An HP 34401A DMM, date code ~1991. Passes all self-tests and seems perfectly fine and accurate on all functions.
Today, I hooked up 10VDC to it and let it sit on VDC for a while. I came back only to find that the 34401A had switched itself to VAC. I thought maybe I inadvertently hit the VAC button, so I put it back to VDC and came back in one hour. Sure enough, the thing had switched to VAC again.
Any clue to what's going on? I guess I should at least scope out the power rails and see if there are spikes.
Hello,
I bought my 34401A in 1990 or 1991, REV 03-01-01, still working fine, so I also got a lot of experience on this DMM.
I find some of the hints given here pretty misleading.
Neither does the 34401A switch its mode, if different signals are applied, especially not from DCV to ACV, nor does it have ACV as boot-up mode. One cannot even store any other boot-up mode than DCV, 5 1/2 digits, R = 10MOhm, that's it. Therefore, a reset would also not bring it into ACV.
The electrolytic caps are for sure not the best any more, but ripple on the power lines probably will not lead to such a specific mode change, as if it was a key press, but to randomly switching to any mode.
There's probably a problem with the display / keyboard PCB.
Most obviously, there might be contamination on the keyboard, especially on the golden fingers of the ACV switch.
The keys of such rubber membranes usually consist of conductive plastics, maybe with imprinted carbon, which may have disintegrated a bit after 30 years, and particles got stuck on the PCB.
Other similar failure mode could apply from general plastic outgassing, or external contamination, depending in which environment the DMM has been operated in the past.
So I would start by carefully disassembling the front panel and cleaning the PCB and each individual switch of the rubber membrane with ISOPROP.
If that does not cure the problem, the µP on the PCB might have a problem on one of its I/O lines.
Others have reported here, that the µP fails fatally after these years, but this would be a total loss, let's not assume that.
Frank