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  #1  
Old 09-25-2018, 01:01 AM
jhiesey jhiesey is offline
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RCA CTC-10 color (and other) issues

Hi everyone, a while ago I picked up a nice CTC-10 (with remote!) and I've been gradually working on fixing it up. This is my first TV restoration, so I might have missed obvious things, but I have been working with solid-state electronics most of my life.

I've gotten most things, including the remote, working well! The biggest issue at this point is that some colors, especially red, seem too far to the right on the screen.


Everything looks fine if I turn the color control down for a black and white image.

My initial thought was that there is something wrong between the color demodulators and the resistor matrix that drives the CRT grids. I did discover a bunch of the Sprague "orange drop" caps had increased in value quite a bit (many 30% to 100% high), so I went and replaced all of those. All of the other resistors and caps in that part of the circuit seem fine as far as I can tell, and I replaced the 12az7 color demodulators and 6cg7 color amplifier tubes as well. None of those have solved the issue. I would think that problems earlier in the color circuit would just cause tint problems instead of making the color shift to the right, but maybe that's not true?



I realize the convergence isn't amazing, but it's not THAT far off. One more oddity: the horizontal hold seems to have significant effect on the tint. Is that normal?

I have the official RCA service manual and also a spare CRT that I got with the set.

There are also a few more issues with the set:
2. There is MAJOR sync buzz in the audio. It is worse when there is lots of color or detail in the picture, so I assume it's the high frequencies in the picture signal getting through the audio IF filter. I followed the instructions in the RCA service manual to align the audio section, but that didn't fix the buzz.

3. Black levels seem inconsistent as the picture changes, and sometimes one or more colors of retrace lines are visible. What should I try before replacing the CRT? As I said, I do have a spare!
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Old 09-25-2018, 10:00 AM
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Brightness contrast and color level look overdriven, and focus looks poor... Some color sets especially if the CRT is getting old will shift in convergence and focus if brightness gets too high and or swings too far.
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  #3  
Old 09-25-2018, 01:03 PM
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To me, this looks like a legitmate alignment problem - color smear, sound buzz both point to that.

Does the color offset disappear when you turn the color all the way down to black and white? Can you make it better by adjusting the fine tuning?

A far-out posibility: you are tuned to the wrong sideband of your source modulator. Make sure the source and the TV are both set to the same channel (3 or 4) and then try adjusting fine tuning.
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Old 09-26-2018, 06:08 PM
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I forgot to mention about the H hold effect on tint. With the color smeared that way, the burst will also be smeared, so when you change the H phase slightly, the burst gate picks up either the early, middle, or late part of the smeared burst. This problem will be reduced greatly once the color smear is fixed.
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Old 09-27-2018, 02:09 AM
jhiesey jhiesey is offline
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Thanks for the help! I'm finally convinced it's an alignment problem!

In the past I just had a cheap consumer modulator that could do channels 3 and 4, and both exhibited the same issue. Recently, I got a professional modulator that can do virtually any frequency and I tried it on channel 8. The color issue is gone! There is no shift and the tint doesn't vary with horizontal hold anymore. The color also looks correct with the color control turned very far down compared to where I had it on channel 3.

No change on the audio buzz though.

The main thing that has been holding me back from doing an alignment is that the TV I have has a KRK-98C tuner, while the manual covers the KRK-97. The main difference is the replacement of the 6fh5 rf amp tube with a nuvistor. Do you think I'll have any trouble with that?

Also, I have equipment, manuals, and a theoretical understanding for aligning things, but virtually no experience. The RCA manual refers to "knifing" the tuner coils to change their inductance. Is this just physically changing the separations between windings?


To answer other questions, with channel 3, the offset is independent of both the color control and fine tuning. Lowering the color makes the offset less noticeable but the position is the same. Same with the fine tuning, where a slight adjustment either way makes the color start to disappear.
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Old 09-27-2018, 03:02 AM
jhiesey jhiesey is offline
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OK, one more update: after getting the picture looking great on channel 8, I decided to try fiddling with the audio alignment again.

Unfortunately, I ran into the same issue as last time: when I adjust the sound if transformer, the sound if amp starts oscillating, making it impossible to tell if it's peaked or just going nuts. Last time I solved this with an oscilloscope to tell whether the frequency is actually 4.5mhz (tuned to real signal) or different (oscillation).

Presumably this circuit isn't supposed to be this close to oscillation, right?
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Old 09-27-2018, 10:56 AM
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Some thoughts: Your audio may be the only thing out of alignment. You may want to tune your professional channel 8 modulator to channel 3/4 and see if those channels look good now...If they do then the tuner is probably fine and the old modulator was at fault.

If you have a Blonder Tongue AM series modulator fun fact: the IF out on the modulator (that loops back to the frequency converter/IF in on the modulator) if the same IF frequency as used in Color TVs. You can check your IF alignment by unplugging the IF input lead from the tuner and connecting it (via an F-to-RCA adapter) into the IF out on a Blonder Tongue modulator...This scheme will bypass the tuner (and any issues the tuner may have) and show you how good the IF alignment really is.
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Old 09-27-2018, 11:24 AM
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1) everything Electronic M said
2) Yes, "knifing" means inserting a tool to spread the windings (only done on small air-wound coils)
3) audio IF oscillating sounds a bit strange to me (but someone with more experience here should comment). Are you doing this with a tuner signal, or injecting 4.5 MHz audio? If injecting 4.5 MHz, be very careful of your cable / wire lengths and grounding.
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Old 09-27-2018, 03:39 PM
jhiesey jhiesey is offline
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On channel 3, both modulators behave the same (weak, smeary color). That means there's definitely an alignment issue in the tuner, right?

The modulator I have isn't a Blonder Tongue; it's a Standard SCM550HP, but it also has an IF loop. Part of the reason I got it was so I could hook the 45.75Mhz IF output directly up to the IF stage of the TV, but I haven't actually tried that yet. I'll do that tonight.

The audio IF oscillation is using a tuner signal. What I observed is that if I tune the IF transformer it goes smoothly from kind of working to oscillating wildly at a bit over 4.5Mhz.

Last edited by jhiesey; 09-27-2018 at 06:34 PM.
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Old 09-27-2018, 10:39 PM
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Quote:
Originally Posted by jhiesey View Post
On channel 3, both modulators behave the same (weak, smeary color). That means there's definitely an alignment issue in the tuner, right?

The modulator I have isn't a Blonder Tongue; it's a Standard SCM550HP, but it also has an IF loop. Part of the reason I got it was so I could hook the 45.75Mhz IF output directly up to the IF stage of the TV, but I haven't actually tried that yet. I'll do that tonight.

The audio IF oscillation is using a tuner signal. What I observed is that if I tune the IF transformer it goes smoothly from kind of working to oscillating wildly at a bit over 4.5Mhz.
Yes, must be the tuner, but only on the tuned RF amp stage(s) for the bad channel(s).

Don't know what to say about the oscillation. Maybe use the scope probe as a handy capacitor and poke around looking for a sensitive point that is supposed to be bypassed and quiet but isn't?
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Old 10-01-2018, 12:18 PM
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Out of curiosity: On channel 8 non-smeary picture, is the hue sensitivity to horizontal hold also fixed as predicted? (Always good to have one's long distance analysis either confirmed or corrected!)
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