[SOLVED] Dark horizontal bars on 5" CRT TV

[Fox Cunning]

Hello folks,

I did post this somewhere else but without any success, so here goes.

The TV is a Samsung CI-125R; it has composite input and controls for brightness, contrast and saturation.

And this is this issue:


Here it is with contrast turned all the way up:


*Initially thought it was the saturation control, but the bars disappear completely if the contrast knob is down all the way.

I've already tested quite a few capacitors that seemed in a relevant part of the circuit with an ESR meter, and replaced those that seemed a bit "suspicious", but no joy.
Before I go any further, maybe some of you experts can give me a better clue? Appreciate any advice

P.S. I couldn't find a service manual or even a piece of schematic. Happy to post pictures of the board if it helps.

[Josh128]

Just a complete guess, but something related to the chroma demuxing circuitry maybe?

[OrdinaryCyborg]

Honestly, I would probably recap everything just to be safe. The caps could be acting differently under a real load. I've had a similar issue with rolling black lines on my Saturn but recapping the psu in it fix the issue.

[Fox Cunning]

Just a complete guess, but something related to the chroma demuxing circuitry maybe?

Thanks, that made me realise it's actually the Contrast control, not the Saturation one causing the problem! Sorry about the confusion.
I'll double-check that section of the circuit now; there should be one diode and one capacitor that I have not replaced there -- I've seen components that tested good out of circuit but still caused issues, in the past.

If all looks fine there, maybe the contrast control inside the PAL decoder chip (TDA3565 by the way) could be the culprit?
Replacing that chip would be a pain in the backside, I'd like to rule everything else out before I do that.

[Fox Cunning]

I can't believe I was looking at the wrong control the whole time!

It was indeed the contrast circuit, not the saturation one.
I traced the correct section now, and it looks like this:


*TDA3565


The capacitor measured inconsistently between 2000pF and 180nF and sure enough it was that leaky bugger that caused the problem.





Replaced with a new good one and voila, nice vibrant image!
Thanks again Josh, maybe a bit indirectly but you put me on the right track

[Josh128]

lol fantastic job man!