NotTaR of Television Sets : Horizontal output transistors keep blowi..  
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Horizontal output transistors keep blowing (or excessively hot)

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 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:

  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 or broken or loose core 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.

  6. 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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