Today's Mystery Guest (RCA CTC-4)

[old_coot88]

Dang, it's almost as if a B+ line is not being bypassed adequately and has got a bunch of junk riding on it. Try scoping some B+ points and see if there's any unusual stuff on it (other than normal low level power supply ripple).

[Phil Nelson]

Not sure if I'm doing this right, but here is the +285V line at the junction of R249/R250 near the 3.58 oscillator tube, with no signal at the receiver:



When I turn on the color bar generator, the trace changes to this:



The traces are somewhat cleaner than what appears in the photos; both of them bounce up and down a bit on my scope.

I got similar results when scoping that line at its origin (junction of R266/C117B in the p-s) and at pin 2 of V117, the horizontal oscillator.

Phil Nelson

[DaveWM]

C201, .1uf good?

[Phil Nelson]

Do you mean C204 (on the RCA schematic)? That was replaced, but I can check it, just in case.

Phil Nelson

[DaveWM]

yes I see that as a plate bypass on the band pass amp, RCA schematic not sams.

[Phil Nelson]

I unsoldered C204 and tested it on my EICO 950B cap checker, and it is fine. I also disconnected the line that I was using to inject a video signal, in case that was introducing noise.

With the IF reconnected and a signal coming in through the antenna, I can see rolling color bars when using a color bar pattern. I was never able to see them before with the IF connected, only the static "colorless bars." The waveform at L42 is still clean and stable, so I conclude the oscillator is definitely running.

Tomorrow is another day . . . .

Phil Nelson

[Phil Nelson]

Maybe there's hope for the old girl yet.



(Ignore the dark horizontal band, which is a camera artifact.)

Today I went on a resistor hunt, checking every resistor in the color signal path, starting at the point where the video signal is picked off from the IF section and working through everything but the demodulators.

I replaced a number of resistors that were out of spec. Then I powered up the set mainly to confirm that I hadn't made things worse. I also wanted to scope the outputs of the quadrature transformer to see if anything like a signal could make it out that way. After several scope photos and a few adjustments, I was surprised to see real color bars for the first time on this TV.

Yes, I know the colors in that pattern are incorrect. There's obviously a lot more work to do, but it's nice to see a little improvement. I'll finish the resistor hunt tomorrow or Saturday.

Phil Nelson