HP-91 recharger problem

I have a friend with a problem in his HP-91 (No, he will not sell it).

1. HP-91 works ok with battery

2. HP AC adapter works ok on another calculator (HP-97)

3. HP-91 does not work when HP AC adapter is connected - it does not turn on. Voltage across battery terminal is 1V when AC adapter is connected and power switch is in "ON".

My friend and I agree that this is probably a recharging circuit problem. I would like to help him, but we are on different continents.

Can someone advise on a solution ? Calculator is fine, except for this problem, so there is no reason for messing too much with it. My friend does not have too much experience with electronic circuits, and he does not possess many electronic tools either, but he can try some simple procedures.




When you measure "1 volt" is that on a DC range of your meter? Take a look at the voltage on AC, I'll bet it's 10+ volts. If so, the problem is one or more bad diodes in the charging circuit on the main board inside the calc. I believe that the 91 uses a full-wave, unfiltered bridge like the 97 does, so there are 4 diodes to test. Replace any open or shorted ones with 1N4001 or equivalent parts -- you can get them at Radio Shack or any of a million other places.



Thank you very much. I forwarded your message to my friend, and asked him to post at this thread.


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