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This unit has clearly seen better days as the pushrods are gone and it looks pretty battered. But looks are not the reason I bought this meter, mainly the legendary performance of the 3458A. Before powering up a used unit that's known to be faulty. It's always important to open it up and have a look, for fear of making a bad situation worse.
[Fix]
On inspection, there are some initial problems that are quite obvious, namely burnt resistors on the main A1 DC measurement board. These have been burned to a crisp, probably due to overvoltage. Looking at the schematics, these are two 10K resistors along the DC input path. Looking at the components, they appear to be CRF65 resistors, very similar to the PTF65. Luckily I had some resistors on hand that could function as a suitable replacement with some artistic creativity. Selftest afterwards reveals seemingly no issue with the DCV section.
[post fix]
![fix](3458-WorkLog/imgA.jpg)
![post fix](3458-WorkLog/imgA.jpg)
## AC trouble
Great! Having sorted out the DCV section so easily, I can now move on to the next error the meter displays on power up; Error 202, Hardware failure - AC board. My clue here is that I don't hear any clicking relays and the fault seems to happen almost instantly. This leads me to suspect that the fault is gitial in nature. Troubleshooting reveals that fuse F701 has blown and the meter now has no 14V supply rail on the AC board. Fixing this by inserting one of the spare fuses allowed me to move on, but the error still persisted.
Next I checked the integrity of the signal chain by applying an input and probing around the amplifier; every area seemed to be measuring, odd... trying all the AC measurement modes also showed me that everything was working. Looking back at the symptoms of a digital fault, I turned to checking the Elantek comparators, but these also seemed to be working, oddly enough. That left me to look at the shift register chain, which is entirely digital in nature, and indeed, bingo!