A good video on basic troubleshooting
For those of you starting out in retro repairs, there are some good channels out there on YouTube.
Personal favourites of mine are Adrians Digital Basement, and Noels Retro Lab.
I thought this video was particularly good for its basic troubleshooting process:
Take a look at what Noel does, starting with voltage checking at the beginning, and how he refers to schematics and looks at the circuits and tests them.
If you don't have a clue, it's a nice little video to start with.