Restoring a HP9830B calculator.

[Tony Duell]

When you press the rewind key, what exactly happens? What should happen is the solenoid should engage, the motor start and the tape rewind until the optical BOT sensor detects the leader.

If the motor runs for as long as the key is held down and stops when you release it, then this generally means a sensor problem. Check the BOT sensor signal (there is an error in my schematic, there should be no feedback resistor on the op-amp, rather a resistor from the -ve input to ground (IIRC)) and the microswitches that detect the cassette and write-enable tab.

Or do you mean that the motor runs and keeps running, but the tape stutters along? That's a mechanical problem. Maybe a bad cassette. Or too much friction in the drive system somewhere. It shouldn't be too hard to find. You imply the tape moves normally during the MARK command, this suggests there is no problem with friction on the head.

As for what the MARK command is waiting for, of course it does writes to the tape, so it's waiting (at least it should be) for a 'next byte' request from the controller. Once you've got it rewinding properly, then check the write clock is running, ditto the bit counter, and so on.