HP-21 - Crazy numbers

[BusyCalc]

Hello Max,

thank you for your reply!

I did as you said: I soldered two wires to the lines leading to the battery terminals and I connected 2 AA Ni-Mh batteries, showing around 2.55V. Unfortunately, the behaviour was the same as before, going crazy after about 1 minute of operation.

I decided to clean deeper the corrosion seen in the traces above the ACT, around the 2.2 uF capacitor to the POR (image). The initial green corrosion was easy to remove using water and vinegar and then isopropyl alcohol, but the blue corrosion is really hard to dissolve.

I also cleaned better the Button 4 connector, because when it wasn't pressed, I could measure about 5-10 MOhm instead of infinite. Also, apparently some rests of the battery leakage was closing slightly the circuit at this point.

After this, I switched the calculator on and it is till now working properly. I cross the fingers!

I have hypothesis for this:

- It was a problem due to the green and blue corrosion, which was joining slightly two lines and introducing noise to the ACT.

- A capacitor was somehow "lazy" after many years without voltage and after several switch on and off it has recovered its properties. I say this because before cleaning better the PCB, I had the impression that the calc was working properly longer after every switch on.

I also had this impression when I restored an HP-65 and tried recording magnetic cards. The first 10-20 attempts were unsuccessful but suddenly, it started to record. I read then that a weak capacitor wouldn't filter the noise of the amplifier and make impossible to record a card.

I'm not sure if this hypothesis of a "dormant" capacitor is feasible.

Regards.

Oscar