SCPH-10010 H: The initial Dualshock 2 is born to corrode if not used for a long period of time. Check all the soldering spot on the PCB and try to find any spot of corroding.

Take the Flexible PCB and look for discoloration of the contact lines.

DIsamble the the Flexible PCB to the hard PCB connector by pulling the two small switches on each side and gently pull the Flexible PCB end. (this doesn't require force, be careful or you will break your controller)

Clean everything with Q-tips and Isopropyl alcohol.