Today's Mystery Guest (RCA CTC-4) - Page 8 - Videokarma.org TV - Video

Electronic M · #**106** 02-25-2015, 05:19 PM

IIRC on mine there is a point where the monochrome is really sharp. On one side of that point mono is blurry, but the audio is really clear, and on the other side if tuned to the ragged edge of the mono, and after sound is lost color comes in....Granted on mine some clod knocked one of the IF cans off the video board and I had to cobble it back on so god only knows what that did to mines alignment.

dtvmcdonald · #**107** 02-25-2015, 06:06 PM

It is frequently said that these old sets don't really need realignment.
I've found that untrue. All my sets ... Pilot TV37, RCA 9-T-249, Emerson 610
and RCA CT-100 had abysmal alignment. True, many of the very sharp
trap frequencies were spot on, but the overall IF shape was in
every case completely wrong. Many RF channels were off too.

Its a lot harder to recognize on a B&W set, especially a narrow band
one like the TV-37. My opinion is that the best test on a B&W set
is the standard 100% color bars pattern (WYCGMRB). If alignment is spot on
you should have no problems with the relative brightness of the
bars versus fine tuning as you tune so the video carrier rides up from the
1/2 point to the 100% point. If its off, you will notice
problems. You can use 75% bars but 100% is a much better test.
This is what I fought with a long time on my CT-100.

stromberg67 · #**108** 02-25-2015, 07:31 PM

I don't really know about other chassis, except that most of my older sets look maybe just OK, but my CTC-4 luminance section probably couldn't be more "sharp", or provide a more excellent picture. Chroma is also very good, but narrow band, as most know. Perhaps just a lucky circumstance, or maybe an extra measure of diligence to provide decent pics to encourage sales.
My 4 also has a correct gamut CRT, before the "gray screen" tubes. Best viewed in a totally dark basement LOL.

Kevin

Electronic M · #**109** 02-25-2015, 08:43 PM

Quote:

Originally Posted by stromberg67

I don't really know about other chassis, except that most of my older sets look maybe just OK, but my CTC-4 luminance section probably couldn't be more "sharp", or provide a more excellent picture. Chroma is also very good, but narrow band, as most know. Perhaps just a lucky circumstance, or maybe an extra measure of diligence to provide decent pics to encourage sales.
My 4 also has a correct gamut CRT, before the "gray screen" tubes. Best viewed in a totally dark basement LOL.

Kevin

My CTC-4 produced one of the most detailed, sharpest monochrome pictures of any vintage set I've owned when fine tuned for best best monochrome picture...

Phil Nelson · #**110** 03-04-2015, 11:22 PM

Before I cry Uncle and throw a tarp over this TV, I scoped the input of the first video amp to see what is getting through the video IF section.

My pattern generator is set to color bars, which the TV displays as gray bars.

The top scope trace is the TV's video IF output. The bottom trace is direct from my pattern generator.

The signal looks pretty scrawny by the time it gets through the video IF chain; the burst signal is basically missing in action. Tell me if I'm wrong, but I'm taking this as a sign that the TV really does need realignment.

Phil Nelson

miniman82 · #**111** 03-05-2015, 04:56 AM

Try injecting a composite signal to the 1st video amp, if you haven't already. That would eliminate the IF as a fault area. If that's not it, it's likely a mistuned trap somewhere.

Phil Nelson · #**112** 03-05-2015, 02:16 PM

Hmm, results inconclusive. I pulled the 3rd video IF tube and injected video from my little Leader pattern generator at the grid of the 1st video amp. The picture had extremely weak sync and it showed a faint image.

Then I looked at the CTC-4 schematic and noticed that the 1st video amp is expecting +2.7V on its grid. That pattern generator certainly doesn't provide a signal that strong.

I tried using my B&K TV analyst as the video source. Cranking its video output control to the max gave me something over 2V, but still not enough sync for a stable picture. Still no true color, either, but if the signal is too weak to pass a usable color burst, I'm not sure what that proves.

Phil Nelson

miniman82 · #**113** 03-05-2015, 03:12 PM

I think I left the IF tubes in when I did it, but it's been so long now I can't remember. You put the signal right in to pin 2 or the 6CL6 and chassis ground, right? If it's not getting a picture, you may have a bad AGC pot on your hands. Mine was completely trashed when I first got the set, wouldn't display much of anything. Check to make sure it's responding, then move on to scoping the signal you're injecting at the CRT cathodes. If it's not coming through, there's still some issue somewhere. My set definitely works when injecting composite.

wa2ise · #**114** 03-05-2015, 09:17 PM

Some vintage color TVs had a separate detector diode for the chroma, separate from the regular video detector. Your oscilloscope display of the luma video would be typical found at the regular video detector of such a set.

