The output voltage was all over the place and not even one one value was correct.
Obviously the switches got corroded or contaminated or just old.
So, I drowned the complete front end of the calibrator in to pure alcohol and moved the switches probably a hundred times each.
A lot of dirt came out to to the front.
Then I had the alcohol evaporate in an oven for a few minutes at 50 degree C
And then I filled the switches with my trusted "
super contact cleaner, with poly phenyl ether" and moved the switches another hundred times each - at least. Then I took compressed air and blew the access lubricant out.
And ... look at this...
All values are working perfectly.
It looks like I do not even need to do a calibration.
But I might have to exchange the potentiometer since it jumps the values a little. But this is an odd little 50k Pot with a tiny shaft.
Ok, time to take some measurements and histogram for each value, to see how stable it is.
Too bad there is no full schematics available for this little unit.
It has a 10.000 V Ref output in the back.
I will take some statistics measurements there as well.