There are many reasons why the horizontal output transistor (HOT) shorted in CRT Monitor. Here are the possible cause:
HOT have life span-it can work for many years but out of a sudden it can just go shorted. Replacing only the HOT solved the probem!
Dry joints in nearby area- Yes, dry joints in the HOT leads, flyback transformer, horizontal driver circuits could cause HOT to blow.
Shorted flyback primary winding. I came across lots of HOT failure due to shorted primary winding in flyback transformer. One of the best example was the HP 17″ 7500 CRT Monitor. Not only it blow the HOT, the B+ coil and the B+ FET blown as well!
Shorted or open internal capacitor in flyback transformer. These problems has contributed lots of HOT failure in CRT Monitor. Normally i will check the internal capacitor first before i replace the HOT and switch “ON” the Monitor.
Too high B+ voltage- A higher than normal B+ voltage will kill the HOT instantly. Make sure the B+ is supplying the right voltage to the primary winding of flyback transformer.
Running at wrong frequency- I once came across the horizontal driver circuit area was running at different frequency. It suppose to run on a 31.5 Khz (depends on resolution in your CPU setting) but this faulty Monitor run at a too low frequency killing the HOT after “ON” for few hours!
Bad electrolytic capacitor in the supply line to horizontal driver transformer- If you found a HOT shorted make sure you check on this capacitor otherwise you may get a call back from your customer saying the Monitor not working again. The capacitor usually developed high ESR value that destroy the HOT in few hours to few weeks time.
Shorted horizontal yoke coils- A shorted horizontal yoke coil could instantly kill the HOT. I always use the dick smith flyback tester to confirm first that the yoke coil is good before the replacement of a new HOT.
Safety capacitor value out- If this safety (holddown) capacitor value dropped to zero, the high voltage produced at the anode could shoot up to over 30 kilo volt thus killing the HOT in a split of seconds.
Wrong replacement-The Monitor could be repaired by someone else and they had fitted a different specification of HOT thus making the HOT won’t perform the best and breakdown after certain of time.
Nearby components- There are some possibilities too that the bad components nearby the HOT could cause the HOT to blow. Check the surrounding components before you switch “On” the Monitor.
Well, hope you can absorb these tips and start repairing CRT Monitor or TV with more confident! Have a brilliant day
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