Possible repair alternative to a dead HP41C

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Possible repair alternative to a dead HP41C

Postby FrostyJW » Thu Mar 01, 2012 4:45 pm

I have been a very happy owner of a HP41C (UK version) together with Card Reader and 3 memory modules since it was purchased in 1980. It was well used and I had several self written programs on cards and in memory, mainly financial routines. Then in about 1992 I found it was not working properly - the display was erratic, characters having segments missing, would not always switch on, could not toggle Alpha or Program mode. Still the same problem with new batteries. In great sadness I put it away (in original packing and with all manuals etc) thinking it would be impractical to repair or too costly. I was - at the time - also a member of the HP41C European users club.

Then 1 month ago, I pulled out the box and my interest was renewed - and happy memories! I found this excellent website and forums and just wondered if I could get it working again (having some little electronic engineering experience) I read part of the service manual in here, and saw that the first section of the circuit is a voltage regulator, to compensate for low batteries, over-voltage and incorrect polarity protection. I know from past experience that voltage regulators can be tricky and fussy, and wondered if it just needed a "kick in the pants!" I have a variable voltage supply unit, capable of 1-20V and anything up to 12amps. I didn't buy any new batteries at this stage as I would not have any other use for them.

Removing the battery cover, I cleaned the four contacts and those within the battery holder. I noticed my version has a small push out plastic cover on the left hand side of the battery compartment, with two spring loaded pins on the base. I assumed (rightly it turned out) that this was for a 6V power supply from an optional accessory at the time. Using these two pins rather than the battery connectors themselves, I connected my external power supply at 4.5 volts. (Note - inside the battery compartment, the actually battery connections are positive (+) on the far right hand side copper connector, and negative (-) on the far left. The external power supply pins are positive + at the BOTTOM lower pin and negative at the top upper pin) Connecting up at 4.5 v yielded nothing at all when switching on. Increasing to 6v also had no effect, still dead. I then increased the supply voltage to 8V and suddenly the HP41C sprang into life, displaying MEMORY LOST - not surprisingly of course! I then reduced the voltage back down to 6V and it continued to work, and then down to 4.5v and still worked. I then bought a fresh set of batteries and it now works like new! (the batteries are not that easy to find in stores in UK, but easy to get via online battery stores here) I am now a very happy owner of newly working HP41C, and have reprogrammed it with the programs I had written. Luckily I had written them all down, though some input characters had me puzzled for a start - and had to read the programming manual again! My next task is to open the card reader, as the rubber wheel is obviously "sticky" and partly jammed. Have read the guide notes in here for that... so wish me luck!

NOTE This method of my "repair" is in NO WAY saying it will work for any other faulty HP41C, which could be due to different problems and for different reasons, but I offer it as a guidance note for anyone else who may wish to experiment with a 'dead' 41C. :)

John W Birmingham England
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Re: Possible repair alternative to a dead HP41C

Postby Garth » Thu Mar 01, 2012 7:32 pm

Did you see what the current was before it came to life? I can't really think of anything that would make it respond like this (although I'm glad it's working!) The only times I've seen voltage regulators be "tricky and fussy" (your description) is the LDO type when they did not have the right input and output capacitors to keep them stable. Otherwise, voltage regulators in the 10,000+ products we've sold have been totally trouble-free, except one type of switching regulator which on very rare occasion gets damaged by static.
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Re: Possible repair alternative to a dead HP41C

Postby FrostyJW » Thu Mar 01, 2012 11:11 pm

Hi Garth, no I didnt see what the current consumption was before it burst into life, as I was only monitoring the voltage. It had not been used for nearly 12 years, and I remember that with new batteries it just didnt want to work on the previous attempts then (and I had cleaned the battery contacts) I had previously tried with the DC power supply some days before, and no luck, but had not risked taking the voltage above 6V. But when I read that there was a voltage regulator, I thought it would probably be safe to take the voltage higher, with nothing to loose! I was surprised to say the least, especially when then reducing the voltage back down to 6 and then to 4.5 that it kept working fine! Of course I have no idea as to WHY - but it works now, so offering my experience in case it helps anyone else!! All the best John
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