Intermittent HP 27

[jupiters_spot]

I thought I simply had poor battery contacts -- I do -- but it appears there's something else going on with my old HP 27. I unpacked my old 27 a month ago and rebuilt the battery packs last week, using some NiMH rechargeable batteries. With appropriate pressure on the battery pack it would turn on and calculate happily. But from time to time when I turned it on, nothing would happen, or I'd get a display of zeros, 'Error', or other nonsensical values, but pressing CLx would clear things and I could calculate again. But as the days passed the number of "turn on and work" instances declined, as did the number of "turn on and see nonsensical values", but the number of times I flip the switch and NOTHING happens is growing. When I do get it to turn on, it'll turn on happily multiple times in a row...and then go back to being a problem.

I'm pretty sure the battery pack is fine -- it happily runs my two HP 21 Woodstocks -- and with the HP 27 open I can validate with my DMM that I'm getting 2.58v to the PCB. I have not been using the HP battery charger.

Has anyone seen this sort of behavior before and have any advice for me? I can solder and do general fixing just fine, but I don't have a circuit diagram for the HP 27.

David