HP-67 Reads cards but does not write

[Trevor Gale]

Thank you again Tony, and yes, I also thought it was time to clear the bench and get this HP-67 connected to the oscilloscope. I made a number of other measurements as well.

Firstly, though - I see a difference in the circuit on my card-reader interface board between that in the MoHPC notes and what I have on both my and my friends' HP-67 - I have attached the circuit diagram of mine to this post (if I have done things correctly).
   
I did several tests with the calculator ready to read a (properly written with a short program) card each time whilst 'scoping the Hx head inputs and the corresponding Rx data outputs; the result was always the same - channel A supplied no RA data while channel B supplied RB data as expected and as shown in your excellent notes document.

'Scoping the head input of *both* channels gave me similar waveforms (+/- 200mV) to your example shown, this tells me that the heads appear to be functioning correctly.

However, what I assume to be the "bias levels" for the amplifier-comparators in the chip seem to be different: for the (working) channel B I find a bias on chip pin 9 of +1.41V which would be in agreement with the waveform crossovers you illustrate. However on channel A I find on chip pin 5 a level of +1.32V which is too low, and generates a constant "high" logic output on RA during a card read attempt.

An obvious test for the channel amplifier was to "persuade" the bias level for channel A to be higher, this was done using a 560K-ohm resistor to the nominal 6V line from pin 19 of the chip to pin 5 - which brought that level to near +1.43V, sufficient to bring it into the threshold region. Having done that, trying to read the card did generate an RA logic signal similar to channel B but with incorrect data which still returns an error (I could not expect to set this "bias" to the exact level required, and in any case doing this kind of modification will change the hysteresis points away from what they are intended to be).

So it is still an enigma, more so by the day...