So I recently got a Keithley 196 digital system multimeter with 6 1/2 digits which was sold as “broken - parts only” for virtually nothing so I figured in the worst case I have something to salvage parts from.
After it arrived I tried to power it on - nothing. Line fuse was okay, so I started checking the power rails, on the analog board the +15V, -15V and +5V were fine. On the digital board however I found the problem: the digital +5V rail was missing, I traced this to the transformer not providing AC on the secondary winding used for the digital supply. Before I started to trace the problem further or bother to repair it, I soldered two wires onto the unregulated DC side of the bridge rectifier for the digital 5V rail and powered that from a laboratory supply - bingo it started right up.
Upon examining the board further I noticed a crispy via on the transformer:
Since the matching trace was on top and covered by the transformer I desoldered the latter. I found that the trace broke off/burned off. It looks like this happened from vibrational damage or a bad solder joint. I fixed this by soldering in a piece of solid wire that I then wound around and soldered onto the transformer tap.
And works again!
I have to dump the EPROMs and the calibration EEPROM sometime, as those are prone to lose data after decades.
50μA reference (uncalibrated)
100μA reference (uncalibrated)
10V reference (REF 01 CPZ)