RCA 8TS30 - no picture

[bigaudioal]

I have been fighting with a friend's 8TS30 for a few weeks now. Set was recapped, gets good audio and a raster. But no picture. I have spent the last few weeks restoring a B&K 1075 for injecting signals. Well today was the day to try injecting a signal into this set to see if I could figure out where the video was getting cut off. Figured I'd start injecting at the video output tube and work backwards through the IF stage. Well I didn't even get past the first injection. The raster did darken a bit injecting into the grid of the 6K6, but no real signal. I made sure I was not overdriving the signal from the 1075 into the set. However I did notice that the contrast control/AGC really does not appear to do much when adjusted. I then ohm'ed out the contrast control and it only measured 2k. Shoukd be 10k. Could an AGC issue affect injecting a signal into the video output tube? I know if the AGC is not right it can overload the IF stage. Trying to figure out why I get no signal from the video output tube and after. Or do I need to back-track further to see a signal? Or does this AGC issue just kill all chances of a signal getting through? First time doing this signal injection, so any advice or insight would be greatly appreciated.

I know the B&K is working well. Injected signals into working sets and looking good. See attached photo.

Thanks guys!!!

[Electronic M]

If the video out is tied to the AGC then it can kill the gain there. The IF will definitely be dead if the AGC is off enough.

[bandersen]

No, you don't need to go further back. I don't believe the contrast or AGC has any effect on the video out.

Did you check the peaking coils for continuity ? Does the brightness control work ?

[Gleb]

Check the negative voltage (-18v) on the contrast control. The video amp gets the bias from that point as well. Significant overvoltage there may completely lock both the IF and the video amp.

[bigaudioal]

Originally Posted by bandersen

No, you don't need to go further back. I don't believe the contrast or AGC has any effect on the video out.

Did you check the peaking coils for continuity ? Does the brightness control work ?

I did check all the coils. All have continuity. The brightness control seems to be working fine.

[bigaudioal]

Originally Posted by Gleb

Check the negative voltage (-18v) on the contrast control. The video amp gets the bias from that point as well. Significant overvoltage there may completely lock both the IF and the video amp.

Thx. I will check that negative voltage. Plan on replacing the control anyway since it is only measuring 2k when it should be a 10k control.

[Penthode]

I do not believe the 8TS30 employs agc. The gain of the if amplifier is controlled by the contrast control.

[bigaudioal]

Originally Posted by Penthode

I do not believe the 8TS30 employs agc. The gain of the if amplifier is controlled by the contrast control.

Was able to track down a replacement contrast control. Now just have to find time to install it and see if it helps.

[bigaudioal]

Last night I BEAT the RCA 8TS30 into submission. Replaced the faulty contrast control. Hooked up my B&K 1075 and tried injecting signals throughout the IF and video stages. Nothing. Suspected the AGC was still messed up. So traced the circuits back towards the tuner from the contrast control. Tested a few resistors along the way (all tested within spec) and finally came to a 270pf mica cap AT the tuner. I usually ignore micas, but decided to replace this one for the hell of it. BINGO!!! That did it. When I turned the set back on, I got normal looking snow instead of just a white raster and the contrast control was finally having the proper affect when adjusting it. Flipped on my B&K and just hooked it up to the antenna leads on the set and got decent picture and audio. I had to tweak the IF cans a bit to match up best picture with best audio while fine tuning, but FINALLY!!!!! Set was on for about 2 hours and seems solid now. This set is going back in its cabinet tonight and waiting for its owner to come get it in a few weeks. Below is a video of the set playing last night.

https://youtu.be/nr8OWvYUOWY

[Kevin Kuehn]

That's looking very good.

[bigaudioal]

Originally Posted by Kevin Kuehn

That's looking very good.

Thanks. I am so relieved this set is working again. Set was bought at ETF in the silent auction in 2015. Kamakiri did the initial recap on it a while ago. Buyer lives near me and I brought the set from Buffalo back to MD last summer/early fall. But then after it sat in my storage facility all winter I could not get a picture. Glad it seems to be squared away now.

[Gleb]

Originally Posted by bigaudioal

Tested a few resistors along the way (all tested within spec) and finally came to a 270pf mica cap AT the tuner. I usually ignore micas, but decided to replace this one for the hell of it. BINGO!!!

What's the capacitor's number according to the schematic diagram? It's strange that there was no any picture when you injected a signal into the video amp.

[bigaudioal]

Originally Posted by Gleb

What's the capacitor's number according to the schematic diagram? It's strange that there was no any picture when you injected a signal into the video amp.

I was thinking the same. All I was getting was a white raster. Contrast control was having zero effect on that raster too. When I tried to inject in the video amp, the raster did change some. Saw a wide dark band (maybe 4-5 inches high) that flipped very slowly vertically. But was not really discernible as a picture.

According to the SAMS (set 54, folder 18) I replaced C21, which is an IF coupling cap.

[Kevin Kuehn]

C21 shorted would place B+ on the grid of the first video IF amp, which would cause it to draw excessive current, thus pulling the B+ line low. That same B+ buss feeds all the video IF tubes,video amp, and the video output. If I'm not mistaken Kamakiri was having issues with something intermittently pulling that B+ low.

[Kamakiri]

Who knows. I've tried so hard to forget all the damn issues I had with that set