Given my experience, and eventual success in repairing my now indispensable Keithley 199 Multimeter, I recently picked up a non-functional Keithley 197 Microvolt multimeter on eBay for $25. Vintage is about the same for this model and the 199, and after looking at the manual (which includes schematics), I figured it was worth the $25 to give it a go at repair. Best case, it was easy and I could turn around and sell it, or give it to a friend.
The unit arrived in relatively short order, and indeed, in all settings, the meter just read “OL”. So, first thing that told me is, processor is working. EEPROM too, more than likely. So, going by the manual, and logic, I started with the voltage rails. The 197 has a few. Basically a ±15V rails, a +10V, a +5V analog, a +5V digital, a -6.4V, and a +2V reference. All of these are regulated by diodes rather than more modern LM78xx/79xx regulators. So, going through the checklist, it soon turned up that the -15V (-V) rail was dead. At first I thought diode, but testing the diode out of circuit resulted in a working diode. So something was pulling that rail up toward ground. So, trace time.