HP-65 motor always on - card reader

[teenix]

The MTRS switch (I think) does 2 things. It lets the CRC chip know if the motor is running or not and it also tells the sense chip to turn the motor on or off. OFF being a HI signal, ON being a LO signal on the MTRS line.

Both of the MTRS pins on the CRC and Sense chip are inputs, however it seems in your case there is some interaction between them causing the MTRS line to (perhaps) pull LO internal to the sense chip, even though you can read HI or LO externally. Maybe the isolation is affecting other pins like RA RB WA and WB which is where the fault may be coming from. It is easy to unsolder and resolder the connecting pins between the two boards.

I don't know which chip has the pullup function on the MTRS line. From isolating the sense board, and the resulting LO level at the sense and Hybrid, it seems it is the sense chip.

As far as I know there is no latching logic on the MTRS line, it is just a switched HI or LO signal.

cheers

Tony