First thing to do -- make sure you have a good connection with no voltage drop between CPU board and power supply (bad contacts = voltage drop).
On CPU board, measure +5V directly across C19 at top of board.
Black lead to negative side of cap, red lead to positive side of cap (positive side is nearest to their J1 connector).

Next - On power supply, measure +5V directly across C2 near J2 connector.
Make sure you measure same voltage at both places. If not, you may need to rebuild a connector or two before proceeding to adjust supply.

Fortunately, Gottlieb used 4.2mm connectors on this series so these connections are much more reliable than the older 0.156" connectors.
Be extremely careful with that adjustment. Too high can bugger things up... and a bad trimmer can also bugger up boards so best not to futz with the voltage regulator's adjustment unless you must.

Yep, very slowly.