Posts: 3
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Joined: Jan 1970
it was one battery in the wrong direction. Thanks a ton
Posts: 1,162
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Joined: Aug 2005
I had this fault recently on a halfnut HP41 (actually a CV, but that's not important).
I knew it wasn't a battery problem as it remained even when I connected a bench PSU to the battery terminals.
I thought it was probably a fault on the display hybrid, but when I took the machine apart I found a dry joint (bad soldered connection) on a diode. If you look at the halfnut PCB, there 2 diodes one on top of the other on the left side. The top one seems to be an afterthought -- the cathode lead is soldered to a piece of red wire that goes across the PCB. The anode lead is soldred to the anode lead of the other diode. It was that connection between the 2 anode leads that was faulty. Without that connection, the
display hybrid didn't get to sense the battey voltage, I guess.