HP97 The journey begins

[teenix]

Does the card motor actually work? Maybe it is loading the sense chip too much if it has a problem. Maybe the capacitor attached to the motor connectors has failed.

Unlikely if it hasn't been moved much, but make sure the ribbon cable traces are not cracked. Maybe wiggle the ribbon in case some high resistance has built up in the connector.

At least with the CPU board, there is storage for 800+ cards so if the card reader is unserviceable, it won't matter.

The [F] segment lit could be a fault in the display, or some corrosion on the display board or an internal short. It is tricky to get the display out but possible. Be careful not to damage the display ribbon cable as it only has single wire strand connections which may break. I would not advise removing the ribbon cable from the connectors as they are held tight and the wires may break. If you have to, be careful.

The driver outputs are only transistors, so if you wanted to you should be able to test the LED display in situ with the calculator switched off, by connecting a (say) 3K3 resistor from the 5V battery and touch the outputs of the anode driver with any cathode output shorted to ground. The upside down construction of the calculator makes things difficult though.

It is hard to isolate the led connections between the circuit boards for testing which is a problem. Please don't be tempted to bend the board to board pins to isolate them, as they "will" break and are also unobtainium. Even with the boards connected, the LED data from ROM0 could be checked to see if [F] is continuously on from there.

I had some small PCBs made that should connect as a replacement for the anode driver in the 97. It is based on MMBT2907 surface mount transistors (which I think are similar to those inside the anode drivers). The only trouble is, I had the transistors upside down :-(

I could get a few new ones made up when I send the (hopefully last) HP97 board design in to the fabricator.


cheers

Tony

(10-17-2021 07:39 PM)hpux735 Wrote:  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