HP2648A tape drive capstans

If that's the tape I'm thinking of, it's used in the 9830, and I think also the 982. There was an external tape drive for these machines that I forget the number of (HP9865???) The tapes are mechanically compatible with audio compact cassettes, but the tape is almost certainly different. These drives detect the clear leader by an optical sensor that reflects off the inside of the cassette shell where you'd expect the capstan/pinch roller to be, you need to use a light-coloured cassette. I've got a few Verbatim digital cassettes that work fine in my 9830. Interesting thing : That drive is mechanically very similar to the one in a Racal Thermionic Digideck. Similar enough that some parts will interchange. And the encoding scheme -- a pulse on one track for a '1', a pulse on the other track for a '0' and a pulse on both tracks for a marker (in the case of the HP, it's a byte marker) is the same too. As is the dact that the thing records 9 bits for each byte, the middle bit is some kind of marker to distinuish headers from data (the advantage of making it the middle bit is that you can read it with the tape moving in either direction. I would guess that the mechanism is not wholely an HP design/build... -tony