RCA 8TS30 Negative Image

[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.

[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!