HP97 The journey begins

[hpux735]

I just finished catching up on this whole thread. What a journey!!

Thanks for all you've been doing Tony. I've inherited an HP-97 from my grandfather-in-law. It requires a fair number of repairs. Your Classic Notes document has proved invaluable.

I've been able to repair the printer, but the LED display and card reader repairs have remained elusive.

I'm pretty certain that the card reader is broken for want of a new Sense Amplifier IC (I've been unable to find replacements anywhere). I've been thinking hard about making a modern replacement of the card reader (WiFi or Bluetooth, similar to your module. The problem, if you're curious, is that the motor doesn't always engage when the !MOT signal is sent by the CPU board. It is sometimes, and the card is pulled through, but often it does nothing. I've attached a trace from my Saleae that is such a case. (MS is the signal from the CPU board to start the motor, and you can see that the motor drive voltage only gets to 1.6 volts. After 1.5 seconds, the CPU throws an error)

Given that you're apparently still working on a new CPU board for the 97, I'm considering just not repairing the card reader and using the new CPU board with the Bluetooth module.

The other issue I have is the LED. It has segment 'f' lit on all the digits, and I believe that implicates the anode driver IC. It seems like this IC is also unobtanium, so I'll probably have to replace it with discrete transistors.

Either way, I've learned that if someone is working on a project such as yours, and you think you might need it some day, buy it immediately. So, if it helps in any way, once your project is ready I'm a certain buyer.

Thanks!
- Will