HP35 problem with a calculator, maybe due to a faulty charger

[aurelio]

(03-21-2021 06:52 PM)teenix Wrote:  

(03-21-2021 04:43 PM)aurelio Wrote:  Well, if the voltage on test point is the one expected it means maybe that the PSU works properly, so what would be the next step in the troubleshooting?
What does it mean that if I use any of the numeric keyes, the calculator turns off, while clicking on the functions the display shows numbers that vary, from time to time, with different criteria?

I would try swapping a known good CPU. If you feel this is too risky, then Harald may still have replacement Classic CPU boards which you can try instead.

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It may be a faulty ROM.

When the keys are scanned, a key code is generated and this value is used as an address offset in ROM so that the processor can execute the code for that key press. Very efficient.

If the keycode is invalid, the Program Counter could be set to an incorrect ROM location causing the code to run in error.

If one or more ROMs have failed then code execution will fail also.

When a key is pressed, the display is turned off before processing begins. It is possible the code is still running but in a dodgy loop that makes the calculator look dead.

cheers

Tony

Thank-you Tony to help me in understanding., I've searched, while, for HP schematics and hardware on the web, reading expecially the Jaques's Laporte site... so it could be as you think a faulty ROM or relared circuit (I read about the reset on power-up). Well to know that it's possible to get replacement CPU boards for the classic, I'have just spare boards for the 41s (after convertion in CL
I've searched on the auction site and found only a few calculators for sale (in to be repaired conditions) .. not cheap and all from overseas with also import fee costs