Phil Nelson · #**115** 03-06-2015, 05:01 PM

Quote:

Originally Posted by wa2ise

Some vintage color TVs had a separate detector diode for the chroma, separate from the regular video detector.

Yes, that trace does look more like a luma-only stairstep pattern, but this set only has the one detector diode.

Quote:

Originally Posted by miniman82

You put the signal right in to pin 2 or the 6CL6 and chassis ground, right?

Yup, signal to pin 2 (or 9) of the 6CL6 and ground to chassis. I re-cleaned the AGC pot and made some cursory measurements that seem to indicate it's OK. Let's hope it is not junk. It is mounted concentric with the 2.5-meg noise threshold pot, and I don't know where you would get a replacement dual 500K/2.5-meg pot nowadays. Worst case, you could drill a new hole and mount a new pot separately, but . . . .

I put the chassis up on the workbench and plugged in the yoke & convergence assembly so that I can do some more systematic voltage & resistance checks, and look at things more easily with a scope.

Phil Nelson

miniman82 · #**116** 03-06-2015, 07:48 PM

I don't think the chroma could be completely missing, it's likely very attenuated after passing though the IF. I'd bet if you used the scope to zoom in on one of the luma bars, you'd still see a bit of 3.58 modulated on them. That's why there's a band pass amp, to bring the 3.58 back up to the point where it can be processed in the set. Same principle as attenuating sound before it messes with the video, then amplifying it in the audio section in intercarrier sets. Does your pattern gen have a means of switching off the chroma when bars are on screen? If so you can probe the band pass amp or demod driver to see if the chroma signal is getting though, switching chroma on and off while you probe gives you a way to make sure you're not just looking at noise. If it isn't getting through, you're probably looking at a slope adjustment to the IF at the very least because the 3.58 has likely slid off to one side and is not being seen by the set anymore. If the chroma is still there, it's likely the color AFC needs attention. You were able to get solid color raster before, but that is a very strong signal and is easy to lock onto. A weaker scene type signal might be throwing it off, so see if doing the zero-beat procedure does anything to improve it. I don't have many more ideas, besides a full on alignment. You could mail me your chassis I guess, I have the rig to do it...

Phil Nelson · #**117** 03-07-2015, 01:59 PM

Quote:

Originally Posted by miniman82

a full on alignment. You could mail me your chassis

Hmm, that's a tempting offer.

Meanwhile, I am checking things around the 6U8 AGC/burst keyer.

Phil Nelson

Username1 · #**118** 03-08-2015, 08:19 AM

Quote:

Does your pattern gen have a means of switching off the chroma when bars are on screen? If so you can probe the band pass amp or demod driver to see if the chroma signal is getting though, switching chroma on and off while you probe gives you a way to make sure you're not just looking at noise. If it isn't getting through, you're probably looking at a slope adjustment to the IF at the very least because the 3.58 has likely slid off to one side and is not being seen by the set anymore.

This is it right here.... Replacing every cap means all tuned circuits need re-alignment.
Chances are each tuned circuit has moved it's center up or down, and as
a unit, is just rolling off before the chroma part and attenuating it..... Easy
fix, but yes be sure everything else is ok....

.

Electronic M · #**119** 03-08-2015, 11:54 AM

Squirrel boy, Most of the caps we replace in sets like this are not in the IF tuned circuits (including the odd paper or two in the IF section of some sets).

Phil Nelson · #**120** 03-08-2015, 01:53 PM

Quote:

Originally Posted by miniman82

Does your pattern gen have a means of switching off the chroma when bars are on screen? If so you can probe the band pass amp or demod driver to see if the chroma signal is getting though, switching chroma on and off while you probe gives you a way to make sure you're not just looking at noise.

Good idea. Yes, my generator has a Chroma Off switch, so that would be easy to do.

Gack. I was measuring something else when HV disappeared and the 6CB5 horizontal output tube began to red-plate

I pulled the 6CB5 and it is toast. No spares for that in the house, so I'll have to order a replacement (or two, for good luck).

Meanwhile, I assume it's OK to power up the chassis on the workbench (with yoke and convergence coils connected) with that tube removed . . . .

Phil Nelson