Restoring a HP9830B calculator.

[Tony Duell]

This is strange....

Let's look at the manual rewind circuit (all references from my schematics). Pressing the rewind key sets U4a on the tape controller PCB (09865-66562). This then turns on the motor, selects reverse direction and high speed via U6a-c.

U4a can be cleared in several ways. One is by a tape command ending (rising edge of runcmd/). It's worth checking this signal isn't toggling.

Of course U5d clears it too. This together with U16b, etc, means a rewind ends when there is a power-on reset, when the cassette is removed, at the end of a seek-for-mark (stoptape/ asserted) or when speedclamp is asserted. The last comes from the BOT/EOT sensor via circuitry on the tape interface PCB.

The fact that removing and reinserting the cassette makes a difference is curious too. This of course will operate the microswitches. It also resets the EOT circuit on the tape interface PCB (U6b).

So, what I would do. Put a logic probe on the 3 signals going to U16b (tape controller PCB) and the output of U16a. Find out just what is ending the rewind.

There is, AFAIK, no overtorque circuit (i.e. nothing that monitors the motor current). I think you can ignore that. The only possibility in that area is that one of the power supply rails is being pulled down, which would cause all sorts of problems. Worth checking, but I think it's unlikely.

At this stage I would be investigtinh those microswitches (which can cause odd faults) and the BOT/EOT sensor (ditto).