Today's Mystery Guest (RCA CTC-4)

[Phil Nelson]

I disconnected C202. The waveform and tone are still present when there is no signal at the receiver. When I turn on the color bar generator, I see strong barberpoling on the screen.

Phil Nelson

[DaveWM]

burst keyer not working right?

[Phil Nelson]

Related to the burst keyer, I never got as far as trying the AFC adjustment procedure listed on page 23 of the RCA service clinic manual (specifically, adjusting T112, the phase detector transformer, or R228, the AFC balance control, which affect the keyer output). Pages 46-48 have more info.

Is it OK to use my digital multimeter for voltage measurements in those procedures, or will it load down the circuits too much? I don't have much faith in my old RCA Voltohmyst, but I could try using the scope for voltage measurements instead.

Phil Nelson

[old_coot88]

Wonder if you coulda cleared a tin whisker or something in the old T115 whilst removing and reinstalling it.
Now that you got barberpoling, try reconnecting C202, and go back further and disconnect R242. If still barberpoling, you should be able to zero beat the osc using the reactance coil, though it won't have any sync.

(Digital meters supposedly are high impedance like a VTVM, so shouldn't cause any loading problem.)

[DaveWM]

I would be tempted to scope and look for clean burst to be coming from the keyer. It like the osc free runs, but adding the control kills it, so maybe the control is not really a nice clean burst, but some video getting in there.

[Phil Nelson]

I reconnected C202 and disconnected R242. With no signal at the receiver, the waveform at L42 looks like the following and I hear buzzing instead of a clear tone from my SW radio tuned to 3586 KHz:



When I turn on the color bar generator, the waveform changes to this, and I see the same old green bars on the screen (no barberpoling). Louder buzzing from the SW radio:

Originally Posted by DaveWM

I would be tempted to scope and look for clean burst to be coming from the keyer. It like the osc free runs, but adding the control kills it, so maybe the control is not really a nice clean burst, but some video getting in there.

If you scroll back to post #142, you can see three scope waveforms from the burst keyer. The first two looked good, the third (cathode) not so much. I can repeat those now, if that's interesting.

Edit: Well, I'll be hornswoggled. I reconnected both C202 and R242 in preparation for scoping the burst keyer. (Everything is now reconnected, in other words.) To my surprise, I saw rolling color bars, which I can make to hover in place by adjusting the reactance coil (L127).



??

Phil Nelson

[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

[IsthmusTV]

Bravo!! That must be very satisfying. That set has proven to be a tough dog and I bet you're learning a lot from it. I'm sure it won't be long now before we see a gorgeous screen shot of Dorothy

-Clark

[old_coot88]

Great work Phil! (Looking for a clapping-hand smiley) What does some reglar programming look like on it at this point?