1965 Zenith Linearity Problem
 

My now ten year ago restored Zenith 25MC33 roundie has developed poor vertical linearity. Everything is stretched when the vertical fills out the screen. If I adjust it for correct circles, the picture has to be pulled in about two inches from the top and bottom. This has happened rather suddenly. Could this be bad capacitors in the vertical circuit? I have never replaced the couplates if that might have an effect on linearity. The picture below does not necessarily show what is going on but just so you can see which set I have. I think it could benefit from some service as the focus is slow to come up much of the time, there is some horizontal jitter and occasional loss of horizontal hold. There is some flashing of green in the raster until the set has been on for about fifteen minutes or so. After restoration the performance was excellent and consistent for a long time. I guess it is time for a good thorough servicing. I suspect some tubes are weak but my tube tester does not list most of them. I guess the tube tester is older than the set.

P.S. I do not know why the picture is rotated 90 degrees. I am doing this from my iPad Pro.

Hmm. This doesn't look like vertical linearity, but too much width??

You describe it as having to reduce the height, but the images you posted seem to have too little height compared to the width already?

The posted pictures do not accurately represent what is happening. Circles are stretched top to bottom.

So, the top is compressed and the bottom is stretched?

Top and bottom appear nearly equally stretched. Something has likely changed in value as this happened fairly suddenly.

Is it possible that a zoom setting on whatever device is feeding the set got changed?

If top and bottom stretch are equal relative to the center line and there is no vertical underscan as it appears in the pictures then the problem is probably too much width (aka excessive horizontal overscan). Other possibilities: vertical section used to have excessive overscan that matched the horizontal (but no longer does), or if your using a digital signal source like a cable/sat or DTV box that the box is sending a distorted image signal to the TV.

Is that picture of the current problem or is it from before the problem manifested?

This model has integrator and feedback "couplates" (3 legged disc cap) in the vertical section. When bad the classic symptom was the picture would pull up from the bottom, the upper half not so noticeable. That symptom (back in the decade) was caused almost all of the time by only one of them going bad. I always changed it good or bad, anytime I worked on a Zenith in the shop. Problem is, I can't remember if it was K1 or K2. It's been many decades, but I think the culprit had a red spot on it.
Here is a link that might help, if you don't have OEM replacements.

http://www.boxcarcabin.com/integrator.html

Good luck
Phil

I have a MC33 also. That set has way too much width. If the integrators are bad, It (as mentioned earlier) will have stretched top and black at the bottom. If I remember right, that set width is adjusted by the blue yoke lead which goes to a terminal strip on top of the chassis next to the high voltage cage. If it is on the blank terminal, it is minimum width.
A crazy idea..... What is the line voltage the set is operating on?
I don't think the signal source is the problem unless its (a dvd source) aspect ratio is set wrong. It should be 4:3. Make sure it isn't in zoom mode.
These sets can have a great picture!

There is no problem with the aspect ratio of the sources, I checked the settings. I took another picture that shows the symptom more accurately. I don’t think there is excessive width but I will be willing to check into it. I don’t know why mi pictures are getting rotated, sorry about that. I guess I need to use a real camera and my laptop, not my phone and tablet.

Your pics of Archie Bunker and the Oreo are showing exactly opposite problems. Is the width or height (or their ratio) changing with picture brightness? Or different on different sources?

Edit: or did you adjust vertical controls between these two images?

No adjustments. The Oreo shows the real problem.

OK. Still asking if it changes with brightness (trying to guess if it could be a regulation problem). If it were my set, I would try replacing tubes in the H and V sweep and high voltage sections first before looking for other problems.

Post a crosshatch, I’m having a tough time seeing what you’re describing though it looks like it may be sort of ‘long’ near the top half of the picture. Worst case you could bring it up here to me, it’s not a bad drive and I have all the test equipment you’ll need to diagnose the problem. Test jig, o-scopes, pattern gens, alignment rig....

Can you adjust height and vertical linearity to make the Oreo more circular?

jr