Fixing a 'broken' Keithley 196

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.

Keithley 196 digital board

Upon examining the board further I noticed a crispy via on the transformer:

Crispy trace

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.

Added wire

And fixed

And works again!

I have to dump the EPROMs and the calibration EEPROM sometime, as those are prone to lose data after decades.

50uA 50μA reference (uncalibrated)

100uA 100μA reference (uncalibrated)

10V 10V reference (REF 01 CPZ)