RCA CTC156/157 warped image

[Jamesk]

Hello,

I have this very nice RCA CTC157 TV that I would very much like to fix to use for vintage gaming systems. The image is warped when using composite video (picture below). The inside was very filthy and had some candle wax poured in it. I didn't see any obviously burned out components, but I will have to pull the board out and give it a closer look. I was hoping someone might have an idea of where and what components I should start looking for problems.

Thank you,
James

[wa2ise]

I'd check the electroltic capacitors involved with the vertical oscillator and defelction circuits. Others can chime in on which specific caps are likely the trouble. Such caps slowly lose capacitence and increase in effective series resistance.

[radiotvnut]

I'd check/replace all electrolytic caps in the vertical sweep circuit. Also, look for any caps that have physically leaked all over the board. This leakage can corrode and eat away the traces on the PC board. If new caps don't fix the problem, it's likely the vertical output IC that's at fault.

[Electronic M]

Another way to spot bad electrolytics is to look for bulging especially at the top. If the can is bulging then it is bad.

[Jamesk]

Thank you for the help. I'll pull out the board and replace all the electrolytic caps with new quality ones. Maybe that's a bit of extra work, but I figure I might as well since I'll have it on the bench anyway.

[Jamesk]

Still struggling with this CTC157

• Replaced electrolytic in vertical circuit
• Replaced electrolytic in power supply
• Replaced Transistors and Diodes in vertical circuit
• Replaced Vertical Output IC LA7831
• Checked resistors in Vertical circuit

[ChrisW6ATV]

Those warped areas make me think the yoke could be bad, unless the pincushion circuits could cause this type of problem.

[Jamesk]

Originally Posted by ChrisW6ATV

Those warped areas make me think the yoke could be bad, unless the pincushion circuits could cause this type of problem.

A bad yoke doesn't sound good. I'm not sure where I could find a replacement.

A buzzing increases around the yoke are when the vertical height is increased. As vertical height increases distortion increases.

Here is a picture with the two vertical outputs switched. The darkness is from the camera. It looks like the top right bend moves to the bottom left. The bend on the bottom moves to the top although it seems to change in shape a bit, it's still compressed.

[ChrisW6ATV]

That was a good test idea to switch the vertical connections. Based on the new picture, I think it is more likely pincushion problems rather than a bad yoke. The best way to do further tests is with an electrolytic capacitor tester (ESR meter). You can test most caps rapidly, without unsoldering them. That is how I have chased down problems somewhat like this.

[Geoff Bourquin]

Is the B+ clean? Maybe there is something riding on top of the 129V that is distorting the sweep. I have seen that 33uf/160V fail, but usually the distortion is more severe than yours. (or was that cap 200V? I don't remember) Or perhaps the +26 scan supply to the vertical is off. There's also a couple diodes near that vertical chip that I would suspect too. The chunk out of the right top corner suggests horizontal sweep issues, and the bottom distortion suggests vertical. Power is common to both of those.
I'm not leaning towards pincushion problems because that usually shows on the sides only and either curved way out or way in on the sides. BUT, you might take a look at the diodes and blue block-shaped caps in the pin circuit. I have replaced a bunch of those caps. They like to bulge on the sides and if they do they're toast.

[Jeffhs]

Maybe the auto degaussing circuit is not shutting off when it should, or at all. I have an HP MX70 17" computer monitor with much the same problem as your RCA TV. When I was using this monitor with my Windows XP computer (I have since replaced it with an HP widescreen), I remember smelling something burning inside the cabinet. Didn't start a fire, but I'm sure it must have affected the degausser (or something in the deflection circuits) in some capacity, as the image had (and still has) a severe bend in it that I can't get rid of. Either something burned out in that circuit (the burning smell I mentioned) or else the switch that's supposed to shut off the degausser welded shut. Doesn't matter to me now that I have the widescreen monitor, but it sure was an annoyance when I was using the MX70 on this system. I use this computer for just about everything, so having to look at a distorted image for any length of time was becoming extremely annoying.

[Jamesk]

Here are the signals with an example from the schematic I have.

Here's 150V



Here's 26V