I saw problems like this in fixed frequency 19"/20" RGB Sony monitors
attached to IBM 5080/5085 CADCAM
workstations. The problem was dried
out caps on a little daughter PCB hanging off the middle of one of the
main PCBs. Sorry, I can't be any more specific than that.
BTW, when
was your monitor manufactured?

Feb 1990. I would imagine your IBM workstations were probably in the
same timeframe-- we may be talking about the same chassis.

There are several "daughterboards" on this mainboard. They are all
soldered on heavy wires-- for robustness I suppose. You said in the
middle-- that does narrow it down and I think I could get to that one's
caps without having to unsolder the whole board-- well maybe not!!

That board, about 3x3.5", is likely to be what the schematics show as
Board "Dc", subtitled "T/B correction". That makes sense, doesn't it?

I think I'll see if I can pull that board without destroying anything--
thanks much, Franc, for the tip!!