HP-91 electronical problems


Hi together,

I have some problems regarding my 91. First the problem description: Connecting the power supply to the calc (yes, full battery inside) does NOT light up the LED's. Removing the power supply and using the battery only let the calculator work just fine. Now for the measurements at the battery cvontacts (battery NOT inside, in brackets I put the values I found on my 97): Switch OFF, power supply connected: 10V (10V), Switch ON, power supply connected 1V (5V). All those symptoms make me think, that this has something to do with a) the charging circuit and/or b) the overvoltage circuit. Unfortunately those two circuits are not the same found in the service manual of the 97 on museum CD5. So my questions: Has anyone encountered the same problem or has anyone a circuit plan or service manual of the 91 or is there anyone, who can help me in any other way. Any help would be very welcome.

Greetings, Uwe

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