Rca ctc12 -
 

Having been a member for a dozen years as of today, I know this should be old hat by now. This is literally a brain trust for early color (any roundy IMHO) present company included these RCAs will test you. I started repairing curb finds when Ford was president!

I have four RCA CTC12 chassis to work on and while I am familiar with the usual task list, the original problem was an arcing vert centering control, causing CB to pop. Easy stuff, right? Not so fast, one has an oddly intermittent short that cause HV to drop out and return (blink), as HOT cathode current jumps from 190 to 270 mA, with flashes on the screen. Never had an early color RCA do this, always either the fly is good or its fried!

This is the customers one I need to fix and came with a parts chassis having a good-testing fly that has the 3A3 cap lead burned off, needing some surgery. I was suspecting an intermittent short in the flyback, so I'm ready to swap them if needed.

I ran it last night for an hour and no blinks occurred, though fly was a bit too warm and some new wax stalagmites dropped. The PS caps are still original, so sync was touchy but it had good video, sweep, color and sound. What I noticed next gave me doubts of any flyback issue. Please share any thoughts you may have on this.

After a second power-up, it began blinking again. Suspecting a shorting disc capacitor between the grid and cathode of the 6BK4, measuring voltages on both pins 1 and 5, the grid voltage drops from 375 to 280 when it blinks, cathode is steady 380v supply. So the boost supply is dropping out, allowing HV to go up by cutting off 6BK4.

Note that Horizontal drive to 6DQ5 is steady 190 vp-p, other tubes swapped were 3A3, 6BK4, 1V2 and 6DW4. The boost supply does not go too many places other than screen controls but there is a .047 maroon drop from boost to 385v supply that is suspect #1.

I would be tempted if the issue is somewhat regular, to run the set with the regulator unplugged and a blank white raster of sufficient brightness to keep HV at spec and see if the issue goes away as a diagnostic tactic.

I have run it w/o 6BK4 cap on, trying to eliminate that part, no change. It does become patternlike, as if something is discharging.

I was trying to pull up the schematic, www.tvhistory.tv Tom Genova site not working?

When it starts blinking, put the service switch to the set up position and check if the high voltage stays up. I've had video output tubes intermittently short bad enough to kill the high voltage.

earlytelevision.org should have the schematic for free...Just go to color then to technical information then scroll down to RCA... They've got literally every RCA roundy schematic there.

Try the blanking tube first. Can cause strange symptoms
Same thing with the PCB mounting grounds.

Zeno

I looked up the schematic for the CTC12. I was trying to see if that chassis used a focus rect tube instead of the stick. I remembered that it uses 6FQ7's instead of 6GU7's. Also a 6DQ5! :scratch2:

IIRC used a tube. Sockets would burn.

Zeno

I replaced one or two of those carbonized messes. It nice when there are obvious clues to where the problem is. Initially, this set had an arcing and lit-up vert centering control.

I like the fact a 12 is similar to a CTC11, (my fav) but rearranged and with some improvements.
The combined luminance/chroma PW700, I do not consider an improvement.

Good idea, I did not monitor the CRT K-G1-G2 while this occurs. This is equally probable to a boost circuit causing it.

I have replaced almost all the electrolytics and the maroon drops, not expected to solve THIS issue.

Also did the cateract, original CRT and it lifted off right away, mine is on right and below has 8 inch circle left after lens broke.
Attachment 205996
A Jig CRT is at right, type 1828P22 and etched "test tube" and the customer's CRT at rear has a darker, greener lens that was OEM.
The CRT with glass left was a 1967 rebuilt Colorama, always more work to de-cateract than the OEM's
Attachment 205997

The last thing I worked on was re-flowing solder on the two 6FQ7 (blanker and color diff amps) and the 12BY7, this alone probably solved the video flashing/HV drop outs. After changing to mylar film caps, then re-tuning efficiency coil, cathode current was down to 185 mA from 210!

This appeared to be a regularly serviced TV (few original tubes and two replaced multi-section caps) but the solder connections under PW700 appeared not baked like they usually are. My other two CTC12 chassis plus a spare chassis appear to have been used much harder.

..."color diff amps."

I like to call 'em "Chroma Outputs" just to be ornery. :tongue:

this is true and the kind of stuff we could put in a glossary, like luminance vs video:scratch2: