flyback is constructed with air gap in core. This makes it behave more like an inductor than transformer as far as the primary drive is concerned. 2. HOT shuts off at end of scan. Current decreases rapidly. Magnetic field collapses inductively coupling to secondary and generates HV pulse. Inductance and capacitance of flyback, snubber capacitors, and parasitic capacitance of circuitry and yoke form a resonant circuit. Ideally, voltage waveform across HOT during flyback (retrace) period will be a single half cycle and is clamped by damper diode across HOT to prevent undershoot. 3. Secondary of flyback is either a single large HV winding with HV rectifiers built in (most often) or an intermediate voltage winding and a voltage multiplier (see the section: "What is a tripler?"). The output will be DC HV pulses. 4, The capacitance of the CRT envelope provides the needed filtering to adequately smooth the HV pulses into a DC voltage. Sometimes there is a separate HV capacitor as well. 5, A high resistance voltage divider provides the several KV focus voltage and sometimes the several hundred volt screen (G2) voltage as well. Often, the adjustments for these voltages are built into the flyback. The focus and screen are generally the top and bottom knobs, respectively. Sometimes they are mounted separately. This or a similar divider may also provide feedback to control high voltage regulation.
In some TVs and monitors, the flyback transformer only generates about 6-10 KV
AC which is then boosted by a capacitor-diode ladder to the 18-30 KV needed
for modern color CRTs. The unit that does this is commonly called a tripler
since it multiplies the flyback output by about 3 times. Some TVs use a
quadrupler instead. However, many TVs and monitors generate the required
HV directly with a winding with the required number of turns inside the
flyback transformer.
Triplers use a diode-capacitor ladder to multiply the 6-10 KV AC to 18-30 KV
DC. Many triplers are separate units, roughly cubical, and are not repairable.
Some triplers are built in to the flyback - it is probably cheaper to
manufacture the HV diodes and capacitors than to wind a direct high voltage
secondary on the flyback core. In either case, failure requires replacement
of the entire unit.
For external multipliers, the terminals are typically marked:
* IN - from flyback (6-10 KV AC).
* OUT - HV to CRT (20-30 KV DC).
* F - focus to CRT (2-8 KV).
* CTL - focus pot (many megohm to ground).
* G, GND, or COM - ground.
Symptoms of tripler failure are: lack of high voltage or insufficient high
voltage, arcing at focus protection spark gap, incorrect focus voltage, other
arcing, overload of HOT and/or flyback, or focus adjustment affecting
brightness (screen) setting or vice-versa.
A monitor that runs for a while or starts to come on but then shuts down may have a problem with the X-ray protection circuitry correctly or incorrectly determining that the high voltage (HV) is too great (risking excessive X-ray emission) and shutting everything down. A side effect of activation of this circuitry is that resetting may require pulling the plug or turning off the real (hard) power switch. Is there anything else unusual about the picture lately that would indicate an actual problem with the HV? For example, has it suddenly gotten brighter than normal or has the size decreased? If this is the case, then there may be some problem with the HV regulation. If not, the shutdown circuit may be overly sensitive or one of its components may be defective - a bad connection of leaky cap (or zener). If the horizontal frequency is not correct (probably low) due to a faulty horizontal oscillator or sync circuit or bad horizontal hold control (should one exist!), HV may increase and trigger shutdown. Of course, the picture won't be worth much either! With a multiscan monitor, this could happen if the mode switching is faulty resulting in incorrect component settings for a given scan rate. A symptom might be HV shutdown when switching into scan ranges. The HV shutdown circuit usually monitors a winding off of the flyback for voltage exceeding some reference and then sets a flip flop shutting the horizontal drive off. On some Sony models, a HV resistive divider performs this function and these do fail - quite often. The red block called a 'HV capacitor' is a common cause of immediate or delayed shutdown on certain Sony monitors and TVs. See the section: "Apple/Sony monitor dies after variable length of time".Go to [Next] segment
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