HP-35 display problems


I have an HP-35 with a bad display: it only shows one digit on two of its segments, like this:

      5    5
3 3
8 8
etc. When I input a long number, the display becomes brighter and brighter, and the numbers blur into an '8'. My guess is that the cathode driver is broken, and keeps on enabling these two digits instead of cycling through the display. Is it likely that this is indeed the cause, or could it also be caused by a bad connection somewhere?


If you have an oscilloscope, look at pin 14 of the main PCB
whether the signal is OK (pin 1 is on the right side when looking into an opened calc), this is the START of the display cycle. The STEP comes from the anode driver IC (1818-1029) pin 6 and goes to cathode driver pin 9. This should allow to find the causation either in the anode driver, cathode driver, contact spokes or the ARC chip. If you happen
to have a HP45, too, you might swap PCBs - they are compatible.



I have seen the caused by old (often invisible) solder flux residue. Clean the keyboard traces around the display (and mayby the CPU card) very well with 91%+ isopropyl alcohol.


I've already cleaned out a lot of green junk (copper oxide?) that was all over the inside of the calculator, maybe that's the problem. I'll try to clean some more, but what exactly do you mean by "keyboard traces"? You mean the traces on the keyboard PCB?

I don't have an oscilloscope, but by using a small loudspeaker with a 5 kOhm resistor I found out that the RCD signal is present, but I heard no "Step" signal (the one from the anode driver to its neigbour). Is it likely that the anode driver breaks in this way, or do you think something else is wrong? If it's just the Step signal, would it be possible to replace it by dividing the system clock or something like that?


I would clean everything in sight. Most of the problems seem to occur around the IC and display chips, but the keyboard is scanned by the display signals. Problems can and do occur in the traces to the keyboard matrix. If you have access to an ultrasonic cleaner, you can try cleaning the circuit boards with it.


I thought that on the classics, the keyboard was scanned by the Control and Timing chip on the logic board, whereas the display was scanned by the cathode driver.

ACT-based machines (Woodstocks, Topcats, etc) use the cathode driver to scan both the keyboard and display, but I didn't think classics did.

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