VIC external power supplies do fail and while they can be repaired, or replaced, owners often worry about powering their VICs and will go out of their way and spend more than thay need just to get the right replacement.

This is not necessary.

A DIN power plug VIC can be powered from any 12V and 5V DC supply port capable of delivering enough current, such as the disk connector on a PC power supply.

You can see what happens to the power inside the Vic 20 in the schematic.

Pins four and five are connected together and supply the regulated +5V. This supply is switched by SW1A.

Pins one, two and three are connected together and make the ground return for the +5V. This connection is not switched.

Pins six and seven supply the 9V AC which is used inside the Vic to generate the cassette motor supply and also feeds to the user port. The feed to pin six is switched by SW1B and the return on pin seven is fused by F1. As this supply is isolated in the standard Vic 20 power unit disconnecting either lead, by the switch or by the fuse blowing, will remove this supply.

A seven pin DIN plug.

A PC hard drive power socket.

An offcut of stripboard.

Some wires.

The offcut of stripboard is pushed into the disk power socket, as the PCB it was mounted on was, and the pins soldered in place, each one straddling two tracks. This gives a greater attachment area and allows more than one wire to be soldered to each pin as was done with the first cable shown.

Alternately one wire can be soldered to each pin of the disk power socket and pins in the DIN plug connected together as was done with this lead.

The 9V AC at the Vic user port can not be used to generate negative voltages because the cassette power bridge rectifier inside the Vic effectively makes 0V the lowest potential.

The 12V DC is connected to the pin 6 9V AC input so that it is switched when the 5V DC is switched. The pin 7 9V AC input that goes through the fuse is not used because is not switched and that would leave the cassette power on at all times. If fuse protection is needed a fuse can be fitted inline with the 12V DC.

The 9V AC when rectified becomes 9V * SQR(2) = 12.7V minus the two diode forward drops of the bridge rectifier which is 12.7V - 0.7V - 0.7V and gives roughly 11.3V.

Using 12V DC instead bypasses one diode of the bridge as the return is through the 0V line so there is only the one diode forward drop giving 12.0V - 0.7V which is also roughly 11.3V.

In spite of its similar pinout this method can not be used to power a C64. The C64's internal power circuit is very different and requires that the 9V AC is actually AC and not DC.

So now you know.