Today's Mystery Guest (RCA CTC-4)

[Phil Nelson]

Originally Posted by old_coot88

test by just lifting one end of the diode.

Well, that led me stumbling down a path that eventually led to a real color picture. After I lifted one end of the diode, I checked out the output network downstream of the video detector (which all looked OK). I experimented with injecting the video at a couple of spots other than the video amp grid, without getting anywhere. I was really tired of messing with injected video at that point, so I decided to replace the old diode and send a signal in through the antenna, just to see how bad things really were.

It didn't occur to me to test the diode until after I removed it. Maybe it has been bad all along, or maybe the heat of removal sent it south. It's definitely bad now, anyhow. I wonder if this 1N64 was a replacement, since the schematic calls for a 1N60:



I replaced it with a 1N34A, since that's what I happened to have on hand, and boom -- there was a color picture. Not a correct color picture, but better than I had seen before.

I noticed a persistent vertical jitter that hadn't been obvious when looking at color bars, and spent a long time getting rid of that. Replacing resistors on the 2nd sync amp as well as the vertical circuits finally took care of it.

Some colors are still wrong, and there's a lot more work to do, but at least you can recognize pictures of humans, with some colors in the right spots.



Tomorrow I'll try the color AFC alignment once again, put the scope on some interesting places, and go from there. The resistor hunt is not over -- it has turned up a lot of bad resistors in this set, and I suspect there are more to find.

Thanks again for the advice & encouragement.

Phil Nelson