HP-82104A Card Reader Repair Questions

[matty_K]

I am trying to repair an HP-82104A card reader. I’ve successfully replaced the rubber on the feed roller, and the reader now pulls cards through “properly”, although it still isn’t actually working.

Write functions appear to execute, but any read operations (including “VER”) fail with a “CARD ERR” error. I’ve tried adjusting the feed roller axle to increase/decrease pressure on the card without any success. The feed switch is triggering (obviously) and the head switch seems to be triggering (based on continuity). Issuing a CAT 2 command gives me the function list, so the ROM is intact, and the CRC seems to be accepting commands and triggering the other circuits. The write-protect system is working properly (I get a “PROTECTED” error if I try to write to a clipped card).

I’ve tried using a ‘scope to see what’s going on the RA/RB and WA/WB lines, and I do see various pulses. I’ll admit, however, that I don’t really know what I’m doing here—I don’t know how I should set my time base or what I should be seeing during reads or writes to confirm that the sense amp is actually reading signals from the magnetic head.

Does anyone have any helpful suggestions for additional testing? I’ve read through all of the related threads I could find, but I’m still coming up short. Also, any suggestions on how to remove the IO connector cover? I’ve read there is a wedge-type lock mid-way in the housing, and the service manual says to insert a spatula to release the lock so the cover can be slid off, but I don’t see how to do this. Since the calculator connectors are sitting right there, I’m very wary of poking tools inside and bending contacts.

Thanks for any help anyone may be able to provide!