Stereo amplifier speaker protection relay fix

This video is simple and general instruction how to fix speaker protection relay problems of an stereo amplifier and home theater systems. Problem is caused by bad relay, because over the time relay contact-surfaces may be worn out, or coil of the relay is not working properly anymore. Bad relay is not connecting signal from the amplifier to the speaker output and one or more channels are silent. This is quite common problem of devices with 15-30 year of age. I will explain how to diagnose the problem and how to fix it.
NOTE: Usually problem starts with one relay, but later other relays will also start to go bad. Relays are cheap, so I recommend to replace all relays at once.
Device I'm working on, is Sony, but many home theater amplifiers build in the 90's and 00's may have similar type of design and basic components.
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.