HP-67 Reads cards but does not write

[teenix]

I measured the sense pins 5 and 9 and got 1.45V on each.

If you have a storage CRO then you may be able to capture an image of the data being read from the faulty channel and compare that to the same data from a known good reader. You would only need maybe a 50mS sample so it's clearly visible for comparison. HP mentions +/- 5% motor variation and +/- 5% CPU clock variations is allowed so slightly altered bit width should be ok within reason.

As I understand it, the sense input is a 200 gain amp, some filtering, a comparator and latch to provide the logic interface. The resistor may not be doing the intended function as it is probably altering the input to an op amp rather than change the comparator trip point.

It is possible that the analog circuitry for that channel inside the sense chip is failing and cannot be fixed. As mentioned, HP said some sense chips that were manufactured were operating with marginal specifications, so if anything is failing inside, then it may just cease to function. In this case the only repair was replacement.

Have you tried replacing the motor capacitor?

cheers

Tony