Question about Apex Digital GT2715DV 27 in. TV/DVD Combo
You'll hear the sound that seem like its going to turn on but it doesn't
TVs and monitors usually incorporate some kind of startup circuit to provide drive to the horizontal output transistor (HOT) until the flyback power supply is running. Yes, TVs and monitors boot just like computers.
There are two typical kinds of symptoms: power on click but nothing else happens or a tick-tick-tick sound indicating cycling of the low voltage (line regulator) but lack of startup horizontal drive.
Check the voltage on the horizontal output transistor (HOT). If no voltage is present, there may be a blown fuse or open fusable resistor - and probably a shorted HOT.
However, if the voltage is normal (or high) - usually 100-150 V, then there is likely a problem with the startup circuit not providing initial base drive to the HOT.
The startup circuits may take several forms:
Discrete multivibrator or other simple transistor circuit to provide base drive to the HOT.
IC which is part of deflection chain powered off of a voltage divider or transformer.
Other type of circuit which operates off of the line which provides some kind of drive to the HOT.
The startup circuit may operate off of the standby power supply or voltage derived from non-isolated input. Be careful - of course, use an isolation transformer whenever working on TVs and especially for power supply problems.
Note that one common way of verifying that this is a startup problem is to inject a 15 kHz signal directly into the HOT base or driver circuit (just for a second or two). If the TV then starts up and continues to run, you know that it is a startup problem. However, this is a risky procedure for both you and the TV.
I have also seen startup circuits that were designed to fail. Turning the TV on and off multiple times would exceed the power ratings of the components in the startup circuit. Some Zenith models have this 'feature'.
When this situation exists, it could be that the circuit is not providing the proper drive or that due to some other circuit condition, the drive is not always sufficient to get the secondary supplies going to the point that the normal circuits take over.
You can still check for bad connections - prod the circuit board with an insulated stick when the problem reoccurs.
Posted on Jun 02, 2010
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Posted on Jan 02, 2017
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