Well poop. The HP 8112A 50MHz Pulse Gen needs a difficult to obtain part...
I have already tested the transistor and logic chip described in the manual for this error code 'E12' yesterday, but I needed a TL072 op-amp to swap in place of the op-amp on the PCB to check it isn't faulty (and I needed some for my stock of parts) so i hit Akihabara for parts and swapped it out tonight but nope, no change.
I don't have any AD7522 D/A converter chips, but there are 6 on the PCB, so I swapped the suspect one with another, but again no change.
So the last piece in the puzzle is the A1U220 chip, a custom HP chip with part number 1DD6-0002. I swapped it with another chip of the same type elsewhere on the PCB and the Error code changed as expected. The chip I swapped the suspect one to should have given me Error E11, and it did, so that confirms a bad chip.
These chips run hot and
need a working fan to keep cool, but the fan in my unit was busted so guess what happened.....
Now, I need to find one of these 1DD6-0002 chips (and a new fan too)... They are a bit pricey on ebay.
Anyone got one spare?