41C repair


I have a 41C which died about 10 years ago. The battery contacts were totally damaged when the batteries ran out.
Now I powered it by an 6V DC power supply and found that
it's running without any problems after a power on reset.
But once I turned it off it never starts again. With a multimeter I measured a current of 0.33 mA in the ON state
which drops to 0.3 µA when I turn it off. After pressing
ON again the current is 0.33 mA again, but nothing happens
in the display.
Any ideas what's wrong with my 41C ?


What sort of 6V power supply is it?

What voltage does it deliver when unloaded?

Does it start again if you remove power and reapply it?

Where do you connect the power supply? To the battery terminals?


1.It's a well regulated adjustable power supply. I connect the ground cable first, then turn on the power supply and when the voltage is 6V then I connect the 41C to the +6V by a banana pin.

2. If I remove the power and reapply it then I mostly get the same results, but sometimes the current becomes so high that the power supply limits it. This looks like a short-circuit, but after a POR everything is fine again.

3. I soldered a wire to BAT+ at one of the diodes on the logic board, so I don't need the I/O-Port-Block for testing.
The ground wire is soldered to one end of C2, that's the 470 µF capacitor.


Do you have any other peripherals atached?

The current draw of an HP41 alone when running id in the order of 20mA. The card reader can require peak currents exceeding 1A

I would suggest that if you're seeing currents in excess of 20 (ish) mA then something is seriously wrong.

Also, I'd be wary of attatching power to anywhere else but the battery terminals.

What current does your powersupply trip out at>


I had to wire batt power ( + to a diode, - to ground) to the CPU board, tired of false contacts, broken posts, etc. It is fine IF YOU ARE SURE ABOUT WHAT YOU ARE DOING.

If memory helps, the positive battery side is series connected to a GERMANIUM diode (it is preferrable to lose 0.3 volts of battery voltage with germanium than losing 0.7 with silicon, another proof of HP good thinking at the time) as a reverse polarity protection. So, if you trace the connection from the battery positive to the keyboard printed circuit board, and then to the CPU board, you may connect to the same point I did.

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