Restoring a HP9830B calculator.

[Tony Duell]

Yes, that BOT/EOT sensor can be touchy at the best of times. Rik Bos has managed to substitute something for a defective LDR, and IIRC that wasn't totally trivial. I have, somewhere, a part number for the original LDR (I have a Racal Thermionic Digideck which uses a closely-related drive mechanism, along with the compent-level service manual). Alas attempts to track down data on said LDR, or even to find the company that made it, have been unsuccessful.

Anyway, it's best to check the microswitches in place. It's not uncommon for them to get misaligned, so that they do not operate correctly against the cassette.

It occurs to me that every tape command apart from MARK starts by doing a read operation (if only to find a file header). MARK can be used on a totally blank tape and thus starts by writing. I think I would start by finding out if it ever tries to do a read operation on, say a TLIST. Does it attempt to move the tape, even for a millisecond (look at the motor control signals coming off the Tape Controller PCB). If not, it's detecting an error much earlier. Perhaps the status, on the SI(0..3) lines is bogus. A failure on the Tape Interface PCB, perhaps.

I have always managed without extender boards. As I mentioned in Datafile, I solder some wires to the appropriate points on the PCB, put the PCB back in, then probe the free ends of the wires.