High Voltage Power Supply Problems.

1.High voltage shutdown due to X-ray protection circuits .

A TV that runs for a while or starts to come on but then shuts down mayhave a problem with the X-ray protection circuitry correctly or incorrectlydetermining that the high voltage (HV) is too great (risking excessiveX-ray emission) and shutting everything down. A side effect of activation of this circuitry is that resetting may requirepulling the plug or turning off the real (hard) power switch. Is there anything else unusual about the picture lately that would indicatean actual problem with the HV? If this is the case, then there may besome 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). One symptom of excessive HV (but not required) is an overly bright picture of reduced size. The HV shutdown circuit usually monitors a winding off of the flybackfor voltage exceeding some reference and then sets a flip flop shuttingthe 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.



2.Low or no high voltage .
Most of these problems are due to faults in the horizontal deflectionsystem - shorted HOT, shorted windings or HV rectifiers in the flyback,defective tripler, or other bad parts on the primary side of the flyback. However, if you discover an inch layer of filth inside the TV, the HVcould simply be shorting out - clean it first. In most cases, these sorts of faults will put an excessive load on thehorizontal output circuits so there may be excessive heating of the HOT or other components. You may hear an audible arcing or sizzling sound from internal shorts in the flyback or tripler. Either of these may bet hot, crack, bulge, or exhibit visible damage if left on with the fault present. Most modern TVs do not regulate HV directly but rather set it viacontrol of the low voltage power supply to the HOT (B+), by snubbercapacitors across the HOT, and the turns ratio of the flyback. TheHV is directly related to the B+ so if this is low, the HV will be lowas well. Faulty snubber capacitors will generally do the opposite – increase the HV and the X-ray protection circuits may kick in. However, low HV is also a possibility. The only way the turns ratio of the flyback can change is from a short which will manifest its presence in other ways as well - excessive heating and load on the horizontal output circuits. While a shorted second anode connection to the CRT is theoreticallypossible, this is quite unlikely (except, as noted, due to dirt).

3.Excessive high voltage.

Any significant increase in HV should cause the X-ray protection circuits to kick in and either shut down the set or modify the deflection in such a way as to render it harmless. Symptoms include arcing/sparking of HV, smaller than normal picture, and under certain scenarios, possible excessive brightness. Causes of the HV being too high are: Excess B+ voltage to the HOT. The likely cause is to a low voltage regulator failure. Open snubber capacitors across the HOT. These are under a lot of stress and are located near hot components so failure is possible. Incorrect excessively long scan drive to HOT caused by failure of horizontal oscillator/sync circuits. However, other things like the HOT will probably blow up first. The picture will definitely be messed up. Failure of HV regulator (tube sets and a few solid state sets - actual HV regulators are relatively uncommon today.) This may result in an underscanned (smaller than normal) picture.



4.Arcing, sparking, or corona from CRT HV anode (red wire/suction cup) .

Symptoms could include a sizzling corona or more likely, an occasionalor rapid series of sharp snaps - possibly quite loud and quite visible – from the anode cap on the CRT to the grounded coating on the outside of the CRT or a chassis ground point (or any other conductor nearby). Corona is a high resistance leakage through the air without total breakdown. The snapping is caused by the sudden and nearly complete discharge of the CRT anode capacitance through a low resistance ionized path similar to lightening. There are two likely causes: Dirt, dust, grime, around and under the suction cup on the CRT are providing a discharge path. This may be more severe in humid weather.Safely discharge the HV and then remove and thoroughly clean the HV suction cup and the area under it and on the CRT for several inches around the HV connection. Make sure there are no loose wires or other possible places for the HV to discharge to in the vicinity. The high voltage has gone through the roof. Usually, the X-ray protection circuitry should kick in but it can fail. If cleaning does not help, this is a likely possibility. See the sections:High voltage shutdown due to X-ray protection circuitsAnd Excessive high voltage.



5.Arcing at CRT spark gaps

This is rarely due to a defective spark gap but rather is a safety mechanism like a fuse designed to protect the internal electrodes of the CRT if the focus or screen voltage should become excessive. The spark gap breaks down first and prevents internal arcing in the CRT. An arcing sparkgap is usually accompanied by total loss of picture or bad focus, brightness or focus fluctuations, or any of a number of similar symptoms. The usual cause is a breakdown inside the focus divider inside the flyback or tripler. Sometimes, it is in a component that can bedisassembled and cleaned but not generally. Replacement of the bad part will be needed.



6.Arcing from flyback or vicinity .

