HP-65 motor always on - card reader

[Trevor Gale]

"Bad news but good news" - the HP-41 cardreader 'sense' chip seems to be able to be used on the HP-65, HP-67 as well as the 41's. Anyway:-

Well, I did finally replace the 'sense' chip, the old one was easy to desolder but it had very clearly been 'fried' - as I gently extracted it the top of the chip came completely loose, revealing all of the internal gold links to the chip die (some loose)!!! I have kept it and I will try to make a macro microphotograph of it!!!

With the new one in, and the board inserted onto the CPU board, I checked voltages and the input bias (1.41V), installed it and ran it with a 56-ohm load instead of the motor to check, and it all worked as expected (with error due to no head input of course).

Before I connected the wiring to the motor, I applied my bench supply at +2.3V (a typical level) to the motor to check that it would pull a card through correctly. However it did not - it ran but very slowly, which was because it was trying to draw 235mA - which I had set as the current-limit on my bench supply... I took out the card and then set the current-limit to 275mA and the motor ran slightly faster but drew all that 275mA. I had a new tantalum capacitor across the motor so that was not the problem - the motor had obviously been damaged by the previous 'sense' chip driving the motor for far too long. Probably what 'fried' the chip in the first place.

I already set the eccentric pin to what felt like the 'softest' drive wheel pressure; I then extracted the motor and checked again on its own, and it still tried to draw excessive current. Thus I am now looking for a new HP-65 reader motor - probably better to try and find a 'broken' HP-65 to take the motor from (in my experience they do not normally seem to fail). So I put this HP-65 aside until I find such a solution - the motor in the HP-65 has differently-spaced mounting screwthread holes to the motors used in the HP-67 and HP-41 series readers..

All is NOT LOST however - I had a 3rd HP-67 with a bad sense chip in it (no data outputs on RA or RB) which I have on my desktop as a "no-card" calculator for occasional use. Since the card reader interface board is the same as the HP-65, I took my 'newly-chipped' interface board and wired that up to this HP-67, installed it, checked voltages etc., wired in the motor, and it seemed to run first-time.

I wrote several simple programs to cards and successfully read all of them back in, then checked that they could all be read by my other HP-67's (they could, no problem) - then read in a 'standard pac' card which also read in okay. Programs written to cards on the other HP-67's were also read in okay in this HP-67 too.

SO I have not solved my HP-65 successfully (yet) but I HAVE gained a fully working spare HP-67 with working card reader!