HP 970A Probe DMM

[markhen]

Recently acquired an HP DMM I've been wanting for my collection - HP 970A from the 1970s. A pretty interesting unit, and rare?

Received with the expected rechargeable battery destruction, but the good news is the PCBA cleaned up very well with a little vinegar. Enclosure (and battery unit) very dirty but can be cleaned up. Interestingly the NiCd battery pack is made up of 8 x 1.2V cells, unlike, say, HP calculators from the same era which operated on 2.4V NiCds.

Powered up the module and immediately it shows a display of "-----" which is the overflow indication. Changing the function (DCV, ACV, kohms) doesn't change this display. In the (included) user manual there is a procedure to check the function selector switch, and it checks out ok. I don't see any other obvious damage or breakage.

Does anyone have experience working on this probe? Google brings up the user manual too, but the manual doesn't have schematics or more detailed troubleshooting info. Virtually all the functionality is in a single hybrid package, so expectations of fixing this thing are low unless it's in the external circuitry?

[factory]

Only other info I can find is in the HP Journal. https://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1973-11.pdf
It does have some simplified diagrams and the block diagram.

David

[markhen]

David - thank you! This will help me debug a bit more!

Cheers -

Mark

[artag]

I have a working one (at least, I hope it still works) if you want to see some waveforms or DC conditions for comparison.

[markhen]

David - thanks again for the article - the circuit diagrams and description do help - but I'm finding that the circuit diagrams are "illustrative" but not "representative"!! For example, the way the input/AC coupling/switch is connected on the PCB is different than shown on the circuit diagram (same function, different placement and components)! Starting to re-draw from the PCB... With all the really important bits in the sealed hybrid module there is only so much to reverse engineer, though.

artag - thanks for offer, I may ask for some DC measurements?! Right now I can't confirm the "1V ohms ref" is correct - looks like mine might be off (like 0.5V). Since this voltage is apparently derived from a single voltage divider chain, if all the Vrefs are off, it could potentially explain my "-----" overflow displays? Apart from the lack of schematic, this beast has 3 ground refs (batt, analog, digital) complicating measurements (and figuring which ground is where on the PCB)!!!

The PCB and user manual all caution against static damage, and although my experience has been good relative to old electronics and static damage, it's a worry that the hybrid module might be "blown"... The fact the digital portion appears to be working to some extent (scans and displays all digits (with dashes)) is tantalizing, though.