HP-41C with 23A batteries - remedy possible?

[Jonas Sandstedt]

Hello, I recently bought a HP 41C calculator, and as stated in the heading I inserted wrong batteries, type 23A instead of type N, such that the circuits were fed 48V instead of 6V. Not good, of course, and now the calculator is dead when fed with proper voltage. Looking at the schematics it seems as if at least the power supply circuit U2 could have been damaged, but could other circuits have been damaged too? I have measured the protective diodes and the transistors Q1+Q2, and they seem OK. I wonder if this has happened to anyone before that has been able to restore the calculator afterwards.
best regards Jonas

[mfleming]

I bought an HP 28C a while back, advertised as not working, that had three of those 12V batteries installed. Thankfully the the calculator survived!

If you have a second 41, try swapping the CPU boards to see if you can isolate the fault to the CPU board and/or to the display. Also check after removing the battery pack to see if there is a residual voltage on the capacitor(s) at the upper left edge of the board. You may (likely!) have exceeded their voltage rating and damaged them. They could be replaced.

HTH,
~Mark

[Jonas Sandstedt]

Thanks for the suggestions, I will start to check the back-up capacitor C1. C1 lies directly between the battery voltage and ground, and is only rated 6V; with a little luck only C1 has gone short-circuit and thereby saved the rest of the circuitry.
best regards Jonas

[Jonas Sandstedt]

Unfortunately C1 seems fine, so I am open for more suggestions. I do not have any more CPU boards, so I cannot swap boards and test.
best regards
Jonas

[acapde]

The power circuitry is composed of diodes CR1, CR5, CR6, zener CR4, inductance/coil L1, some electrolytic capacitors (470uF, 47uF) and a bipolar 14-pin IC (U2), all these component should be around U2. You should be able to check all the diodes fairly easy with a DMM and the electrolytic capacitors if you have a meter with such feature. Also the coil can be checked with the ohmmeter functionality of the DMM. if all the passive components are okay then I would say the problem might be U2.

Hope this helps.

[Jonas Sandstedt]

Thanks, all those perpiheral Components seem fine, and unfortunately only U2 remains. When installed, the batteries slowly lose voltage, so there is some kind of Power drainage. When measuring Vbat it is 4.8V, probably voltage drop over CR6. I will try to replace U2 which Ithink is HP specific.
best regards Jonas