Restoring a HP9830B calculator.

[MattisLind]

Tony, you're absolutely right. It is a model B.

I was debugging the printer when the strobe pulse suddenly stopped coming from the calculator itself. I then checked if other I/O things like the tape drive was working. It wasn't... Remember that I have swapped (yes yes I know...) in the card from the 9810 into the 9830 since it was working better. But apparently now also this one failed.

Back to debugging the I/O interface board. After reading in Tony's manual and also checking the schematics I had an idea that it was the tape flag that wasn't detected properly. The tape flag is coming in through the /Bus IO Rd signal. Then via a 7410 to a 74121 mono stable flip flop onto a 7474 flip flop and then generates the I/O flag at the output of a 7453. Thus I hooked up my analyzer probes at U7, U8, U9 and U13:



I had my HP1664 analyzer trigger on U7 pin 9 negative edge. The output from pin 1 of U8, the 74121 was rather interesting:



It seems that this signal always is low, but the non inverted out works fine. Replacing it with a the 74121 from the other board (rather annoying that my rather big stash of 74XX didn't include a 74121, but 74122 and 74123) gave a much better output signal:



And tape commands worked as they have done before with the card borrowed from the 9810 (before it broke down). Now I can continue debugging the printer!