My VIC-20 switches itself off after about 20 minutes, and there’s a wire on the board I don’t think is original

**Update: it looks like the power supply. I ran it on a spare C64 supply I’d repaired earlier with a little clock program so a freeze would show straight away. It stayed on past 40 minutes with 4.94V at the user port (the old brick gave 4.74V) and it came back cleanly after a restart too. One run doesn’t prove it, so the old brick is waiting for a repair of its own. The black wire** **is a factory bodge wire carrying 9V AC as many of you said. Thanks to everyone who chipped in!**

I decided to make a start on its restoration so I got my boxed VIC-20 on the bench this week. Before turned it on I checked the power supply, the plug and the fuse and measured the output 4.7V DC. It came out fine. The cursor froze once, but after a restart all the keys, sound worked and the original joystick worked too, which I didn’t expect.

About 20 minutes in it just switched itself off. Power LED has gone off and black screen. I turned it off and on again, it came back and a couple of minutes later it went off again. So I unplugged it and started the strip-down.

The board (250403 Rev D) is cleaner than I expected. No burn marks, no water marks, nothing obviously resoldered. The one odd thing is this black wire running across it, from one of the outputs over to the power side. I’m not sure it was fitted at the factory.

Has anyone seen this wire on their VIC-20? And does the switching off sound like the power supply to you or something on the board? I’ll post updates as I go.