View Full Version : 1971 Zenith 20BC50


consoleguy67
11-22-2015, 04:46 PM
Hi all,
I have a 1971 Zenith 23" color set with the 20BC50 chassis. What would cause loss of color? I've adjusted the tuning, and that didn't help. If I turn the set off and back on, the color comes back. Any suggestions? I've ordered new tubes, as I believe that the set could benefit from being retubed.


Thanks,
Frank

Electronic M
11-22-2015, 05:27 PM
Probably uses a 6GH8 for the color osc. like my 1971 12B13C52 chassis Zenith....When that gets weak color goes away, or does not want to sync which is easy to see if you put color bars on the screen (the bars get RGB barber poleing). Other issues that can cause it include miss-adjusted fine tuning or color killer.

Username1
11-22-2015, 10:18 PM
It could be anything really..... From the color Osc, Burst Amp ~ All the way beck to the
tuner..... Turning it off and on vibrates stuff, so could be a bad or dirty tube socket....
Next time start with an Onslow style whack..... See if it's a lose thingie...

.

consoleguy67
11-22-2015, 10:22 PM
Thank you, Tom. It does use the 6GHA for the color osc. It is one of the tubes I ordered. I'll replace it and see if that helps.

consoleguy67
11-22-2015, 10:24 PM
Thanks, Username 1. I'm going to check the socket, and see if maybe the pins/socket are dirty.

consoleguy67
12-01-2015, 09:15 PM
Hi All,
Now I have another problem. What would cause the focus to suddenly go blurry, and then come back? I've adjusted the focus control, and the Peak Picture control, and it will help. Sometimes the picture is crystal clear.

Electronic M
12-01-2015, 10:35 PM
Hi All,
Now I have another problem. What would cause the focus to suddenly go blurry, and then come back? I've adjusted the focus control, and the Peak Picture control, and it will help. Sometimes the picture is crystal clear.

Does your set use that MASSIVE resistor to divide the 25KV HV line down to obtain focus voltage? If so it may be drifting.....Mine did a bit of mild fluctuation then stuck at bad focus, I cured it by adding some resistance to the resistor from the focus pot to ground, then it relapsed.....About three lather, rinse, repeat cycles later it stabilized long term (been good 3-4 years now) and no longer drifts every 2-6 months.

The correct way to fix it is to replace the big one, but since my 12B13C52 chassis is a daily driver, and I was broke during that time it was faster (try finding a part like the HV divider in less than a day), cheaper and easier than hunting up a proper replacement for the real culprit.

TUD1
12-01-2015, 11:54 PM
If you're going to be "retubing" it, I'd suggest going to Radioshack and getting some DeOxit to put on the tube sockets and tuning/color adjusters. It's not cheap, but it's done wonders for me.

Findm-Keepm
12-02-2015, 12:19 AM
Probably uses a 6GH8 for the color osc. like my 1971 12B13C52 chassis Zenith....When that gets weak color goes away, or does not want to sync which is easy to see if you put color bars on the screen (the bars get RGB barber poleing). Other issues that can cause it include miss-adjusted fine tuning or color killer.

If replacing the 6GH8 tube fixes the color, SAVE THE OLD 6GH8 - they will work great in some circuits, but fail in Color Oscillator/Burst Amp applications. We could fix some sets just by swapping 6GH8's within the set, no tubes replaced, just swapped around.

zeno
12-03-2015, 08:14 AM
Zeniths this vintage had a lot of bad CRT sockets.
The focus pin would arc to ground. Often the pin
area would turn dark & have green crap on it.
It will also smell bad & make noise.
To test pull all the CRT ground straps from the socket
only & put a HV meter on it. Should be 0 volts.

73 Zeno:smoke: