Interesting HP-65 card reader failure mode

[John Garza (3665)]

Just a final update to close this out.

The problem was a faulty sens/amp chip. It exhibited triangular waves instead of square waves. I do know triangles can be converted to squares via a simple RC differentiator circuit. However, the timing of the pulses would have to be correct for that to work, which may or may not be the case depending on the extent of the damage to the chip. So instead of investigating that possibility, I just swapped in a complete sense/amp board from a 67 parts machine. Slightly different chip (different serial number), but the 65 takes it just fine.

One tip to make it easy on anyone else going this route - the 65 head wires are soldered to the board. Late 67's connect via little plugs. Instead of trying to tin the tips of the head wires perfectly straight, just solder a small section of a clipped component lead (diode, resistor, capacitor, etc) at a right angle to the head wire. You now have a little L shaped straight pin to insert into the plug on the sense/amp board with tweezers.


And thanks everyone for your help!
It's one of the things that makes this Forum great.

-John