I want to give a huge thumbs up to the Ray Carlsen "Power Saver" cable that's available for the C64. It's a short dongle that goes between the C64 power brick and the C64 console. If the power supply is good, the light on the dongle lights green and everything is good. But if the 5V line is running high, the LED is red and no power goes to the computer. A high 5V line can destroy chips on the board. What's bad is that it could go high at any time, and you'd never know until your computer starts to die.
In my case, I tested a power adapter I was going to include with a C64 trade for a forum member here...and it immediately gave a red light! I like to think we saved a C64 with this cable.
I paid $35 + $8 shipping for the cable, and I'm so glad I did. Google search Ray Carlsen Power Saver to learn more and where to order.
HIGHLY RECOMMENDED by RickR for anyone using a Commodore 64.