KV-29RS22C only raster with retrace lines no video

[willyrivero]

Hi.

I'm trying to fix this TV. It shows raster with retrace lines but no video. I have no schematics. Supply voltages are fine on IC CXA2025AS.
CXA2025 is the Y/C/RGB/SYNC/ DEFLECTION.
With my scope I traced the video signal to the pins 4 and 5 (Yin and Cin respectably) the signal is always there. but there's no signal out from pins 20, 22, and 24 which are Rout, Gout, and Bout.
I checked the xtal's too, they're fine
The funny thing is that the video signal randomly comes out from the IC and the screen comes back to life again sometimes for hours but then video signal dissappears again just like that, without any cause. This "on" and "off" all the time got me confussed.
Am I missing something, Can anyone help?.

 

[MAGNETRON]

Hi
Have you checked for dry joints around the I.C,are the supply rails there when fault develops,these are some of the questions that need answering,possibly Chip intermittent .
Intermittent CRT Short possible too.

 

[willyrivero]

Thanks. I checked and resolder all the pins of the IC. and the supply voltage is there when at fault. I haven't checked the CRT for shorts though. How can I know if the CRT is shorted? You mean in its socket pins?

 

[MAGNETRON]

The CRT could have intermittent shorts inside the CRT ie Cathode to filament/grid,unlikely though as usually it is not all 3 guns.
Try A new Chip.

 

[willyrivero]

Yeah, I'll try a new chip and see what happens. Thanks for your help.

 

[3lectrokid]

have you looked for buldges in the caps. it could be a cap that has dried out

 

[willyrivero]

Yeah, I did that thanks. I checked them with my ESR meter. But I did resolder all the pins of the IC "again". The picture came out one more time. twisted different parts of the board and everything was fine. I tested the TV for a few days on and off and it was in good shape.
I installed the cover, brought the TV to my living room turned it on and nothing, the picture was off. But that gave me a clue. It is one joint and is somewhere else. I just focused too much in one area.

 

[knaaphix]

The CRT could have intermittent shorts inside the CRT ie Cathode to filament Try checking crt board

 

[willyrivero]

Thanks.

By far I have three possible causes:

- An intermitent CRT ( which I'll check in more detail).
- An intermitent IC ( I probably could have changed it by now. But it's not readily available).
- A bad joint that acts up now and then ( I'll resolder all the joints around the IC area including the CRT board). The problem "seemed" to have come back this time when I installed the tv back cover.

One question that comes to my mind though. Would a short at the CRT would cancel out the 3 signals that come out the IC?. Because my scope shows no signals out of it when the problem arises eventhough source voltages are fine.

Well. I'll post later to tell you guys if I solved it or just gave up on it. Thanks again.

I'll

 

[Nigel Goodwin]

One question that comes to my mind though. Would a short at the CRT would cancel out the 3 signals that come out the IC?. Because my scope shows no signals out of it when the problem arises eventhough source voltages are fine.

Yes, there's feedback from the output stage to the IC - so if the CRT is H/K short the IC will shut down totally as it tries to let the cathode voltages go high.

 

[willyrivero]

Thanks Nigel. One less doubt to worry about.