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NotTaR of Television Sets : TV doesn't work after being in storage
Copyright ©
1994-2007, Samuel M. Goldwasser. All Rights Reserved. Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying.
I may be contacted via the Sci.Electronics.Repair FAQ (www.repairfaq.org) Email Links Page.
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So the TV you carefully stuffed in a corner of the garages is now totally
dead. You swear it was working perfectly a year ago.
Assuming there was absolutely no action when you turned it on, this has
all the classic symptoms of a bad connection. These could be cold/cracked
solder joints at large components like transformers, power resistors, or
connectors and connectors that need to be cleaned or reseated. By 'no action'
I mean not even a tweet, bleep, or crackle from anything.
To narrow it down further, if careful prodding of the circuit board(s) and
various large components with a well insulated stick does not induce the set
to come on, even momentarily, check the following:
- Locate the horizontal output transistor. It will be in a TO3 metal
(most likely on an older set) or TOP3 plastic package on a heat sink.
With the set unplugged, confirm that there is no voltage across C to E and
then measure between them with an ohmmeter. In at least one direction it
should be fairly high - 1K or more. This confirms that the HOT is
probably good.
(There is also a slight chance that there is a low voltage regulator
in addition to the horizontal output, so don't get them confused. The
horizontal output transistor will be near the flyback transformer and
yoke connector.)
- Trace back from the HOT collector to the flyback and through the flyback
to the B+ feed from the power supply. Clip a voltmeter between this point
and the HOT emitter. Make sure the leads are well insulated and can't
accidentally short to anything. (This test can be performed across C to E
of the HOT but if the horizontal deflection were to start up unexpectadly,
the meter could be damaged by the high voltage pulses on the HOT collector.
But if you can't find the B+ source, it may be worth the risk.) Plug it in
and turn it on.
- If the problem is in the low voltage (line) power supply, there will
be no substantial voltage across C to E.
You should be able to trace from the power line forward to find the bad
part though a schematic will help greatly.
- If the problem is in the startup circuit or horizontal oscillator/driver,
then there will be something on the order of 100 to 160 V across C to E.
In this case, a schematic may be essential.
There is also a slight chance that there is a low voltage regulator
in addition to the horizontal output, so don't get them confused. The
horizontal output transistor will be near the flyback transformer and
yoke connector.
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