One of the parts that fails a lot on a Commodore 64 is the power switch.
After 40 years the contacts of those switches seems to get oxidated or dirty.
The correct switch is hard to find, or sold for to much money.
One solution is to open the switch, careful bend the metal shield away of the plastic housing.
Then clean the contacts and reassemble.
Only disadvantage of this is that the plastic can crack and reassembly of the switch can be a hassle.
There is another trick…
You can order a switch almost the same as used in the Commodore 64, and swap the black knob.
when you search on the internet for a TR-80 switch, you see that those switches are very common, but the knob is to large.
The knob does have the same mounting as the Commodore power switch, so when you can swap the knobs, so you have a “new” switch with correct knob.