HP-65 motor always on - card reader

[Trevor Gale]

Hello again - this time I am being confused by my HP-65 which I am trying to repair - and from an electronics engineers' view it is a puzzle. Again! All the usual is completed, no gummy wheel, no loose soldered wires, etc.

The problem is that when the calculator is switched on, the reader motor immediately starts and never stops. Everything else in the calculator works (there was a keyboard error but that was due to a small break in one of the key strips, and that's repaired).

First-off, on opening the unit I temporarily replaced the motor with a 56-ohm resistor to (a) provide a load and (b) to avoid over-running th poor motor!).

Then I checked the contacts for the card finger-switches MTRS, WPS and HDS and they function as expected and in order (while moving a card in, first checked with a multimeter to ground, no supply, then with battery in and voltages measured).

Here's the puzzling part - (conditons: calculator in 'run' mode, no card inserted, no key pressed) with the CRC interface board plugged into the main CPU board as normal, upon power-up, the motor supply from the 'sense chip' pin 1 goes to +2.56V (which would drive the motor). Measuring the MTRS signal at pin 5 of the hybrid, pin 9 of the CPU, pin 2 of the CRC interface board, sits at +3.7V = HI, and WPS & HDS are also HI. VSS to the 'sense' chip is at +6V as expected, and the 'ground' pins are at 0V as expected.

Pushing a card in pulls the MTRS, WPS and HDS signals LO as they should. However, doing the same with the interface board unplugged from the CPU board does NOT result in the motor drive voltage being sent out. To be clear, I checked further with VSS and GND wired to the reader interface board, but I still find that MTRS is now at a LO level (0.49V) from the CPU board and is also LO on the interface board.

In the first case, pulling HDS low does not stop the motor drive voltage (I wondered if that transition would be latched by some other logic) yet pulling MTRS low on the interface card does NOT cause the motor drive voltage being set to the +/- 2.5V either!

Does anyone know if there is some kind of interaction with the data lines and the switching of the motor? One thing I have not yet done is to determine if the 'sense' chip is pulling the MTRS signal high or the CPU board is doing it - I have to cut a trace on the interface board to find that out, which I might do in the meantime.

Any ideas?