RCA CTC-31 & 38 chroma lock problem

[Chad Hauris]

There must be some kind of common problem on RCA CTC-31's and to a lesser extent 38's...it is a loss of chroma sync.
I've got 3 31's and they all have it...2 of them pretty much have it from turn-on, and another is OK when cold but when it heats up will drift on certain scenes but then other times is OK.
Also on a 38 I just got back to working it was drifting but I was able to lock in chroma with an adjustment of the piston trimmer capacitor....the tint control action still doesn't seem right. There is another 38 I have at my parents house that I got in high school that also had the drift problem.

I probably will need to get the scope and try to check out the waveforms in the circuit...but maybe there is some common problem you have noted in these. I have tried different 6GH8's but this usually made the problem worse (I don't have any brand new 6GH8's).

The weird thing is that none of the portables I have that use an all tube chroma circuit (CTC-22 and 36) have the chroma problem...they use 5GH8's but maybe the circuit is different enough that the problem doesn't crop up?

[captainmoody]

On that chassis it is a must that the 6GH8's are new replacements, I went round and round on this years ago! If you still have trouble after this, PM me as I have a lot of info from when I was repairing them in the late 70's (common chroma probs I ran into)

[Chad Hauris]

Thanks captainmoody...I will need to order some new 6GH8's. At our repair shop we have a large stock of tubes from other shops that have closed down but strangely enough there are few if any 6GH8's...maybe they went into sets so much that there was little stock left over when people quit re-stocking tubes. And that CTC-31 sure uses those 6GH8's, there must be 6 or 7 of them in there.

[bgadow]

I gave up on my -31 for now, until I get smarter! One problem is the chroma lock, it will stay still for a few seconds with some fiddling but then it goes off. Can't adjust the tint without setting it spinning. The main reason I shelved that set was an arc that keeps attacking in the chroma circuitry, and I don't understand why, but it apparently was the culprit that burnt out a coil, and I don't want to burn out another. When I do finally tackle it again I'll remember you, captain!

(btw, I bought a full set of NOS GE 6GH8's for mine)

[Chad Hauris]

Bryan, I had a loose connection from a wire wrap terminal to the circuit board in a 38, this resulted in no horizontal osc. plate voltage: resoldering the terminal post fixed it. The bad connection was not evident from a visual inspection, but touching the terminal indicated some play in the solder joint...this was confirmed with the voltmeter. You may want to check for this on your set...perhaps a similar condition exists in your chroma area. If it is one one of the screen controls, there is 1040 volts there, so you could get a fairly strong arc.

[kc8adu]

iirc the common cause for weak burst is a resistor in the plate circut in the burst amp going high.