Commodore Plus/4 Power Connector
The power supply for the Plus/4 is the same as the C-64, namely the non-repairable "black brick" which outputs two voltages: 5 volts DC at about 1 Amp, and 9 volts AC. Although some Plus/4s were built with the round DIN type connector, most have a square 4 pin type, similar in appearance to the C-128. The pinout is as follows (viewed facing rear of computer):
----v---- 9VAC ------|-0 0-|------ 9VAC | | +5VDC ------|-0 0-|------ Ground |_________|
Note: you can use a standard power connector removed from a dead C64 motherboard to replace this unavailable square DIN type. It fits in the same place on the Plus/4 board. Then you can use a standard C64 power supply, but as rare as the Plus/4 is, I would recommend a more reliable power pack than the old Commodore black brick.
Source: Ray Carlsen's C= Repair Articles