Something that old, certainly by now has some cold solder connections. I
would reflow the whole board, especially if you say alkaline leaked out of
the batteries onto the board and the cleaning had some positive effect. If
there are any electrolytic capacitors on the board, change them too.



"Bill Bowden" wrote in message
...
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