Recently aquired a HP-25 programmable calculator (1975) that had dead batteries and wouldn't power up correctly. Sometimes the display indicated 12 zeros and other times the display was blank and a few keys would indicate a single "0" in various places.
I took the thing apart. cleaned the board and keyboard with alcohol and toothbrush and contact cleaner and managed to get it working for a couple hours using a couple alkaline AA batteries.
Put it all back together and everything seemed to work right but the next day it went back to the original erratic operation. I read an article indicating possible noise problems on the data line that was fixed with a 20k resistor termination on some particular line. I tried that idea on a few pins without sucess and cannot get the thing to power up with further cleaning. Haven't looked at the clock signals on a scope yet. Hard to find a schematic for old HP calculators.
Any ideas?
-Bill