COBY 24" LCD TV - 2 seconds to black

[jc2468]

Hi,

I hope someone can point me in the right direction. Originally the TV came to me w/o any signs of power. I've since recapped it. Everything was fine, got power, display working out of the case. After I reassembled the TV, it displays the 2 second to black symptoms. Tested out of the case again, same 2 seconds to black.

I've checked:
- CCFL tubes ok, swapped with known working ones
- inverter transformer tested ok
- reflowed transformer and some SMT components
- when black screen, all voltages are present 12+,5+, 3+ on the enable pin

I've isolated the issue to the CCFL connector JP4. If I plug in a smaller size bulb, the TV works and the screen stays on. However, it is only at that connector this occurs. If I try the other connectors. I still get 2 seconds to black.

Anyone point me in the right direction for things to check next?

Thnx

[alexanna]

Re: COBY 24" LCD TV - 2 seconds to black

I have experienced the same symptoms you describe; a shorter length CCFL will correct a 2 sec. to black problem.
There are a lot of things that could cause this, what I have found on several occasions is a problem with one of the small switching diodes usually a “BAV99”.On some monitors these are directly tied to the CCFL returns.
I can't trace the circuit from your pictures, You may be able to tell if this type of circuit is used in your inverter by tracing the return pin of the CCFL connector to the small diodes.

[Longbow]

Re: COBY 24" LCD TV - 2 seconds to black

This sounds like an interesting problem, especially since others have seen it happen also. The photo is of the main supply - isn't there a separate CFL supply (pictures?). This symptom is telling you that the current draw from the tubes is triggering a shutdown. Most of these inverters are going to be very similar...perhaps there are some generic inverter schematics on the forum we could refer to. Normally, I have seen this type of behavior when primary parts change value. There can be a low ohm resistor that feeds the switching device source pin, called a current sense resistor. These resistors go up in value especially if the switcher fails. Normal current draw is then sensed as being a problem and the supply goes into protection mode. Let us know.

[ukvince]

Re: COBY 24" LCD TV - 2 seconds to black

Is that a damaged track or a scratch near FX1