Marantz CD 72 problem

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.

I would check the solder joint where the RCA socket meets the PCB. With pulling interconnects on and off it's not unusual for these joints to crack, or even for a small amount of trace to lift from the PCB.

Marantz CD 72 has two sets of RCA jacks, one fixed and the other variable. So, four RCA connectors in total . Among these four, only two left channels (fixed and variable, which are on the top) work while neither of the two right connectors(on the bottom) sings. I opened the CD player and couldn't find anything wrong with the connector--they look tight and sturdy. No loose sold joint can be easily indentified either.

Left fixed variable
Right fixed variable