Excessive HP-42S battery drain

[aj04062]

I've just purchased a semi working 42s on TAS.

It drains batteries at an excessive rate. New batteries are unusable in about 1hr. I don't have a current probe, but I can watch the voltage drop on my multimeter

I've seen two possible root causes, bad capacitor or bad processor.

Does anyone know of another reason for this kind of issue?

[Raymond Del Tondo]

Not sure if the normal Pioneers also suffer from the burnt zener diode like the HP 48 calcs do. If you insert the batts into an HP 48 with wrong polarity, there's a good chance the zener diode will melt, leading to an excessive battery drain.

HTH

Ray

[aj04062]

Ah yes, I shall have to check that too.

Update:
Swapped out two CAPs and Zener from a known previously working 17-BII. No change. You can watch the voltage drop like a countdown.

I guess the CPU must be bad as I cannot find any corrosion that would allow leakage. If anyone has a working board in a trashed case, I'd buy it or you can get this case. It is in very good condition!

Edited: 13 Jan 2013, 10:12 a.m.

[robert rozee]

just a long shot, could it be the 'on' button is "stuck down" due to a leakage path across it? a test would be to carefully lift away the pcb away from the keyboard contacts while the calculator is running.

i've had an hp200lx with a key 'shorted' due to flakes of the carbon contact material detaching.

[aj04062]

Good thought. I'll try the board in another body and see what happens.