Help with HP 29C

This otherwise beautiful working calculator suddenly developed a fault displaying 'error' followed by all LEDs randomly lighting some very bright. It then just shuts down with no vital signs. Given a while it again powers-up normally and works for a 20 30 sec until LED flashing starts again. (Batteries are new Li-Ion and well charged). All suggestions welcome. thanks


Until you get some more answers, try substituting the LiIon batteries with some NiCd or NiMH.

The voltage of the LiIon are 1.60 each for a total of 3.2 volts. The design called for NiCd at 1.22 topping out a 2.44 volts.

Normally this would not be a problem but the increased voltage may be to much for the aged circuits. It does sound like a circuit is heating up and then shutting down.

I had a very old Casio calculator (their first battery model). It would only work for 5 minutes until the IC heated and then it shut down. I could get it to work indefinitely if I sprayed the IC with a can of "Cold" (can't remember the brand name) for testing circuits.

My first home computer was a 486 which froze 20 minutes after start up. Shut it down and allow it to cool and it worked again for 20 minutes. Turns out the CPU had 'crept' out of its socket and the heating was enough to disconnect one of the pins. Had to reseat the CPU and all was well.

Cheers, Geoff


thanks Geof

the batteries are in fact NiMH ( 1.2V) not Li sorry for the red herring




It still sounds like a power supply problem and we all know how finicky the Woodstocks are.

I would try tracing the circiuts back from the battery contacts.

Try spraying with a can of Cold spray, various power supply components, when the display appears correct you may have isolated the problem.

cold spray link

It does not sound like the Logic CPU as it cold starts correctly.

Cheers, Geoff

Edited: 8 Sept 2009, 2:38 p.m.



filling the display with say, 8888888888, it remains ok for about 20 sec pretty much consitently then gives up until powered down briefly after which it resets nicely. However if you reapeat this enough times it will eventually latch-off for a good while after the display goes erratic which makes me suspect the LED driver chip - or even the clock. I had it open a few days ago nothing obvious at all like dry joints or failed components. Does the Woodstock PSU steps up from 2.4V to 5 or so ?


Re 29C problem I found this in an archive seems very close to what my 29 is doing- did any one follow this one up ?

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