Shorting the Switch Matrix to +50 volts.
When in a hurry, many make an under playfield adjustment with the game turned on. Doing a switch adjustment with the power on could easily short a coil lead (+25/50 volts) to a switch lead. This will immediately blow the switch matrix, and can fry most anything up to and including the 6821 PIA at U38.

Start with the switch column connector 1J8 which goes to eight 2N3904 transistors Q42-Q49, as these are first inline (and test easily with a DMM set to diode test). Often that is all that gets damaged, but it can go further. The output of the 2n3904's goes to a 74LS244 chip on the CPU board at U40. And the output of this chip then goes to a 6821 PIA at U38.

Depending on what switch contact was shorted to coil power, if the switch matrix still does not work, check the switch row logic path. The switch rows are controlled through CPU board connector 1J10, which connects to 4011 chips at U30 and U39 (or U51/U52 respectively on system9), where U30 (U51 on sys9) are rows 1-4 and U39 (or U52 on s9) are rows 5-8. This then connects to the same 6821 PIA at U38.

If after replacing the suspect components, disconnect the switch input plugs from 1J8 and 1J10 at the bottom of the CPU board. Put the game into switch test mode, and none of the switches should be activated! If a whole row of switches is activated, that would mean that something in the row chain is still bad.