HP-45 one digit only

[geologie]

Hi, i have an HP-45 that only register one digit number. If i enter, for exemple, 20, the display only shows 2 and if i square the number i get 4. If i continue squaring, i get the good answer on the LED. The display is all ok, the keyboard is all ok. I resolder the main board connection without any change.

A friend that also have a 45 bring me his board and if i put it in my calculator everything is fine, so my main board is bad. Now that i have another good board with me, what can i check and compare to repair my board?

Thanks