Tech help repairing an HP-41 card reader


I would like to see if there is anyone here who might have experience repairing the 41's card reader and can give me a suggestion for repairing mine. Naturally it was only intermittent when I started and now it is totally failed.

I had intended only to repair a bad roller and clean the head. It was intermittently reading properly, it wrote cards correctly most of the time, and the roller was just starting to turn gummy. Since this was my first attempt at really working on this type of reader, I ended up disassembling, working on, and reassembling several times. After trying the tubing fix, then the o-ring fix, then back to the tubing fix, the mechanical aspects of the reader seem fine. It can push the cards through just fine. However, when putting a known good card through for reading, it almost immediately displays "CARD ERR". Sometimes it displays "MALFUNCTION". It never reads correctly. And if I attempt to record onto a card and read the card on another good reader the card is corrupt.

I have interchanged the module with the head and motor assembly from a good reader and it works fine. Therefore I assume the problem is in that part of the reader. There isn't much in that assembly. The motor, a noise suppression capacitor, and the head.

What is likely to be the problem? I haven't tried to change the cap yet, but I'm thinking that might be my next step. The motor seems to turn about the right speed. It is smooth and the card pulls through apparently ok.

Any help from experienced repair folks is much appreciated.



My own card reader failed because one of the windings in the magnetc head went oppen circuit. I suggest you disconnect the head and measure the resistance of each winding. They should be 30 - 40 ohms. Also check the connection of the head wires into the small sockets on the pcb. The solder tinned wire ends are just a push fit into the sockets and a measure of each winding resistance with the wires fitted might show if there is a bad connection, but connect your meter to the body of the sockets.


I don;t know exactly how much you swapped out, so some of the suggestions might apply to sections you know to be good.
Did you swap out the sense amplifier PCB (the little one standing vertically at the back), or does it work with the one from the 'dead' reader. Obviously faults here can affect reading and writing.
What about the switch PCB and the contacts on top of the reader mechanism? Problems there -- the swtiches not closing at the right time -- often causes the MALFUNCTION message. You might want to check this -- make sure the contacts
are correctly possitioned, that the card sensing cams are in place, that the contacts are clean, etc.
Make sure you're connecting the head wires to the sense amplifier PCB correctly -- only one of the 120 ways of doing this will work! Check the head windings for continuity too (check between the red and yellow wires (one of the tracks) and then between the blue and orange wires (the other track))
An open-circuit hrad is not common, but it can happen.
I suppose you should also go over all the mechanical parts -- you've had this thing apart, and we can all make mistakes...


When I swapped the modules from the good reader to the bad one, I swapped the entire module that has the motor. That would include the switch assembly, basically everything that comes off when you unscrew the two screws from the bottom that are angled into the module. The only thing I had to do with the module from the good reader was unplug the 5 pins that go into the PC board on the angle, the two wires from the motor, and the five from the head. I was careful to put the wires back correctly, using the documentation here and refering to the notes I took before removing them.

None of the circuit boards were swapped, so I think the ones in the bad reader are ok as they worked fine with the other motor/head module.

However, in working on the bad reader before that I had disassembled completely and reassembled it several times. So if the switches can be done incorrectly and the card still be pulled through, maybe that's what I messed up. I'll go look at that again.

Thanks, Tony.


YEs, I would now look very carefully at the switches. Make sure that (a) the card detect cams are in place and the right way round, (b) the contact strips are clean, correctly hooked into the frame and the cams, (c) the contact strips make contact with the PCB properly and (d) the PCB
contacts are clean. Also check that the 5 (?) pins from the mainboard to the switch PCB are making proper contact.
There are 2 swtiches and a photodetector on that PCB. The photodetector is for write protect, the 2 switches are basically card-inserted (which turns on the power to the reader, motor, etc) and card-at-head (start reading or writing).


My bet is an open circuit to the head. Check the winding resistance. Two pairs of wires should show the same value (usually around 40-50 ohms). The black wire should show zero ohms to the metal case around the head.


Just wanted to thank Tony, Dave, and Gordon for their suggestions. I checked the resistance on the head, both windings, and they were both in the 50-60 ohms range. Seemed that was ok.

Then I worked on the copper switches. They both worked ok, so that left me with the small piece of copper strip that appears to apply some pressure to the back of the card right in line with the head. I assume that it is to make sure the card is in propper contact with the head. Comparing it to a good one, it appeared that I must have bent it a bit in my past attempts to make the thing work. I straightened it a bit, trying to make it shaped as close to the working one as I could. That did the trick!

The reader works perfectly. And while it was opened, I replaced the rubber wheel which was not quite gummy yet, but not really as it should be. Thanks, Randy, for the tubing! Works like a charm!

So, thanks again guys. I really appreciate the help and support of you all. This forum is a great place.

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