You're lucky, you have a good channel to look at. With a scope
or signal tracer, compare the L chan to the R chan, going from
input to output, with an audio signal into both channels, until they
don't match. Ken

You can get that same signal by tuning to a mono signal or switching the tuner
to mono.

One "obvious" thing to check... Check your cable, and check the connections of
the right-channel jack to the board.