HP-21 works as a fast counter

[Ache]

Hi everyone!

It is the first time that I write in this forum and I don't know if I am carrying out the procedure correctly.
I have been following this forum for a long time and it has solved many problems for me with these wonderful calculators.

I recently got my hands on an HP-21 in very good external condition. Internally, the circuitry doesn't have much corrosion and appears to be in good condition. When I turn it on, all the digits are set to 0 and it starts working as an up counter from the rightmost digit. The counting frequency of the first digit is about 20Hz. I've left it on for a while and it won't stop counting... I got tired at 000000093894 or so... the first digit is hard to read because of the speed at which it changes.

It also does not pay attention to any key. I have tested the keyboard on another HP-21 and it works more or less well.

I suppose it is due to a failure of the ACT, but I had not read any case like this.

Does anyone know anything else???

Thanks and best regards!!!