HP-21 - Crazy numbers

[BusyCalc]

Hello,

I've purchased an HP-21, which shows a strange behavior when operating it.

If the calc was not used for several hours, it shows 0.00 after switch on and I can key in. For example if 1/0 is entered, it shows correctly the expected "Error" word on display.

But after few seconds, the calc becomes crazy and it starts showing odd numbers (zeros, commas), usually endind with a random number like "0185934170".

After this, if I switch off and on the calc, the wrong operation starts from the very beginning, I don't get the 0.00 and I can't key in anything. Instead of it, crazy numbers are showed.

If I let the device off for hours, then again it starts properly for few seconds.

Background of the calc: I opened it because the old battery pack had a small leak, mainly under the Batt Plus contactor. Some adjacent printed circuits were light corroded with white, green and blue stuff. The 4 button contactor was also light corroded and didn't contact.

All this was cleaned and it is now in good visual shape.

I also checked several points of the ACT:

- Pin 1 Vss: around 6V.
- Pin 2 Vgg: around -12V.
- Pins 16 and 17 Phi2/Phi1: around 180kHz rate.
- Pin 20 Sync: around 1,6kHz rate.

Could it be related to the tantalum caps? I find strange that after a "nap", it starts properly for few seconds, but afterthat it becomes crazy and a further reset doesn't help. I was wondering if any component remains loaded and it blocks the normal operation.

Could you please help me with tips to check further?

Thanks!

Oscar