HP97 Card Reader Motor Replacement?

[GanjaTron]

Hi everyone,

I have refurbished the card readers in my two HP97s (gummy wheel --> tygon or silicone tubing), but one of them has been seriously acting up recently. It sporadically stalls and won't start up at all, or run very slowly, sounding severely laboured and irregular.

After removing and hooking the motor up to a PSU, I immediately noticed it squeals and vibrates intermittently. If I tap or shake it, the squealing stops briefly, only to resume after seconds. In contrast, the motor in the working card reader is practically silent. I compared the current draw of both motors with a bench supply at 2V and got the following (note the motors are from different manufacturers, both Swiss):

Escap (squeals):
Outside card reader: 20-25 mA (intermittent), 50-60 mA (squealing)
Installed, unloaded: 110-120 mA (240 mA startup!)
Installed, card loaded: 240-250 mA (!)

Maxon (silent):
Outside card reader: 15-20 mA
Installed, unloaded: 20 mA (25 mA startup)
Installed, card loaded: 160 mA

According to the service manual, the motor shouldn't draw more than 50mA unloaded, and 180 mA under load, so the Escap appears to have worn bearings or brushes, on top of which it draws an almost tenfold current just to get going!

Interestingly, if the motor is installed in the card reader and powered by the calc, it will only start up when a small resistance is placed in series with one of the motor leads; I noticed this when I hooked up my multimeter to measure current. The motor will then sporadically come up to full speed for 2-3x successive card reads before slowing down and stalling again for a while. Once stalled, the voltage across the motor is just 0.1V or so (instead of >2V) before the calc gives up and issues an error. That leads me to also suspect a fault with the Sense Amp on the card reader PCB, or can the worn motor bearings alone cause this behaviour? (I haven't tested the Maxon in the faulty card reader yet -- TODO).

So in a nutshell: has anyone had problems with their card reader motor and found a viable replacement? Furthermore, has anyone experienced flaky motor voltages coming from the Sense Amp?

Thanks for any hints,

--Roland