Dying Hp-42s needs doctor


Anyone tell me where I could send an Hp42s calculator for repair (I > > live in the UK)... this one crashed spectacularly when I changed the batteries recently... though that might not have much to do with it, it could have been dead already..


Andy W


I've had a couple of scary low-battery events: the beeper beeps, everything's locked...it's horrible. Years ago I phoned HP tech support in the UK and a very helpful person suggested taking the batteries out for a few minutes, turning the machine on so it completely discharged the temporary storage, then replacing the batteries. (Make sure the new ones are good!) It will display "memory clear" but I was thrilled that the machine was as good as new. Once it went like this after flying...cosmic rays?



I would also try shorting off the battery contacts with a coin.


I would also try shorting off the battery contacts with a coin.

I'm sure that this goes without saying, but just in case-- Make sure you remove the batteries before shorting the contacts... ;^)

(Howdy, Iqbal. How's things?)


Actually some of HP's manuals recommend briefly shorting the contacts WITH the batteries installed. Also installing the batteries backwards for a brief time.


If there is a diode to protect the circuitry for reverse poalrity, reverse batteries won't do a thing. If there is no diode, ICs are very likely to be damaged, particularly CMOS RAM.


Also it is nonsense from an electronics engineering point of view to short circuit the batteries (except to help them discharge and overheat, something we are not supposed to look for).

It makes sense to REMOVE the batteries and short circuit the terminals, to discharge capacitors that may be inside, so to fully erase all memory, registers, etc. and force a cold reset.

If we are entering the non-scientific-approaches land, perhaps someone should suggest *which* type and value of coin will offer the best results. For magic purposes, I'd suggest a Krugerrand, or Canada gold coins, but any *gold* coin will do, as long as it is in *mint* condition...

Just my 10 pennies


Since all diodes have some reverse leakage current, I think HP intended this trick to help discharge the internal capacitor fully. I know the '41 has reverse diodes and the HP recovery techniques for the 41 say it is OK to reverse the batteries.

The '42 and Voyager machines mentioned shorting the batteries while installed... strange, but a temporary short of the button cells won't damage them or even effect their life much. I still don't know why HP recommended it though.


I think the reason HP says to leave the batteries in place is to prevent possible damage from electrostatic discharge. Normally the user never gets near any electrical signal inside the calculator except when the battery cover is off. The act of touching the battery terminals with a metal object is an opportunity for an electrostatic discharge to get on the printed circuit board. With the batteries in place (and if you have followed HP's instructions in order, you will have fresh batteries in place) a discharge to either battery terminal will be shunted to the other battery terminal through the low impedance of the batteries, bringing all nodes in the circuit to about the same potential. Without batteries in place a dicharge might be conducted onto either the + or - power supply bus, then might find a path through one of the chips to continue on its way to ground. I think one of the battery terminals is connected to the shield ground of the calculator - in the Voyager series this is the metal back plate and the metal keyboard bezel (connected together by springs). Without batteries, a discharge to the other battery terminal might pass through any of the chips to the ground side of the power supply, then to the shield, into your hand, to ground.

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