Inside the motor of the HP-41 Card Reader

[damaltor]

First of all, i am sorry to tell you that i do not have any pictures, as i did not have a camera or phone handy. Sorry!

I disassembled an HP-41 Card Reader a few days ago, which i got from TAS. Before testing it, i wanted to be sure that the gummy wheel is okay. Of course it was not, but it was still in place and not everywhere in the housing, so i made the standard approach and replaced it with two rubber o-rings. The fix was quick, and i started reassembling it while chatting with my collegues. That was the start of an odyssey.

While chatting, i wondered why the screw holding the motor in place needed so much force to screw into the motor. Looking down, i suddenly realized: I was just screwing the loooong screw, which should go into the small metal flap holding the two halves of the reader housing together, directly into the motor. The screw is about 10mm long, and the screw which should have gone in there, merely two. I was shocked.

Of course i unscrewed it immediately, but to no avail: I put the card reader back together, and plugged it into a 41. After putting in a card, the calculator immediately switched off. After disassembling the reader again, i tested the motor alone after desoldering it. It took about 850 mA, but did not move at all. At this point i started to swear. And to sweat.

After my collegue was finished laughing at me, we thought about the next steps. The motor was obviously screwed (haha). I looked at faulhabers website (the company who produced the motor), which actually has a motor that might fit for a replacement - but i did not yet want to give up. I did the following:

I tried to open the motor itself. After Trying to bend up the small dents in the steel housing (bad idea) with a sharp knive, i cut myself into the finger. Great. After giving this up after a while, my collegue had another idea:

I put the motor onto a big mill, and milled away all four dents. I bent the contacts straight, removed the small plastic cap and then the bigger plastic end cap, which contains a miniature commutator made of six hair-thin wires. I wanted to take out the axis, but that was impossible because there is a small press-fit ring on the axis, which holds it in the front cap.

I slowly pressed the axis out of the ring in a miniature vise, and tapped it with a M0.5 screw to get it off. Now i had the axis with a beautiful, flat system of five overlapping wire wound copper inductivities. The ultra thin wire looked fine, not broken, and i measured that all of them were okay.

I tried to spin it inside the housing, but it felt like there was something in the way.

Then i put the rest of the motor under a drill press (not spinning) to press out the magnet with the front plastic cap.

Now i saw the culprit: The long screw went through the motor without damaging anything. The only thing it did was breaking the top cap, and making a tiny little dent, maybe 0.3 or 0.4mm high, into the way of the outer parts of the axis. Enough to keep it from spinning, as it is about 0.5mm thick and has a place about 0.8mm wide.

I cut away the dent with a knife, and pressed the cap with the magnet back on with the vise. Of course, i forgot to mark the direction of the cap, and i had no idea how the magnet was oriented inside the housing (In case you did not know that: The orientation of the magnet in relation to the housing and the commutator defines the strength and the rotating direction of the motor). I put in the axis and the back cap with the commutator. While one of my collegues held the motor and a small power supply to make it spin, i noticed that i bent the thin commutator wires. Nothing was spinning yet.

I put the cummutator under a microscope and slowly bent the wires back in place with a needle. Second try.

Now the motor started spinning, slowly, unwillingly, stopping every now and then. With my third hand, i started rotating the top cap with the magnet. And behold: The motor started to spin faster and faster!

Now i was starting to gain some hope again. after putting the motor back together, press-fitting everything in its place (the hardest part probably was the tiny press-fit ring on the axis which i pressed on with a few M1 nuts), the motor seemed to work again!

I put the card reader back together, concentrating on putting the right screws in the right places. I put it into the 41... put a card in... The motor started to spin! Still, the card would not move any millimeter, and the 41 only said MALFUNCTION.

After trying a few times i noticed the problem: The motor was spinning, but in the wrong direction (remember that magnet thing?). It tried to push the card backwards.

I disassembled the card reader a third time, and changed the + and - cables from the motor. After reassembling, i tried it again.... The Card Reader does now work perfectly again, with just about no mis-read cards.

The whole process took about six hours, because i always had to find an idea for the next step... but i guess it was worth it

[factor]

Good work! Thanks for sharing.

[jebem]

(05-06-2016 09:30 PM)damaltor Wrote:  First of all, i am sorry to tell you that i do not have any pictures, as i did not have a camera or phone handy. Sorry!
(...)
I put the cummutator under a microscope and slowly bent the wires back in place with a needle. Second try.
(...)
The whole process took about six hours, because i always had to find an idea for the next step... but i guess it was worth it

Excellent repair description with humor and a happy end!
Thanks for sharing this extra information.
Pity about the missing pictures, though.

[BobVA]

This truly merits the term "epic"!

[damaltor]

Well, if i ever get another broken motor, i will happily make a few pictures Thank you!