[drh4683]

Do you have a tuner subber or a B+K analyst? Let the set cool down for a few hours then start it back up with the subber in place, or the B+K analyst set to IF output and hook it up to the IF input cable to the chassis. See if the set still acts quirky. If it does, the tuner is cleared and you have some kind of IF or AGC fault within the chassis.

One of the toughest yet easiest to fix problems for me was with Zenith 20CC50 chassis. Id turn on the set and it would work, then go blank intermittently. I looked around and checked the circuits and monitored voltages until it failed again. Couldnt pin it down. I adjusted the AGC and the other controls that I "knew" about. Then I looked at the front apron drawing on the schematic. There I see an "AGC Delay" control. Forgot about that one. With the flip down door down, the lable does not indicate that there is an AGC delay, they list all the other non-customer adjustables but not that, but look through the hole and there it was! So I adjust it and the picture starts flipping in and out like crazy. All the running around and confusion isolated to an darn intermittent dirty pot.
Intermittents are the true test of your ability and they can drive you nuts. I always hated having a really tough problem to fix and thinking about it at work. While you're at work you think what to check and then the light goes off and you know where the problem should be. Excited to get home to check, you find out you were wrong!!!!!!!!! Thats the worst!!! Then not being able to go to sleep because its not fixed yet and the gears are still turning. I used to be up till 3AM on a friday night until the job is done. I get so focused and into it that breaking my concentration is the worst thing to do. I am much better now. Ive learned to take a break now and its amazing how much better you feel when you come back to it. Like in my case, you find out its a god dang intermittent pot and you feel like a total idiot! Or in another case, an intermittent molex connector contact was causing weak color on my every day 12A12C52 zenith color set. That one had me running in circles too. So sometimes the easiest stuff causes the most rediculous problems. Hopefully you'll find its some control or tube or socket and make it easy. As much of a pain they can be, I love working on them.

Also in your case, check that 6JU8 color AFC tube. Those are troublemakers too, almost always responsible for color problems in zeniths.