Service Manaual for Marantz MM 500 needed

I have a Marantz MM 500, 5 channel power amp that is causing me some troubles 😉

All channels work, but then from time to time the sound in the right channel simply disappears! .... the next day it is back again.
I have measured if there is some DC on the output, and there is not ... so the output is not burned ... also as the sound returns 😉

I'm thinking that it might be the relay, or some muting in the input .... not sure ...

Have been trying to locate a service manual on the internet, but only found the sites pushing (public knowledge) for money ... not supporting that.

Thanks in advance