[trinescope]

Originally Posted by DavGoodlin

This problem of vertical output transformers has been reported on L and M series chassis, but not the 24MC32 and 24NC31/Z, the last roundies in the narrower 1966 and 1967 chassis. While this seems to be an age problem, a difference in circuit designs may give clues.

Checking and comparing the schematics, there are differences between say a 27KC20 using a 6EM7 and a 25LC20 using a 6HE5. Primarily the 6HE5 has a higher b+ voltage on the plate and the sweep pulse is much higher amplitude. I have 2 1964 models using 25LC20 sets and I'm not ready to plug them in due to this fact.

Borrowing a piece of medium voltage equipment called a "Megger insulation tester," I was able to actually measure a VOT's primary to secondary winding leakage in the Gig-ohms range, using approximately 1000 volts DC. Using this as a baseline reading, I should try the following methods to increase this value:

1. Pass just enough DC current through the lower-resistance secondary winding to heat up and drive moisture out of winding using heat without a potential difference applied.
2. Dip the entire transformer in insulating heated varnish, in hopes that voids where moisture may have attacked exposed copper windings are filled.
3. Retesting leakage after using any combination of the above methods.

It happens on the rectangular sets as well.