Predicta Debutante - voltage rail being dragged down

[cj_reha]

Originally Posted by Electronic M

The vertical linearity pot is SUPPOSED to have only one terminal. The wiper and bottom terminal are grounded...They are grounded by internally connecting them to the metal case of the pot that then is riveted to the grounded metal chassis...most pots of that cosmetic style were made that way...it saved production time by eliminating 2 hand wired solder connections...the set uses couplets for the same reason. If you haven't rotated the pot through its range or sprayed cleaner in it you might want to try it. It could as easily be the pot is good but has a dead spot as it could also be the pot is dead open.

I tend to unhook loads from shorted B+ rails to track down shorts. The lowest resistance path to ground is often the culpret... thought some shorts only happen when warm so sometimes I simply see which load says the B+ when connected.

Just looked at my parts chassis and that pot only has a single terminal as well, not sure how I noticed it before...thanks for clearing that up. Will definitely try using some contact cleaner, I hope the pot is good after all. Would save me the trouble of drilling it out.

As for disconnecting loads, the 160V rail only splits off to the IF board and one point on the PCB. Disconnecting the PCB point (as described in the OP) gives me a stable voltage, so it is a problem on the PCB; however, this point measures 10K to ground, and this is correct according to the official Philco service manual. My current theory is something shorts while heating up, or is at least excessively leaking electrically. Could be a defective component as there is not much I have not changed, but I'll likely figure this out once I start taking stuff out of circuit. Hopefully it's not something super difficult to find.