Marantz CD 72 problem

Hello

check yours RCA cable or change it

RCA cables are fine and RCA jacks look sturdy too.

Try the easy way by swapping the left & right RCA cable to really confirm its not the RCA problem first.
If its confirmed not the problem, then most probably its an open circuit on that channel somewhere.

I would suggest opening up the casing & trace the connection from that affected channel.

Shouldn't be difficult unless its the PCB board.

Just trying to help.

Not sure about the 72 but I had the same problem with my Marantz 63 mk2. Seems that not only did Marantz put the RCA connectors too close together on the back of the chassis but for some reason the female RCA's they used do not make a good tight connection (center conductor)with most male RCA jacks. They are mounted directly to the circuit board so it will require a little soldering to fit a better set if that's the problem.