Tbh actual fixing slipped even for me.
I'd say it was more system repair, than individual parts troubleshooting. That's the excuse I have, not to be able reporting "it was bad opamp U153 which caused problem A, and bad resistor R272 which cause problem B"
As received, box was throwing all the possible errors, reading jibberish. Cleaned mess and replaced few parts in area where electrolyte from capacitors spread.
That fixed few self-test items, but still were bad reading.
Then I had it sitting few months, forgot what was done already.Switch replacement - got conditions when it sometimes work, sometimes not.
Then sitting few months, again, forgot what was done already.Recently issue was found to be one of infamous 2512 resistor on input FE circuitry, which got cracked due to PCB stress and barely making contact.
Replaced all of them with 25K PTF56 25ppm/K ones, no randomness anymore. But new issue - meter started to hang from time to time and reset itself from no reason.
Replaced SRAM chips, replaced firmware ROMs to 39SF020 Flash with A20 FW. Hangs/reboots seem to be gone.
Again sitting few months..Vref voltage sometimes drop to some low voltage, causing selftest errors from 200.x and so on. So I started to check circuitry around VREF. That's when "Transformer died". Actually it was OK, just mains fuse got blown quietly without any magic smoke, as I messed up on power polarity while replacing LT1124 opamp near reference
. Don't know how that happen. New LT1124 installed, all back to normal (sorta).
Again sitting a month..After assembly meter pass self-test once it's cold, but after warmup it fails undocumented 306.2/306.4 tests (which were added since A19 or around that fw?).
Since it seem not to affect actual measurements, and I ran out of time, I let it be and calibrated box as is.
Violated my own rule of documenting everything, and here's the result in it's glory. So people, don't skimp on documentation, it's often more important than actual fixing.
Now it's almost arrive to new user, which has no equipment to debug or test it, so I'd expect no further work to be done.