HP-65 motor always on - card reader

[Trevor Gale]

I think there is a pull-up resistor internal to the chip for MTRS, and that's what might be at fault (or it simply relies upon the input already being pulled up by another resisitor). I have also "mimicked" the head input signals (i.e. two separate 180mV signals over two 47-ohm resistors with a 1.35V bias) connected in place of the real head connections) and found that the comparator outputs RA and RB do not toggle correctly either. So it looks as if the 'sense' chip may be fried.

I am wondering if the 'sense' chip is similar to the one used in the HP-41 card readers - I have a spare unassembled HP-41 reader that is well beyond repair but the small electronic boards are undamaged - and the connections seem to be the same (head connectons, VSS, GND, gb etc.) so if it is I could replace the chip on the HP-65 reader board. (I do not have any "spare" HP-67 boards or chips!).

I very much doubt that there is any fault with the CPU board on the HP-65 since all other operations and expected voltages are correct.

Any ideas? And thanks, yet again!