RCA CTC-120D screen pinching

[Firble Man]

I’m so excited to finally be on here!
I have a couple 1948 Admirals I look forward to restoring, but I’m currently working on a 1986 RCA ColorTrak 610520611. I believe the chassis number is CTC-120D.
This TV has a pretty strong picture, especially for not being re-capped. But there is a weird consistent issue where the screen is pinched in the middle. It doesn’t stay in one spot vertically. It moves up or down depending on the source signal.

Could it be related to the pincushion board? And does anyone know what the ratings are on those caps?
I don’t know if it’s the PWPIN(pincushion) board though…The pinch moves down when playing NES, but it moves up when playing a videotape.
With no signal, it moves at such a speed, the static screen looks complete, but you can catch the pinch moving very fast with your eyes.
This leads me to believe it might be related to the signal interpreter circuit.

This isn’t a nice enough TV to bother re-capping entirely, and I kinda need to get it going for the Christmas season.
Does anybody have a theory as to which capacitor or couple caps need to be replaced to restore the screen?

Thanks for the help!!
-Firble Man-

[damen]

Ive seen that problem at least 100 times. It's the metal electrolytic cap strapped on the top of the back of the chassis.

[Firble Man]

Thank you!
I will give that a shot. I think I have one from a very similar '89 Sylvania I parted out last year. I just need to find it.

[zeno]

Originally Posted by damen

Ive seen that problem at least 100 times. It's the metal electrolytic cap strapped on the top of the back of the chassis.

Ditto
When its really bad will look like a Christmas tree.
Zeno

[vol.2]

Originally Posted by Firble Man

Thank you!
I will give that a shot. I think I have one from a very similar '89 Sylvania I parted out last year. I just need to find it.

You should be able to sub for normal caps. When I've done this, the metal cap had a legend on it to interpret the values and the terminals. I don't know how crucial the actual values are at whatever point that is in the circuit for your particular set, but using discrete caps in place of the old one might be a better choice than using a substitute metal cap of different values.

[Firble Man]

Thanks for all the info!
I couldn’t find the ‘89 board, so your suggestion to just use separate caps gives me hope.
I hope to desolder the big guy after work tonight and then I can get his values. Hopefully I’ve got some with the same ratings.

[vol.2]

Originally Posted by Firble Man

Thanks for all the info!
I couldn’t find the ‘89 board, so your suggestion to just use separate caps gives me hope.
I hope to desolder the big guy after work tonight and then I can get his values. Hopefully I’ve got some with the same ratings.

Just make sure you use the correct voltage ratings as well. You can post an image of the metal cap's markings if you need help deciphering the terminals.

[Firble Man]

Thanks!
It was pretty clear.
One ciruit, 200V 600μf, has a semicircle cutout by it, and the other, 250V 30μf, has a triangle cutout by it.

I know it's safe to go with a higher voltage cap, but which is a better direction to go with the capacitance, if I have something close?

If I don't have anything close to those ratings, I might need a lead on where to buy single caps as cheap as possible.

[old_tv_nut]

Many old electrolytics with unspecifed tolerance were in filter circuits where the main requrement was a minimum capacitance, so tolerance if unspecified was something like -20% and +50% (or more). This reduced the cost because anything that came off the capacitor maker's line was acceptable.

It is theoretically possible to overstress the rectifier that drives these capacitors if they are too large (causing a large peak charging current), but usually the rectifiers are spec'd to allow a huge + tolerance on the capacitor size.

[Electronic M]

Mouser and digikey are good sources for cheap new caps...You can find NOS cheaper sometimes but caps can fail with age even if unused so caveat emptor....

[Firble Man]

Since this is the first time I've ever bought capacitors, are these adequate replacements?

250 V, 600 µF, ±10%, Polar

250 V, 30 µF, ±20%, Polar

Original Cap:

[old_tv_nut]

Originally Posted by Firble Man

Since this is the first time I've ever bought capacitors, are these adequate replacements?

250 V, 600 µF, ±10%, Polar

250 V, 30 µF, ±20%, Polar

Original Cap:

Those should work just fine.

[Firble Man]

Great. Thank you!
I'll give an update once they arrive and I have installed them.

[Firble Man]

Thank you everyone who said it was the big cap in back!
It’s all working now!!

I replaced it with capacitors of the same two ratings, prayed to God, turned it on, and it’s back in service!
Thanks again for the help and knowledge!