I have a 19" System 3 with Space Phone. Love the set, and it has a great picture. However it has developed a problem which I hope doesn't mean I have to retire it, with my limited repair skills.
This morning I noticed it has a solid 3" block that travels vertically up the screen. It's not that noticable, but the "block" is blacker than the rest of the screen, and I seein the center is a bright white dot.
There is no distortion in image as that block moves up the screen. (it reminds me of those channel guide stations) Like I said... the block is about 3" high, and covers the entire screen as it travels vertically.
Is it a cap in the power module? Has anyone seen this issue before?

Does it happen on one channel, or more then one channel?

Does it happen with just static on the screen?

Does it happen if you hook up a VCR or game console and run it?

The first thing I'd check would be the dual section filter cap on the power supply module. You'll probably have to use individual caps to replace the original. I'd also resolder all the heavy parts on that power supply board while I had it out.

This could very well be interference from something external. If you have cable tv it could be something far removed from your house, even. I would try to eliminate that first. You could unhook the antenna connection and play a tape or dvd and see how it looks.

I wanted to report back that I found the problem. Of ALL things it was a clock radio I bought recently and placed on top of the set. I didn't even figure it out untill I carried the set downstairs to "retire" it. I fired it up and problem gone. I put 2+2 together, carried it back upstairs, unplugged the radio and it worked. The clock radio is a late 70's GE with a blue florissant display. I don't know why the Zenith would have picked up something from that, but it did.....

RF interference. At least it was a simple fix.

If you want to hear something crazy check this out: About 17 years ago I was called out on a service call to a friends house that had (at that time) a late model Magnavox (Philips) set that when you set down in a easy chair across the room, set would go to "snow". Yes it did! When called, I still didn't know what they were talking about until I saw it for myself and I brought a Magnavox engineer friend to see it too. Set was operated off rabbit ears then-no cable. Of course it turned out to be the tuner with a very sensitive defective transistor in it somewhere. Yes, sitting down in a chair developes a static/RF charge that went across the room (about 20 feet) and shut the tuner down! Turning off the set would "reset" it and you could do it all over again! Marconi demonstrating his theory all over again! Magnavox was still here then and we gave bad tuner to their engineering department for research.