I was fixing an old mixer Rodec MX180 and after repair was done, i noticed that there was quite a loud hum (50/60Hz tone) when connecting the audio output (RCA cable) to the audio amplifier.
This noise completely disappeared when I connected the 0 V of the audio output to the Earth (Chassis).
I didn't expect this, since that 0 V was in actually already connected to Earth but trough a 10 Ohm resistor and 10 nF cap in parallel.
Basically bypassing this 2 components, the Hum completely disappeared.

For what purpose those components have been placed there?

 

Check those two R C components first. Soldered/screwed contacts to them ,too

 

they look good to me. Also their contacts are ok. Voltmeter in continuity mode shows good contact.

 

In fact, those R C components are used to reduce the electric noise...
Replace the capacitor to 0.1 uF . If this has no effect, replace the resistor to 4.7 Ohm