Quick repair on this one. Obtained from a hamfest. Unloved and uninterested. 4.5 digit manual range bench multimeter from HP. This one circa 1979. Felt sorry for it so here it is.
Of course when I got it, it was broken. Drifting on all ranges and a severe offset towards one polarity on every measurement. Checked the power supply and reference voltages and all good. So I spent some time staring at the wrong version of the service manual. Of course there are a couple of revisions of this DMM and the components are in different places. Eventually found the correct one, which was the really badly scanned one from Keysight. Ugh.
First diagnostics concentrated on the auto zeroing circuit. This was eliminated after grounding the input FET and getting sensible zeroing. The front end was buzzed out and impedances checked. All good. Hmm. Eventually I was poking around the Hi-Z parts of the input chain and one of the input clamping diodes fell to bits!
As you can see the leg has disintegrated. When this is out of circuit it behaves properly! The diode does look like a diode out of circuit but clearly has some leakage problems going on.
This is an HP 1901-0586 which is completely undocumented and expensive to get hold of. The service manual specifies it as a general purpose diode 30V 25mA, so pretty much anything will do here apparently. Well that's not good enough for this as it's very high impedance IMHO and it was clearly affected by leakage of some sort. I'm not paying for an original diode so after doing some research and simulations of the clamp in LTspice, the B-E diode in a 2N3904 is sufficient for the job so I will snip the collector off one and solder it in, then carefully clean it and re-test.
That I will do tomorrow.
Calibration is slightly off which is probably my fault for turning the wrong pot inside when I was checking out the amp balance.