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?