HP 41 card reader repair questions

[Dave Britten]

(06-23-2020 11:36 PM)GreyUser Wrote:  The malfunction message is an usually an indication that one of those switches is not making contact.

The most common cause is mis-alignment of the two small whiskers that protrude from each leaf with respect to the circuit board. If the board slides up or down before being located on the alignment pins, the whiskers don’t align correctly with board contact points and the switches do not function, ohmically. You can usually coax the whiskers into the proper position with a small strip of heavy paper by sweeping the paper strip in the direction of card travel. You must check the switch function with an ohmmeter if you don’t want to go nuts assembling and disassembling to get things working.

Aide from the obvious mechanical mis-alignment issues, oxidation on the leaf switch contact points or the board contact side can cause the same problem. This one is typically caused by outgassing of the failed urethane drive wheel. My usual approach during wheel replacement would be to clean the gold board contact points with a clean pink eraser and use de-oxit D100 on the leaf spring contact points.

Additionally, it can due to the whiskers or the leaf itself having been bent out of true during a previous repair attempt but this isn’t common, the parts are pretty robust, given their size.

You did set the motor current with the eccentric cam, correct? Can you point me to repair procedure that you used?

I think my money is on the two spring switches being fussy. Maybe I'll open it up again tomorrow and try cleaning them with some Deoxit, and also making sure they're straight. They seemed to be responding okay according to the continuity test on my meter, but there could be some bounce or noise happening as the card passes through. Sometimes I get "MALFUNCTION" almost immediately, and sometimes it'll take about 50-75% of the card before reporting "CHECKSUM ERR". The motor and card movement are super smooth, and I cleaned the heads with 91% isopropyl alcohol.

I haven't checked the motor current, but I did turn the eccentric cam to the position that seemed to be the loosest (slot perpendicular to the card path, raised dot pointing away from motor). Getting an ammeter wired in there will be a pain, so I'll wait until I've ruled out the spring switches.

I used a combination of the information and photos in these two articles, as well as any other useful info I could find on discussion boards (namely this one).

https://www.keesvandersanden.nl/calculat...Geiser.php

https://www.hpmuseum.org/cgi-sys/cgiwrap...gi?read=57