HP82104A Card Reader Repair

I've acquired an HP82104A card reader but I'm having some difficulty reading magnetic cards. I've replaced the gummy wheel. Taken it apart and cleaned everything, including the read/write head. Confirmed that spring contacts and cams seem to be working correctly. The motor and clutch seem to be working fine. I've looked at MoHPC and the web looking for all references but need some help. I'd like to replace the power supply cap, but I'm not sure which one it is. Also, I'm stumped on how to remove the bottom pcb that extends into the main connector that plugs into the HP41C. There appears to be a screw that's inaccessible that attaches the pcb at the rear of the HP82104A housing. The connector plastic housing appears to have two halves that can separate, but I don't want to risk breaking the plastic to try to pry it apart. I've included a pic showing the pcb caps as a reference. Can someone provide some guidance to which one is the power supply cap? Also I'd like to know how to remove that bottom pcb from the card reader housing. Any other repair suggestions would be greatly appreciated.

Edited: 14 Apr 2012, 8:42 a.m.



About the connector cap: it slides foward, but it has a molded interlock, no hidden screws of any kind. The interlock is kind of a small upside down plastic ramp in the middle of the upper cap. As you insert it, the slanted part of the ramp pulls it up slightly, but when it is in place, it 'clicks' in the vertical side of the ramp, locking it. Pulling it forward usually releases it, but it demands some controlled strength. (hope I have explained it well. I'd have done it better in Portuguese, though...). About the power cap: I myself would like to know, have never needed to replace any so far... 8^)


Luiz (Brazil)

Edited: 14 Apr 2012, 1:59 p.m.


Thanks Luiz. Your explanation was perfect. I thought that HP may have used this method to attach the top plastic shell to the bottom of the card reader case. I'll let you know if I'm successful in moving the top shell to expose the pcb.


The cap in question is mounted directly on the motor. A good quality tantalum 3.3 uf 10-15 volts.

The only thing of interest under the removable cover pcb area is the motor speed adjustment pot which is present only on early machines and requires a service rom to adjust.

IMO, the problem is most likely one of these possibilities:

  1. Wrong size o-rings (should be us size 005)
  2. O-rings not glued on (slipping under load)
  3. Wheel eccentric adjustment (should be set for ~200 ma of motor current, 3 volt supply, card stalled)
  4. Leaf spring contact problems (verify with ohm meter with card inserted)
  5. Dirty head

99% of the time, it is a mechanical issue above, not a bad component or speed adjustment. Turning pots and replacing parts as a shot in the dark usually leaves you... still surrounded by darkness.

Edited: 15 Apr 2012, 8:17 a.m.


Thanks Randy for the very helpful hints and useful information. Going back to the o-rings as a possible first source of the issue. Will work through your other suggestions as I eliminate the other issues. The leaf springs have been verified with an ohm meter and the head was cleaned very well with isopropyl alcohol.


  • O-rings not glued on (slipping under load)

  • Hi Randy, this is the first time I read that O-rings must be glued and actually never I did it, fixing such a issue. When I met this kind o f problem I thougt it could be due to the abnormal size of the rings......but now I've to think again about that, I was sure that sliding was unpossible, cause of the shape and the nature of the rings

    EDIT: I've re-read carefully M.Hoskins instruction and I realize that I mess a point where is written to glue them with superglue, sorry for that, apologize and hope to have not to re-open now the devices for fix again the issue....just I cross my fingers..

    Thank-you Randy

    Edited: 15 Apr 2012, 6:04 p.m.


    It far easier to test the motor assembly out of the reader with a good 3 vdc power supply. Insert a card, hold it once under the head and measure current. Release, adjust the eccentric if needed, repeat as required. This is the assembly test procedure that is outlined in the HP 97 service manual. The normal, or starting position of the eccentric should have the dot on the top facing the outlet side of card path. O-rings usually cause higher current than spec'ed by HP so you set it as low as you can get it.

    If the card speed varies, and you can usually see this visually, the motor to lead screw connection is suspect. It is not a "clutch" as is widely purported, the screw is firmly attached to the motor shaft with Loctite. Any slippage will result in read and write errors.

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