21" Zenith 20BC50 - Horizontal Lines Problem
 

I've got a 1970 21" Zenith 20BC50 color television set with thin horizontal lines running across the screen. A lot like the video below. Except for the lines the picture is fine. Does anyone know what is causing the problem, and suggest some possible solutions? Thanks.

https://www.youtube.com/watch?v=ZEG721wb4-M

Retrace lines.
could be a few things.
Screen drives too high, OR more likely, the blanking circuits are not working correctly ( vert /horz ) not shutting off the electron beam on the back-sweep, hard to tell w/o seeing your set's video

Thanks. I'm thinking it may be a bad capacitor or the screen voltage is set too high. I'm still trying to find the G2 dial. It's marked on the chassis but the arrows point to the color control dials. I'll find it eventually.

Don't know this chassis, but, could those "color dials" be individual R, G, and B G2 controls?

That could be, though adjusting them have no impact on the retrace lines. Maybe it is a capacitor. I managed to find an inexpensive copy of the service manual on eBay and will be receiving it within the next few days. Fixing the problem may be more involved than I was hoping.

Zeniths of that era typically have 3 G2 controls (1 for each color), plus 2-3 color drive controls.

I use G2 and screens interchangeably below.

To set the screens turn brightness to min, flip the service switch on back to service position (collapsing the vertical to a line) turn down all the screens till the line vanishes , turn red up till its line is barely visible, turn green up till you get yellow, turn blue up till you get white, return service switch then brightness to their normal positions. Turn color level to min and if white areas on screen are tinted adjust drives to balance it, return color to normal.

If no line or light after flipping the service switch even if red G2 is at max increase brightness till it appears...If any gun won't produce a line with max G2 turn-up brightness till you get that color and turn the other colors G2s down to match.

:D That's the real odd-ball model!
It was built between the 12A and the 12B models. I think they built it using the floor-sweepings from the plant. :D

Hybrids always were a bit weird...

my parents RCA CTC-53 Mural was an odd one as well.

The CTC53 Mural was an ex-hotel-motel set. Nasty einzell lens CRT.
The Zenith was just an odd set. Chassis number was even unconventional will the rest of the Zenith line. It didn't have 20 tubes. :thumbsdn:

Voltages are apx !
This is a delta CRT so there are 3 G-2 adjustments. Newer in line CRT's
have just one. The G-2 controls go to 300VDC on one side & 800 VDC boost
on the other. You can vary them in that range ( 300-800 ) A good CRT wont need much. The more G-2 voltage the brighter that color & if too high
you will get the retrace lines.
IIRC this had a transistor for blanking. If it was a sudden fail suspect that.
If a tube blanker its probably a 6KT8, Zeniths alternative to the junky
6GH8.

The B model year was interesting. You could get 14, 16, 18, 19, 20 & 23
and 25 inch sets. Included is the much loved 4 tube hybrid intro.

73 Zeno:smoke:
LFOD !

This is the cross over year for chassis numbers.
Used to be tube count then model year.
Switched to screen size then model year.
1971 had both formats. Model numbers also started to change to
year letter - nonsense screen size ( 40## =19", 47##= 25" etc )
To real screen size but that change took at least 5 yrs before it
was across the board.
So anyhows the format for this chassis is
20 = screen size
B = model year 1971 ( released fall of 1970)
C = color TV
50= consecutive number, the higher the # the higher the feature level.

73 Zeno:smoke:
LFOD !

It should be a really good set when you get it working. I have a 20CC50 (next model year of the chassis with minor revisions) and had a 20BC50 parts chassis (on loan to a friend currently) My 20CC50 makes a picture that is among the better I've seen on a delta-gun CRT set.
https://farm3.staticflickr.com/2815/...1205ff6a_z.jpg https://farm3.staticflickr.com/2812/...2bacb684_z.jpgDSCN2566 by Tom Carlson, on Flickr

The biggest difference between the chassis I noticed was instead of a round color IC on a 9PIN tube base-socket the CC uses a chassis-socketed DIP chip (there is something I'd never have expected to exist in a TV till I saw it) for the color.

Thanks. The G2 color knobs were turned too high. I followed your instructions and the problem is fixed. Thanks to Zeno as well.

My first new tv was a 20cc50. Outstanding pix quality. Used it maybe 10 years and then upgraded to a used 25cc50. These chassis's are massive and were the best this country could build.

Also, check to make sure the AGC control is not set too high.