Hey guys.
Well, you guessed it, already having issues with my ctc-16.. It seems that a week ago I had some flickering of the screen, especially seen on a black raster, like before the movie starts os between commercials. It is a bright area of the screen, gets brighter in areas of the top or bottom with a hint of retrace lines. This will flicker is on the top or bottom of the screen or sometimes the middle. It's odd, it almost looks like voltage issues as the area will move, up or down then go away. Thought at first it was the transformer as it feels warm to hot at 1 hour intervals. Havent rechecked tubes yet.. I recapped the set already. Tonight I could swear I heard a faint humming sound coming from the set. These look like 6" bars of bright areas flowing up the screen, flickers, then moves to a different part. Any ideas? I also noticed that the top of the screen, white letters or images, but mainly noticed with letters are more out of focus than the middle or bottom of the screen. Dont think it's related as the set has always been this way, but looks like its getting worse, focus has no affect...

does this set use a 12by7 or a 12gn7 if so change it and go from there. steve

Yes 12by7... The raster almiost goes white.. Ill heck it tonight, Thanks Steve

if your raster is going almost white i would look at the i.f. tubes & tuner. the 12by7a is just your video amplifier. it would make the pix dark or pop in and out if it were bad or leaky. steve

I noticed yesterday if i put a fan in back of the set the almost white raster goes away. Gotta get my tester fixed. my sockets have worn out...

Using a small wood stick or pencil, tap on the IF, tuner, video and AGC tubes. Have the set in front of a mirror so you can see if there are any changes. If the tubes have any loose elements you'll will see the problem get worse almost immediately.

When I restored my Zenith I had trouble with flashing in the picture. I had to replace the video output tube. Tapping the tube with a screwdriver handle revealed intermittent shorting of the elements. It is not uncommon shortly after restoration to have a failure. Those parts have been sitting idle for twenty-five years or more. Stick with it though, the results are worth it. Once you get all the bugs worked out, you should have a great and reliable daily watcher.

Hopefully whatever the problem is, it won't be as frustrating as trying to track down those smearing issues you had. I think it's a bad tube myself. Like compucat said: those tubes are probably quite old.

Do these change when you put a fan on them?

Sounds like it must be a thermal problem...the fan probably blows away enough heat to let the parts cool down. May be a micro-crack in the solder, perhaps at a tube socket or a wire wrap post connection, or a thermally intermittent capacitor/resistor. Somehow a lot of heat gets trapped under the CTC-15 and 16 chassis as I have seen a lot of burned wiring/terminal strips.
You might try blowing heat into the suspected area, carefully, from a hair dryer or heat gun to try and make the problem occur when the set is cold.

Actually I take that back, problems getting worse, was doing it right at powerup tonight, noticed the focus is getting worse on white images, other images still in focus, just white lettering when it's flickering or not, it doesnt matter.

got a screen shot of the symptom? perhaps its some strange HV defect. Does giggling the focus control or HV adjust have any effect?

It may also be a peaking coil issue...changes in video frequency response can simulate focus problems (like when a sharpness control is adjusted).
Cannot remember if this set had a peaking switch/control but you might try adjusting it to see how the problem changes, and check the peaking coils in the video circuits for opens or intermittents.

Well last night I replaced some tubes I found went bad, the 1v2 focus tube, a 6gh8a I forgot ehich one that controls, but it was bad, and the tuner tube is weak, but I have to order it. Focus is much better! Still a little off on the top of the screen but the bottom is good. Think I found the issue. The 6BK4 Shunt regulator tube has flashing under test and the needle jumps around. Think this tube may have air in it, going to order it today and see if it fixes the issue..

BTW, I did check all the peaking coils and resoldered them anyway.

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Cool thanks for the tip, I figured it wouldn't be good to run it due to the stress factor as well. Theres this guy here in Sacramento called Mr.Tubehead, he has all these tubes usually handy. Its good to see people in the tube business still! He usually repairs guitar amps but he wants me to help him with his 1948 RCA TV, so in turn he will give me a full set of NOS tubes for my 16..


A bad 6BK4 will cause all kinds of focus and brightness problems. Don't run the set until you replace it because it can stress the flyback. Fortunately, it's a really cheap tube.