HP3497A data acquisition unit repair notes

[trobbins]

Some notes from getting a HP3497A back on to a logging project.

The various power supply circuits are prone to failing, and identify themselves as not passing the self-test or poor DVM operation.  Probing for rail levels is easy as the front panel swings open for access to the power supply pcb and probe points.  I'd recommend replacing the e-caps associated with the two doubler rectifier circuits related to the +30VL and -18VL rails.  I also had a LM317T fail, and a quirky failure of what could have been like 2 diodes in a bridge, but fault-finding is fairly easy.

I had three 010 option 20x mux relay cards to check.  I put shorts on all channel inputs of a terminal card and swapped that card between relay cards and slots to confirm ok operation of cards and backplane.  I noticed a few channels were not showing all zeroes as I scanned through the channels with a python script and 82357B GPIB-USB interface, but there was some inconsistency between channels that showed non-zero readings, and slot locations, and the three relay cards showed different numbers of suspect channels.  I measured the contact resistances of one relay that consistently showed non-zero readings and one of the 3 contacts was noticeably higher in resistance at about 0.6 ohm.  The relays are 5V COTO or EAC mercury wetted reed relays type 0490 1231.  I was able to bring them all back to ok operation by giving each coil a few 5V kicks (which can be done without removing the pcb from the backing plate).  I think the contacts must have needed some robust wiping, as the coil voltage is likely a bit lower in normal operation due to a 74LS145 controlling one end of the coil and a transistor at the other end.