Fluke 8100A Nixie Tube DMM - Broken!

[doombot1]

Howdy all!

I've got 2 Fluke 8100A DMMs. They're pretty neat things - love the nixie tube display. One of them has always been a bit wonky, and the other one, at least until a month ago, worked wonderfully. A 50-plus year old DMM that was still accurate to +/- 0.005. Incredible! Anyways... I brought my girlfriend downstairs to show her, and tried to turn it on, and ... nothing. The tubes no longer light up. Busted out the thermal cam, and one of the transistors seemed to be getting kinda hot - swapped it out, and that fixed nothing. (and of course, checked all fuses).

The other DMM does turn on, and all of the tubes work - but the issue is that they all show zero, regardless of the inputs on the front panel. When a voltage is applied, they sometimes briefly (like, almost too fast to capture at 120fps slo-mo) flash to a different number. Not sure if it's to the correct number, though. But - that one's tubes turn on. So - I tried swapping both the LV and HV transformers from both units (separately), and they still work identically to beforehand. Also tried swapping all three of the DIP chips, and same result. So... I ruled out the easy things.

Short of literally desoldering and checking every single component one-by-one, does anyone have any ideas as to what could be wrong with either meter?? Manuals for these can be found on Ko4bb. Before the good one died, I had both meters turned on and on my desk for about 3 hours, and was using a scope & DMM to probe points in both DMMs, in an attempt to fix the broken one (which is now the more-working of the two; the one with the tubes that turn on but show zero). So maybe the (previously) good one died later because I'd just been using it too long or something? It didn't die while it was on... again I used it for 2-3 hours, turned it off and put it on a shelf, and an hour or two later, pulled it back out to turn it on and got nothing. The line switch does work - I do get voltages in the circuit, but they are all over the place - and I can't leave it on for too long to test because the transistor gets hot (Q5).

Any ideas? Thanks for the time of day - any help would be greatly appreciated. And of course, let me know if there is any other information that would be of use.

[Kleinstein]

There is a manual around, at least for the 8100B, that is very similar. This include schemantics and quite some troubleschooting information.  One could check the supplies and a few more voltages. Especilly the unit that shows zeros one can check the ADC input and intermediate signals at the ADC to see if the problem is before the ADC, at the ADC or in the display part.