CTC-5 Service Info Sought

[bgadow]

Fixing up old tv sets is a constant learning experience and challenge for me. No professional training here, just what I have picked up from years of tinkering, and moreso, what I glean from here and the ARF.

Right now I am working on my CTC-5 (Deluxe chassis) which had a couple issues I had put off fixing. Figured it would be nice to have this one in tip-top shape for the digital switchover. An issue since I got it going: close interaction between the horiz hold/contrast/agc/brightness controls. Any of them can throw the horiz (and sometimes the hv) out of whack. I decided it was time to get serious about this. To start with, my best picture was looking pretty washed out. Tubes, including the crt, are good; I checked hv and it was hovering around 17kv, about 3kv too low by my references. By tweaking the front panel controls I can get a better pic, corresponding with a dead-on 20kv, but it is unstable, tends to jump out of sync or overload.

I have a pretty good library of TV repair books (including color TV) so I dug through them but I haven't found anything that addresses horizontal sweep alignment on these sets. Sams lists a brief set of setup instructions which says to adjust the horiz waveform so that the rounded and sharp points on the waveform are at an equal height, and it shows a picture of the proper wave. (I'm not great with a scope, but am working on it) I can get close but not exact. If I get the 2 peaks even I lose most of the valley; this is at the end of the coils travel and also makes the horiz unstable; it is prone to jumping out of frequency, so much so as to push HOT cathode current way over the line. I have expermented with various combos of waveform/horiz hold position. I would like to read more about this circuit but only the brief write-up in the Sams fits this chassis. Newer books cover the CTC-12/16 well but don't go back and hit the different circuit used here. I actually have some CT-100/21CT55 era stuff but it's no help, either.

Did the factory service manual for this set cover it very well? What might I need to do to correct that waveform? The horiz osc board has been recapped. I had replaced the coil bypass cap with a .015 when it should have been a .01, but have now corrected that. Any advice is appreciated. This set has a lot of potential and I'd really like to see it reach that higher level.

[yagosaga]

Hi Bryan,

I have the original RCA service manual for the models 21-CS-7815, 21-CS-7817, 21-CS-7815U, and 21-CS-7817U. Chassis No. CTC5 or CTC5A, Is it, what you need?

Regards,
Eckhard

[roundscreen]

Originally Posted by bgadow

Fixing up old tv sets is a constant learning experience and challenge for me. No professional training here, just what I have picked up from years of tinkering, and moreso, what I glean from here and the ARF.

Right now I am working on my CTC-5 (Deluxe chassis) which had a couple issues I had put off fixing. Figured it would be nice to have this one in tip-top shape for the digital switchover. An issue since I got it going: close interaction between the horiz hold/contrast/agc/brightness controls. Any of them can throw the horiz (and sometimes the hv) out of whack. I decided it was time to get serious about this. To start with, my best picture was looking pretty washed out. Tubes, including the crt, are good; I checked hv and it was hovering around 17kv, about 3kv too low by my references. By tweaking the front panel controls I can get a better pic, corresponding with a dead-on 20kv, but it is unstable, tends to jump out of sync or overload.

I have a pretty good library of TV repair books (including color TV) so I dug through them but I haven't found anything that addresses horizontal sweep alignment on these sets. Sams lists a brief set of setup instructions which says to adjust the horiz waveform so that the rounded and sharp points on the waveform are at an equal height, and it shows a picture of the proper wave. (I'm not great with a scope, but am working on it) I can get close but not exact. If I get the 2 peaks even I lose most of the valley; this is at the end of the coils travel and also makes the horiz unstable; it is prone to jumping out of frequency, so much so as to push HOT cathode current way over the line. I have expermented with various combos of waveform/horiz hold position. I would like to read more about this circuit but only the brief write-up in the Sams fits this chassis. Newer books cover the CTC-12/16 well but don't go back and hit the different circuit used here. I actually have some CT-100/21CT55 era stuff but it's no help, either.

Did the factory service manual for this set cover it very well? What might I need to do to correct that waveform? The horiz osc board has been recapped. I had replaced the coil bypass cap with a .015 when it should have been a .01, but have now corrected that. Any advice is appreciated. This set has a lot of potential and I'd really like to see it reach that higher level.

Did you replace the mica caps? If not then you should start there.
I have a rca field service manual and the caps are listed as c605-12pf, c611-82pf, c610-82pf, c606-330pf, c604-1000pf. After you replace the caps do an alignment. The horz osc on the five is vary stable and you should not have to adjust it again unless something fails.
Good luck, The rca ctc5 is a cool set.
Ed

[kx250rider]

I've worked on quite a few of those, and I've always noticed that the AGC is very unstable. I remember that even when I worked on a few CTC-5s for the original owners in the early 80s. I think part of the problem is design, but new mica caps may help. RCA sets use those in the front end, and they're often shot. Actually, the reddish and brown "Micamold" ones often aren't even real mica caps... They're paper caps in a mica-looking package!

Charles

[bgadow]

Thanks, fellas. Back when I recapped it I had replaced a couple of the micas on the board, but not all. I noticed that one that I replaced wasn't a perfect match. At the time I had a no HV situation and I was shotgunning everything trying to remedy it. (turns out I made a soldering error on that board which took me many years to catch) I'll go ahead and replace everything that's left. Yagosaga, my set is model 21CD7895, but I don't know why that manual wouldn't work for what I need to do. Is there a page or two that covers the circuit, maybe something that could be readily scanned? I appreciate the help. This set was working very, very well a few years ago, but it could just be that some of those remaining caps have finally given up.

