HP-41 card reader repair


I have repaired quite a few card readers with gummy wheels in the last years. Yesterday I encountered the same problem for the second time. The card reader work after the repair but sometimes the HP-41 is corrupted showing weird symbols or even MEMORY LOST. Sometimes it doesn't let me do a MEMORY LOST manually. But most of the time the machine runs just ok. The HP-41 on its own works perfect. I suspected a bad connection in the module bay. Cleaning didn't bring any success. As this is the second time I tought to ask if anyone experienced the same. It looks like an electronic issue although I thought the card reader only had the usual gummy wheel problem.
Thanks for any help on this


Can you trace the signals with a logic analyzer /oscilloscope? A bad capacitor could cause some noise on these lines.


Hi Klaus,

Many thanks for the hint. I'll check the signals with the scope. I also have the schematics. I have tested the unit further in the last 12h and it didn't fail again. Actually the failures slowly got less and less. It seems that the capacitors haven't been charged for a long time and it looks they have comme back to life with usage. I'll still check the signals and maybe I can preventively change some capacitors.

Regards, Daniel

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