Chroma w/out Luminance in CTC-11? - Videokarma.org TV - Video

Phil Nelson · #1 09-09-2009, 08:28 PM

I was about to ask for advice on this when the forum moved, and I guess the old thread is gone.

Long story short, I have started working on my $1 RCA CTC-11 "roundie." The set was dead until I replaced a fried thermistor. I also patched up the horizontal hold control, whose shaft was broken off and which was bent and popped loose inside. I cleaned the usual things with DeOxit. The tuner is still awful; I can only tune in a station between channels.

Now it has great audio and the vert/horiz holds are fine. The picture is awfully dim. First photo is this set. Second one is my restored CTC-11 for comparison.

The problem is not in the CRT or convergence assembly. I hooked up this chassis to those parts from my restored CTC-11, and got the same bad picture. My CRT tester also shows emission on all three guns.

The color controls have an effect, but brightness and contrast do not. It was suggested by oldcoot88 that I have chroma, but virtually no luminance.

Looking at the video amp/output board, I subbed known-good tubes for the 12BY7A (video out) and 6AW8A (video amp & color killer). No change there. I tested their voltages, which are generally in the ballpark except for pin 8 of 6AW8A, which shows 232 volts instead of 120.

I will continue checking things, but I thought I'd throw this out in case I'm missing something obvious.

Phil Nelson

cbenham · #2 09-09-2009, 08:42 PM

Quote:

Originally Posted by Phil Nelson

I tested their voltages, which are generally in the ballpark except for pin 8 of 6AW8A, which shows 232 volts instead of 120.

This is purely a guess because I don't have a CTC11 schematic. If the plate voltage is high, look to see if the tube's cathode resistor is open or if something is driving the first grid way negative and cutting off the tube.

Best I can do in the dark. #;^) Cliff

Phil Nelson · #3 09-09-2009, 10:05 PM

Here's that part of the schematic (in two pieces, because my copy of the schematic is on two pages).

C2D is part of a four-section electrolytic can, so I'll have to get to it eventually. I already replaced C5 near the 6AW8A because it's on the PC board and easy to reach.

Phil

http://antiqueradio.org/art/temp/VidBoardSchem1.jpg

http://antiqueradio.org/art/temp/VidBoardSchem2.jpg

old_tv_nut · #4 09-09-2009, 10:33 PM

Check R49 (330k) - could be open (?)

Phil Nelson · #5 09-09-2009, 11:23 PM

R49 checks at 315K, but I'll replace it anyway, now that one leg's lifted out.

Phil

Zenith26kc20 · #6 09-10-2009, 01:53 PM

Try following the video signal path with a scope. It sounds like a peaking coil may be open. I'll look at my schematic when I get home tonight.
Also, carefully look at the printed circuit around the video stages. They were sometimes notorious for crystallized connections on tube socket pins

*zenithfan1* · #7 09-10-2009, 02:12 PM

Yep, those peaking coils are a PITA sometimes, moisture gets in and corrodes the tiny wire.

freakaftr8 · #8 09-10-2009, 02:43 PM

Also, I dont remember if the CTC11 had this or not but check kine bias pot for function. Or video peak...

rcaman · #9 09-11-2009, 02:28 PM

does the service switch function correctly. you may have a dirty service switch.
steve

freakaftr8 · #10 09-11-2009, 02:45 PM

Yeah, those service switches get nasty! Had that problem on a CTC16. Had to Deoxit the crap outta it..

sampson159 · #11 09-11-2009, 04:53 PM

having the same issue with a ctc16.very low brightness.does look to be a peaking coil in the video.set brightness at max and kine bias at max to get a dark picture.lots of bad solder joints and those big tuner assemblies!i love these old rca s but why didn t they have a set of tuner plug connectors?i always break a wire or two when i pull a chassis.but then again,it wouldn t be as much fun ,would it?

Phil Nelson · #12 09-12-2009, 08:55 AM

Bill (aka oldcoot88 aka jughead) won this week's Swami prize by suggesting a dirty service switch in another forum while this one slept.

http://groups.google.com/group/rec.a...89616d42a2119#

Thanks for all of the tips. I like these easy fixes, even when I don't think of them first.

This one looks promising. Nice bright CRT, and the convergence is already better than on my "restored" CTC-11, which I slaved for hours to improve.

Now I am on to other issues, like replacing electrolytics and trying to fix the blasted tuner, which only receives stations when I turn it between channels. DeOxit didn't improve it, and I'm loath to try scrubbing the contacts for fear of making it worse. My other 11 has the same tuner problem.

Phil

freakaftr8 · #13 09-12-2009, 04:29 PM

Curious to what causes tuner drift like that between channels.

I thought tuners were on a keyed shaft and wont allow the contacts to drift off. If someone twisted the tuner knob hard I suppose it could bend the shaft causing an alignment problem between the key and the contacts. Mabye an adjustment of some type.

Quote:

Originally Posted by Phil Nelson

Bill (aka oldcoot88 aka jughead) won this week's Swami prize by suggesting a dirty service switch in another forum while this one slept.

http://groups.google.com/group/rec.a...89616d42a2119#

Thanks for all of the tips. I like these easy fixes, even when I don't think of them first.

This one looks promising. Nice bright CRT, and the convergence is already better than on my "restored" CTC-11, which I slaved for hours to improve.

Now I am on to other issues, like replacing electrolytics and trying to fix the blasted tuner, which only receives stations when I turn it between channels. DeOxit didn't improve it, and I'm loath to try scrubbing the contacts for fear of making it worse. My other 11 has the same tuner problem.

Phil

old_coot88 · #14 09-12-2009, 09:07 PM

Quote:

Originally Posted by freakaftr8

Curious to what causes tuner drift like that between channels.

I thought tuners were on a keyed shaft and wont allow the contacts to drift off...

By golly it's a shot in the dark, but I have seen a few cases where a wafer was loose on the shaft, that is, the rotary part of the switch was "un-keyed" from the shaft. You have to get in there with good light and see if each rotor is turning in solidarity with the shaft. If one is loose, you have to get it into proper position on the shaft and epoxy it.
Bill(oc)

Phil Nelson · #15 09-12-2009, 10:57 PM

Probably worth checking out. I can imagine that you could wear out so much contact material in the regular positions that you wouldn't get a decent signal through. But getting the best reception in the "humps" between channels does suggest some kind of misalignment. Seems like you should get nothing, or snow at best, in between positions.

Now that I have an agile modulator, I can check every VHF channel, not just 3 or 4. If wear is the real problem, you'd think there would be at least one channel where it wasn't worn down to the bone.

Phil