[old_tv_nut]

I appreciate the info about the (possibly fake) mica caps. I have finished replacing the paper caps in my CTC-5, and am just about to do the 'lytics, thinking I would be done.

Bryan, Television Engineering by Fink has an evaluation of this circuit but unfortunately no clues on how to adjust it or what the waveforms should be. It does say that this circuit has disadvantages of being sensitive to the signal level input - and is also sensitive to component values.

I think you should not see such interaction with contrast and other settings. So, I would trace the signal levels with a scope to see if there is a weak stage.

(Following comments refer to the super chassis, but I believe the deluxe is the same except for adding a noise gate stage.) Since you have SAMS, you could trace the signal levels with your scope, starting by setting the AGC for the correct video level (W1, grid of the video amp), then check waveforms for AGC keyer, sync separator and sync amp. These stages should have enough gain that the horizontal circuit is getting constant amplitude pulses with slight changes in AGC setting and any setting of brightness and contrast. You should have clean pulses like W7 shows at the input to the sync amp.

Good luck, and let us know what you find, because after reading the posted info, I have a feeling I may be doing the same thing soon!

[bgadow]

I spent some time tonight digging through some old trade magazines. An invaluable resource. I have one box of articles, cut and pasted then organized in folders, all done by some very enterprising repairman way back when. One good article I found is from March/April '72, a Carl Babcoke review of color horizontal circuits. For whatever reason he chose to base it on a CTC7AA. The basics are very close, close enough for me to learn a lot. I don't know how much this will help me...one suggestion is to short the waveform coil while locking the horiz hold. Simple so I'll have to try it and see what happens. Meanwhile, our friend Frenchy had posted over on ARF regarding a possible contrast circuit mod. The -5 uses a different arrangement from later sets, variable bias instead of variable degeneration. I only know the technical jargon because I found an article from '65 that outlines how to rewire it to the newer circuit. Not sure if I want to do that; I'll work on fixing this problem first. From the brief description in the article this just might have some relation to my problem.

Another question: what type of caps should I use for replacing those low value micas? Looking at what I did in the past on that board, with some I was able to use an orange drop, on one I used a ceramic disc. I'm a little leary of that disc-my reasoning is that RCA could have used one back in '56 and they didn't, and there must have been a reason. I checked out the Mouser catalog and I see some muRata radial hv caps that look like they would work. One concern is that a couple of the old ones were rated at 1kv, something the muRatas can handle.

[old_tv_nut]

I dug out the field service guide schematics for the CTC-5 and -7. The contrast control in the -7 is coupled through a capacitor, so it won't change the DC gain of the video output. This would make it more stable against video output tube variations, but it also means you get a variable percent of DC restoration with the -7 circuit, depending on the contrast setting. The contrast control in the -5 is directly coupled to the video output cathode, so it varies the gain of the whole signal, AC and DC alike. However, when turned towards max contrast, it will also magnify any DC video voltage errors that are due to the operating point of the video output tube.

[bgadow]

Here is the article I mentioned above. Would be interested to hear opinions on it.
http://i184.photobucket.com/albums/x...ontrastMod.jpg

[reeferman]

taken from Color tv servicing made easy (Lemons & Babcoke 6th printing 1966)

[old_tv_nut]

Originally Posted by reeferman

taken from Color tv servicing made easy (Lemons & Babcoke 6th printing 1966)

thanks

[old_tv_nut]

Originally Posted by bgadow

Here is the article I mentioned above. Would be interested to hear opinions on it.
http://i184.photobucket.com/albums/x...ontrastMod.jpg

If the CTC-5 original configuration increases the black level with contrast increase, then it does need redesign. However, this wouldn't be the only way to fix it. A bias network attached to the cathode could fix that without reducing the DC coupling.

If the black level is keeping constant with contrast setting, and the problem is just asking for too much drive (beam current) at high contrast setting, then a reasonable fix could be to wire in a contrast limiting variable resistor in series with the customer control. This would give you an adjustment to compensate for video output tube variations.

It'll be interesting to see what mine does when I finally get it going.

[bgadow]

Something I have learned is that Babcoke & Lemons really knew their stuff. Easy to read and understand. The RCA color service guide that Babcoke did for TAB books is my most referenced. This weekend I came across an article Lemons did in '72, predicting what TV sets (and TV servicing) would be like in '92. Will have to scan it sometime.

[old_tv_nut]

Bryan,

I put the scope on mine tonight and found that the horizontal waveform was not equal peaks. I adjusted that, and then was unable to get stable horiz sync. I then found that the horizontal frequency was way off (low). The waveform was bad because the frequency was off, and adjusting for the right waveform (equal peaks) only made the frequency worse. I turned the waveform coil several turns the other way, which got the frequency back up, then adjusted the horiz hold control, then back to the waveform to get it closer, and after several times back and forth got both good sync and the proper waveform. SO - these two things interact considerably!

Also, when it was initially out of whack, the other controls like color and contrast would affect it, as you observed (I'm guessing this is mainly due to it being sensitive to B+ when it is not properly locking.)

I have not replaced the mica caps, but this might still be your problem, I guess.

- Wayne

[old_tv_nut]

Bryan,

Let me clarify - the case when I had weak lock, the H frequency was off by some fixed amount, which would correspond to multiple stationary diagonal bars if I had been able to see it on the CRT. Once I hooked up both channels of the scope, one to the H waveform and the other to video, I realized I was off frequency. When I got back to the proper combination of frequency and waveform, the lock became very stable.

If you are seeing a proper picture that is on frequency, but the lock is unstable, then it's something else that isn't bad in my set. Maybe those micas or maybe a bad input signal level.