'About' 20 trimpots is a bit unspecific. But the identification of two clearly discernible rows does probably correspond to the X and Y channel. So looking at the components would be the next thing to do. And you should identify the points in the circuit diagram of the basic unit to which the GPIB-module interfaces. Maybe do some measurements on those signals in order to find out how they behave. I am a bit irritated that the circuit diagram does not even hint of a connector which would carry those signals. Are they wired singly into various points of the regulator PCBs?
I opened it up again and there are two staggered rows of nine trimmers:
Channel X? | Channel Y?
|
25k 100k 25k 2k2 |100k 4k7 25k 4k7 10k
100k 4k7 25k 4k7 10k | 25k 100k 25k 2k2
They aren't laid out as two identical rows, and I think they are more like a bank of pots at one end are for channel X and the other end for the Y channel. There is also a bank of 50 resistors directly below the trimmers with four reference zenners below that. The zenners are spaced evenly along the bank of resistors.
4-4016 (Quad analog switches)
2-4066 - (Quad bilateral analog switches)
6-4015 (Dual 4 bit shift registers)
So I'm thinking that yes it does look like some form of DAC resistor network using a shift register. Then I look up the last unknown part on the board:
Ferranti ZN429E (8 bit R2R DAC) There are four of these.
So I think it must use a combination of shift register to set a coarse level with the DAC as fine control?
I did start tracing the circuit but it would take me a considerable time to try to ascertain what function each of the trimmers has? I could just tweek a bit and see what happens, but I'm apt to make it worse than it is.
Schematic or calibration instructions would be the easy way. Someone must have a copy?
I may well spend an evening tracing it out (probably two at my speed) now that I see the split at least I only have to figure out nine of the pots........The biggest pain are the vias hidden under the row of resistors.