The meter is one of the early units from around 1989 and upon arrival of course it did not work. It gave the typical RAM TEST 1 HIGH error message meaning that at least one of the NVRAMs had dead battery. Further, it also complained about 202 - HW FAILURE: SLAVE TEST (DC BOARD) meaning that the DC board had something in it that it did not like.
NVRAMs (from 1988) were easy enough to take care of, I used some old but still good ones from an another unit that had it's NVRAMs replaced with new ones, for now. CALRAM was ok, it had datecode from 2009 so obviously it has been replaced before. FW revision was 3, I put in version 8 in new EPROMs. The meter was also completely stripped down and cleaned of all the dust that had accumulated over the last 27 years.
Then to the DC-board error, root cause was a U1 (TI CD4094B) that was outputting only ones to the next in chain. Great, that ensured that all ohms ranges were selected at the same time and no surprise, ohms current source was faulty with CA3082 that had pin 15 (common collector) open and Q307 blown up.
Took some work to find out those faults but easy enough to fix, replacing those three made the self test pass.
At the same time I also removed all electrolytics as they had datecodes from 1988. That turned out to be unnecessary as all measured ok with only minor difference to new ones. Seems that the old electrolytics were made very well and they had also epoxy sealed bottoms so no leakage. Nevertheless, I put in new ones as the olds were off the board. The two Y-capacitors on outguard power supply board got replacements too as these had their casings cracking already. Line filter and fan got replaced too.
On the AC-board fuses F701 and F702 were 1/16A types. CLIP states these to be 1/8A types so I replaced them. I think this was a factory screw up as they definitely were not replaced before and even the spare fuses F001-F003 were also 1/16A types. F702 was also blown and it had a normal 20mm fuse bodged on it.
Has anyone encountered the same issue with those fuses?
The faulty DC board had obviously been replaced before as it was from 2007. The peculiarity in this instrument is that CALRAM has about 7.18V for the zener but I remember measuring it to be about 7.15V, albeit with a Fluke 87 while debugging the board.
The difference also shows in measurements, they are off about the same amount relatively. The meter is now hooked to a Fluke 732A and seems stable so far except for the gain error.
Now, what could have possibly caused the reference to change operating point?
In this case, I think nothing. The DC board has probably been changed from a newer unit where it failed. Then whoever changed the working DC board from this unit to whatever they had sent the fixed unit for cal and this unit made it's way to surplus store.
Well, thanks for the newer DC-board with VHP101 40k reference resistor. It looks like this meter needs only proper adjustment and calibration now.
Why is this message in metrology section? Plan is to see if this was really fixed and stays that way. That "SELF TEST PASSED" does not mean much - stability is of more concern.