RCA 8TS30 Negative Image

[Chris K]

Working on my first KCS 20J-1 chassis. Replaced all electrolytic and wax caps and put a pattern generator on the set. I get a clear, focused, somewhat stable negative image. If I manipulate the focus coil, loosen the wing nuts to move it and fool around with it, occasionally it'll snap into the right B&W configuration for a second but then reverts. Any idea where I should start looking? I have not changed any resistors yet nor have I tested any.

[jr_tech]

Do you have another CRT try?

jr

[Chris K]

Just tried it. The issue is still there and I think it's the contrast pot. I have to turn it 2/3 of the way down to get a non-negative display. Wiggling it causes it to be intermittent. It's been cleaned with Deoxit Fader as have all the other pots. All NOS tubes. The picture is very twitchy both in the horizontal and vertical. I have very rarely gotten a steady pattern. Seems to be sensitive to many things...moving the tuner knob...pulling and rocking the vertical and horizontal pots etc. Maybe it's a bigger systemic issue? IDK. The only caps I haven't replaced are the safety caps off the line.

I still have the big bleeder resistors in place including one that's a Candohm resistor. I can't see how replacing them would make a difference but at the least, the metal resistor needs to go.

[stuben]

Definitely replace those safety caps across the line! They look like mica but they aren’t. Just molded paper and prone to failing! Also what are your voltages like in the bleeder box?

[Penthode]

Your set does not have AGC. With the contrast control fully clockwise with a strongish signal, it is supposed to go negative

[Chris K]

Originally Posted by Penthode

Your set does not have AGC. With the contrast control fully clockwise with a strongish signal, it is supposed to go negative

My pattern generator has adjustable RF output gain. If I lower that will it help? I don't think my contrast pot is working anymore. It was making a slight grinding sound. Now it's really not doing anything.

The issue now is I can't get a picture that's stable. Twitches...momentary brightness and focus changes...great picture if I wiggle no specific control for a second...then back to twitchy crap...tap on the focus coil, picture looks good for a second then goes to crap etc. There's just a general instability I can't track down. It's not the CRT...same thing on 2 different 10BP4s (and on my 5" tech CRT). It's not the pattern generator...I've used 3 different ones...same thing. Maybe I should try a broadcast signal? I don't think that's going to cure the symptom. It seems to be an issue at a much more basic level. Tuner has been cleaned...tube sockets have been cleaned...very frustrating.

I wish there was some way to post a video of how it's behaving. It's a lot easier to display than it is to describe.

[Chris K]

Originally Posted by stuben

Definitely replace those safety caps across the line! They look like mica but they aren’t. Just molded paper and prone to failing! Also what are your voltages like in the bleeder box?

I will check the voltages this evening and change those caps. Yes, they are those nasty, awful mica impersonators.

[Yamamaya42]

Originally Posted by stuben

Definitely replace those safety caps across the line! They look like mica but they aren’t. Just molded paper and prone to failing! Also what are your voltages like in the bleeder box?

I found out the hard way about those "fake look like micas" FADA, when one of them decided to explode! :O

[Penthode]

Originally Posted by Chris K

My pattern generator has adjustable RF output gain. If I lower that will it help? I don't think my contrast pot is working anymore. It was making a slight grinding sound. Now it's really not doing anything.

The issue now is I can't get a picture that's stable. Twitches...momentary brightness and focus changes...great picture if I wiggle no specific control for a second...then back to twitchy crap...tap on the focus coil, picture looks good for a second then goes to crap etc. There's just a general instability I can't track down. It's not the CRT...same thing on 2 different 10BP4s (and on my 5" tech CRT). It's not the pattern generator...I've used 3 different ones...same thing. Maybe I should try a broadcast signal? I don't think that's going to cure the symptom. It seems to be an issue at a much more basic level. Tuner has been cleaned...tube sockets have been cleaned...very frustrating.

I wish there was some way to post a video of how it's behaving. It's a lot easier to display than it is to describe.

You need to learn and understand how it is supposed to work. There are many books on basic TV theory. I would suggest and old copy of the books by Grob and Kiver.

The contrast control simply varies the grid bias to the video IF and tuner stages. Have you measured it?

Do you have a smartphone? Can you take a few snapshots and post them?

[Chris K]

I wish I had the time to go to school and learn and read all I could get my hands on regarding post war TVs and electronic theory. I have many of the basic texts and when I retire in a few years, I intend to take this to another level. Just don't have the time right now.

Posting video would be a better way to demonstrate this because it is not something I can capture in a photo. Forget about the contrast issue...I get that and I actually cut in a temporary 10K pot as a substitute with no change. I have an image and it's decent. The focus is soft but I'm using the 5" technician CRT and it's going through the focus coil that it doesn't need. I'm not concerned with the focus...that will clear up with the 10BP4 installed. The image isn't stable but it's not like it loses horizontal or vertical lock. Unfortunately, the best word I can come up with is "twitching". I'll try to capture in some photos and post. Thank you for your interest!

[Chris K]

I got it. It wasn't a problem in the contrast pot or in any pot for that matter. A 270pf mica cap in the video IF was noisy. I remember Paul Carlson doing a restoration on a very high end RCA receiver that was incredibly sensitive and he traced a noise issue to a couple of mica caps that tested fine but when he put his self-made "Super Probe" on it, you could hear the noise in the cap very clearly. Same thing with the issue I had. The twitchy picture was being caused by one of these capacitors. Paul said restorers are going to have to start paying closer attention to 80 year old mica caps. The lower quality ones are now starting to break down and cause issues. Seems like he was right, at least in this TV.

[bandersen]

Know issue with these sets. I recommend replacing all the 270pF IF coupling caps in the early (46-47) RCAs.

[Chris K]

Thank you Bob

[Yamamaya42]

Originally Posted by bandersen

Know issue with these sets. I recommend replacing all the 270pF IF coupling caps in the early (46-47) RCAs.

I was told not to touch those mica caps when I did my FADA, as it would all but guarantee a IF alignment would be needed, but I did replace the 1500pf ones on the cathodes.

[Electronic M]

My approach is change papers, out of tolerance resistors and if it still has problems and they relate to stages with micas change the micas last.