RCA cables are fine and RCA jacks look sturdy too.
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