If the arc is coming from a specific point on the flyback - a crack orpinhole - this may be patched using high voltage sealer or even a fewlayers of plastic electrical tape. This may prove to be a permanentrepair although starting the search for a source for a new flybackwould not hurt just in case. The arc most likely did damage the insulation internally which may or may not be a problem in the future. In some cases, the pinhole or crack is an indication of a more seriousproblem - overheating due to shorted windings in the flyback or excessive secondary load. First, clean the areas around the arc thoroughly and then try severallayers of plastic electrical tape. If the TV works normally for say,an hour, then there is probably nothing else wrong and you can try fora proper sealing job or hope that tape holds out (put a few more layers on - each is good for about 8-10 KV theoretically). If the arc is from one of the sparkgaps around the CRT, this could alsobe a flyback problem indicating internal shorts in the focus/screen network. Once I had a TV where the main problem was a cracked flyback arcingbut this took out one of the fusable resistors for the power supply tothe VERTICAL output so the symptoms included a single horizontal line.Don't ask me to explain - replacing that resistor and the flyback (theflyback tested good, but this was for someone else) fixed the TV. In another case, a pinhole developed in the flyback casing probablydue to poor plastic molding at the time of manufacture. This resulted in a most spectacular case of sparking to a nearby bracket. A few layers of electrical tape was all that was needed to affect a permanent repair.



7.Ozone smell and/or smoke from TV .

Smoking is just as bad for TVs as for people and usually more quicklyterminal. White acrid smoke may indicate a failed electrolytic capacitor in thepower supply probably in conjunction with a shorted rectifier. Needless to say, pull the plug at once. A visual inspection should be able to easily confirm the bad capacitor as it will probably be bulging and have condensed residue nearby. Check the rectifier diodes or bridge rectifier with an ohmmeter. Resistance across any pair of leads should be more than a few ohms in at least one direction. Remove from the circuit to confirm. Both the faulty diode(s) and capacitor should be replaced (though the capacitor may work well enough to test with new diode(s). If a visual inspection fails to identify the smoking part, you can probably plug the set in for a few seconds until the source of the smoke is obvious but be prepared to pull the plug in a real hurry. If the smell/smoke is coming from the flyback, then it has probably gone belly up. You may be able to see a crack or bulge in the case. While the flyback will definitely need to be replaced, it is likely that nothing else is wrong. However, it might be prudent to use a Variac when performing initial testing with the replacement just in case there is a secondary short circuit or excess HV problem.



Should I be worried about X-ray exposure while servicing a TV or monitor?

The only source of X-rays in a modern TV or monitor is from the CRT.X-rays are generated when a high velocity electron bean strikes aheavy metal target. For anything you are likely to encounter, this canonly happen in a vacuum - thus inside the CRT. The higher the voltage, the greater the velocity and potential danger. The thick front CRT faceplate protects users adequately but there may be some emission from the thinner sides. At 25-30 KV (quite low as X-ray energies go) X-rays will be stopped by almost any metal so what you have to worry about is where there are no shields. However, realistically, there is very little and I would not worry aboutexposure unless you plan to be sitting for hours on the sides, behind, or under the TV or monitor - with a picture (there will be none if the screen is black).



8.Fly back shot by 4 year old .
Your 4 year son shot the Sony in the flyback transformer. Smoke and sparks everywhere. Great aim! Who says these FAQs cannot be funny? Needless to say, unplug the set immediately. Inspect around the target area for obviously blown or damaged components. Test fuses and fusable resistors. Repair burnt solder connections and circuit board traces. Once the set is entirely dried out, power it up - preferably through a series light bulb and/or Variac until you are sure nothing else will let loose. Look, listen, and smell for any unusual behavior. If it now works, then consider yourself lucky. If not, there may be damage to transistors, ICs, or other components. (The following from: edison@nelson.planet.org.nz (Richard Symonds)). We're seeing another 'hazard' these days, people cleaning theirtelevision screens with window cleaner - no problem in the days ofseparate chassis but with the entire p.c.b. jammed under the tube onmost t.v.s these days just a few drips and its all over. Some havejust corroded the switch banks (had one recently just got into theA/V switch - when you walked around the room the set changed to A/Vand back by itself!) but a few have got around the microprocessors and surface mount components and resulted in complete write-offs. Isuppose the damage is the opposite of electroplating as the microp'shave constant voltage to them. Never mind, they'll be a good sourceof parts for future use.


REFERENCE:
http://www.stedmundsbury.gov.uk/sebc/play/tvprobs.cfm.
http://www.epanorama.net/links/repair.html#tv.