Picked up this set from a local collector, who shot this video. I've never seen an issue like this before, take a look:

https://www.youtube.com/watch?v=nGyboqrG8ic

Swapped vertical boards out of another set, and that's not it. About all I can think of is a regulation issue. There's a 470 uF cap on the board which I've heard is problematic, so I'm going to replace that.

Before I finish up my Mouser order, any thoughts or input?

...

A few things to watch on early XL100's
Cold joints around the pin transformer.
Cold grounds from main chassis to mother board.
Module sockets crack & go int.
If any of these it should act up when tapping around.
On the 17" sets vert sockets were a BIG problem. They
would crack & cause the module PCB to burn up.
New socket & a module with no dud value was costly.
If none of that start with 'lytics from the output then
back.

73 Zeno:smoke:

I've seen that before but i can't remember when, looks like a bad filter somewhere.

I'd try jumping the filter caps (set off) one at a time and see if it goes away.