Question on a Hickok 505A oscilloscope

[Kamakiri]

Trying to do some work with my scope, which has up until this point, never been used for, well, really anything.

It seems to function decently well, with one exception that I'll get to in a moment. I changed out all of the wax caps that were left in there....it seems that someone worked over this unit at some point and put mostly Spragues in it, and JAN tubes. I'll get to the Spragues at my next capacitor order. Tubes tested good.

Problem is, the vertical gain control does nothing. I can move the dot, or sweep line, all the way around the screen with the controls, but I just can't get a heck of a lot of height out of it.....maybe just shy of an inch at best.

Now I realize that I'm going to have to end up replacing the caps, but before I chase a ghost, there are 8 screws on the back, with jumper wires between them. The attached wires seem out of place. Numbers on them are D1 through D4. I've found a copy of the manual online, but it doesn't refer to the proper positioning of these wires, much less what they're for.

Can anyone shed some light on this? Thanks!