ADM 3a with no horizontal scan

[Seth Morabito]

OK, I made a very interesting discovery today.

When I'm working on a project on my workbench, I normally have it
plugged into a Variac. This ADM 3a is no exception. I bring up voltage
slowly on any unknown old electronics, and then let it sit at our
normal wall voltage - which currently reads 121V.

Today, for whatever reason, my Variac was turned down to 110V, and I
didn't notice. I turned on the ADM 3a, and... it's fine. It beeped, it
has a cursor, and scan looks perfect.

I turned the Variac slowly up to 121V, and at around 118V the 3a
goes nuts and loses horizontal scan, I just get the vertical line.

So, some part is really marginal, and I need to figure out which one
it is. What fun!

-Seth