Some old things literally aren't worth repairing...

Although no success, at least you have a video that helps you understand how to sorta work on these guys! I gave this a good effort here, unfortunately it didn't end as I wanted.

[@douglashoff95]

The metal box is called an AMPLIFIER. By the way WD40 works great to soften up the grease on moving parts of the turntable (Possibly hardened grease is making things stick). Cartridge possibly is bad. See if you get sound from the speaker by feeding a signal into the wires that come from the cartridge. Then you would know the electronics are working correctly. Since you have the schematic it will make things easier for you. I'll bet you'll have that playing in no time.

[@indebtperhapsiam8893]

I did exactly so and we have sound. The amp works, I tested it. That's how I know the cartridge is bad.