If basic steps fail, you can test the PSU output pins. The N64 PSU outputs two main voltages: (for the logic/CPU) and approximately 12V (for the video/audio).
The N64 is a tank. The power supply is its only weak spot. Nine times out of ten, the console is waiting patiently for you to solve the PSU problem. Follow this guide, and you’ll be playing Mario 64 before lunch. nintendo 64 power supply not working