Function switches, perhaps? I don't see a relay on the schematic. The signals head right out of the filter after the stereo demodulator and into the function / input selector switches. It seems that everything beyond there is O.K. or you might notice it with other sources.
If you have a signal tracer you might probe backward from the L & R outputs at LC1 for any loss of signal. There ARE a couple of 1µF electrolytic in front of LC1 isolating it from the output of the HA1156 demodulator. I'm not saying they are bad, but old caps are always a little suspicious, and there isn't much else in that circuit after the 1156. There could also be adjustment issues, but that's opening a different can of worms.