I am planing on testing my donkey kong four board set but the power supply on the Schematic says "pp-7a" but my power supply says "pp-7b" . Is there a difference in voltage?
No difference really, they are the same specifications. Some component values and board layouts are different inside the power supplies, but in the end, they have the same connectors, pinout, and voltages. Completely interchangeable.
The TKG3-UP manual says "PP-7A" but (according to the Camo Murph DK-data spreadsheet) a lot of the TKG-3UPs came with PP-7Bs. Nintendo did keep using the black PT-8A transformers for quite a while before switching to the PT-821B.