Premise: If the right channel for both the headphones and also the main outputs does not work then that indicates its probably not a jack issue. Im not intimately familiar with the RX15, but I believe most/all of the stereo-only models of that Yamaha vintage like the RX7, RX15, and RX21 (RX21L) use a single 8-pin dual op-amp to drive the audio outputs. Look for a small DIP-8 chip by the output stage possibly labelled NJM4556D, YM4556D or similar
Statement: Look for a tiny DIP-8 chip by the output stage possibly labelled NJM4556D, YM4556D or similar
Relationship: entailment