HP 21 not powering on - repair/troubleshooting advice

[dmh]

Ok, a bit surprised as visually the terminals looked ok but some light abrasion to the terminals has given the following:

- one powers on and works fine except for the '4' button. It clicks like all the other buttons. Any suggestions?

- one shows some screen activity as the power switch is turned off or on but never stays on. Shows either one '0' on left or right or three '0' across the screen. Is this the ACT chip or could it just be the power switch?

- last one still does nothing

Thanks!