Sony Cassette deck diagnose and repair (slip clutch)

Instruction video for diagnosing and fixing old-school Sony stereo system cassette deck.
I present general info and diagnostic, but also telling one easy fix for slip clutch which seems to be common problem with these cassette decks.
Side note:
I'm a mechanical engineer, but what it comes to electronics, I'm mostly self-learned. Do not try to fix your electronic devices if you're not sure what you're doing. If you handle electrical devices carelessly, you may get electrical shock or cause malfunction to the device.