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Full Version: Hp-21 problems after battery leak.
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I just bought a HP-21 that had major corrusion from an oöd battery pack. i cleand it all up and it worked fine for a few minutes. After i turned it off and on again, it didn't.
If i turn it on and off several times, it sometimes does wired things like flashing numbers in the display or countin 0-9 at the last digit.
Sometimes it works correct too.
I thought it might not be producing the 4Volts for the display. Where can i measure those 4 Volts if that part of the circuit works fine?

Sorry for my bad english and thanks in advance,

i found 6.2 Volts at the capacitor below the top left IC. So that "step up" (?) regulator is working.
Any ideas what to check next?


Stand the bottom of the circuit board in a glass of vinegar for around 30 minutes. The vinegar should not get to the level of the small power supply coil. Rinse the circuit board very well, shake off the excess water, and then dry it overnight in a low oven (say 120-140 degrees F). This will remove any battery crud from under the main chips, etc.

I will let you know if it worked


I'm out of ideas. It must be a faulty chip :(


Hi, Harry;

please, send me your e-mail so I can send send you some info (images) I cannot post here.

Best regards.

Well, it sounds like one of two other likely problems. The first (and most likely) is a bad ACT chip (the 22 pin 0.4 inch wide chip at the bottom of the circuit board).

The other is a bad filter capacitor in the power supply. If you have an oscilloscope check the 6V line for a nice DC waveform. A bad cap can cause this voltage to be a square wave. A voltmeter can show a normal looking voltage. You should also check the other two voltages running around the machine.