Today's Mystery Guest (RCA CTC-4)

[Phil Nelson]

OK, after pushing my CTC-4 project aside for a long winter's nap, I hauled it back out.

Last year, I had decided to build a video preamp in hopes of injecting a usable video signal at the 1st video amp, so that I could bypass the set's RF/IF stages and debug my obvious color problem in a "clean" environment.

Long story short -- Yes, I did eventually build a video preamp; No, it didn't work as expected (owing to the inevitable leetle wiring mistake by me); and then I got disgusted and worked on other projects.

Recently, I got a helpful email from ChrisW6ATV, who asked whether I had ever tried injecting a video signal from my Sencore VA62A, using the Drive Output jacks at the lower right of the front panel. Chris mentioned that you can set the Drive Range level to 3VPP and basically accomplish what I wanted to build a video preamp for (that is, inject a somewhat stronger-than-standard video signal). He reported getting good color on his CTC-4 when injecting video in this manner.

Duh!

So, here we go. The VA62A makes good color bars, using the 10-bar pattern referenced in the RCA service manual ( http://antiqueradio.org/art/RCA CTC-...ice Manual.pdf ):



Here is that video signal scoped at the injection point (terminal J on the IF board; the junction of L105/R126/R324 in the RCA schematic):



Here's the screen image when that signal is injected (after tweaking L127, reactance coil, to slow down the rolling enough for a photo). Yes, we have seen this barberpole effect many times before:



Chris noted that you can pipe an external video source through the VA62 using its video jack in the rear. To try that out, I set a second generator to display NTSC color bars and sent that signal through the VA62:



Of course, you can also inject a DVD video signal through the VA62 in that manner, but I'll spare you any more ghastly-looking scenes from the Wizard of Oz. Trust me, they look awful.

Now that I can inject a known-good video signal at the right level, I'll back up and:

-- Run through color setup procedures as best I can
-- Check voltages throughout the color sections
-- Scope signals throughout the color sections

These things have been done (or sort-of done) before, but now that I can inject a nice strong 10-bar color signal, it should be possible to work through the demodulator phase adjustment procedure on page 49 of the RCA service manual. Other parts of the manual that reference that pattern may also make more sense.

As always, I'm open to other ideas. And thanks again to Chris for this bit of advice.

Regards,

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[Penthode]

Phil,

Are you connecting the Sencore from terminal J to ground? The detector circuit output is across J and L. I looked at the CTC4 schematic and the connection of the generator from J to ground may not work because L is floating. Note that L is the sync separator feed. L also attaches to the cathode of the first video stage. V110 is a cathode follower.
The first cathode follower video stage does not invert the signal nor does it provide any voltage gain. It only provides an impedance match to the delay line.
So why not try and attach the generator from L to ground and see if that works? (You might want to DC isolate the connection to L with a 2uF 50v electrolytic with "+" to the cathode of v110).

Cheers,

Terry

[Phil Nelson]

Originally Posted by Penthode

So why not try and attach the generator from L to ground and see if that works?

I tried connecting the generator from L to ground and the picture became completely unstable: horizontal wouldn't really lock, and I couldn't tell whether vertical locked or not. When I connect to J, the image is stable apart from rolling colors.

Originally Posted by Penthode

(You might want to DC isolate the connection to L with a 2uF 50v electrolytic with "+" to the cathode of v110).

Not sure what this means. Can you explain what is connected to what?

Phil Nelson

[ChrisW6ATV]

Originally Posted by Phil Nelson

And thanks again to Chris for this bit of advice.

Phil, after all of the great advice and tips I have learned from many of your restoration pages, I am glad to be able to offer even a little tidbit back in return.

Originally Posted by Penthode

Phil, Are you connecting the Sencore from terminal J to ground?

Terry, I also connected my VA62's video between point J on the RCA CTC-4 schematic and ground, and in my case, I got near-perfect video right from the start. The signal was, if I remember right, inverted video at about 3 volts peak-to-peak. Feeding it at point J seems to also put the needed signal to point L, through the nearby components. I also tried between point L and ground and did not get a usable signal; I did not try between points J and L (but did not need to, as noted above).