Unfortunately, these sorts of problems are often difficult to definitively
diagnose and repair and will often involve expensive component swapping.
|NotTaR of Television Sets : Horizontal output transistors keep blowi..
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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 set 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:
- 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 or broken or loose core can all affect
- Excessive voltage on HOT collector - check LV regulator (and line
voltage if this is a field repair), if any.
- Defective safety capacitors or damper diode around HOT. (Though
this usually results in instant destruction with little heating).
- New transistor not mounted properly to heat sink - probably needs mica
washer and heat sink compound.
- 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.
- CRT shorting internally. If this happens only once in two weeks, it may
be diffuclt to track down :-(.
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 set 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.
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