I have similar issue, just been given a Technics EH600 system, when turning the volume dial it does nothing, I'm left with a low volume at all times, I have disassembled the separate enough to get to the circuit board behind the knob, but it's fine I re-soldered two joints but they were just the fixing points for the knob to the circuit board & didn't really have any electrical contact value, The other joints looked fine too, It's such a shame as the sytem is lovely but there's no remote with it if there was that would be a different matter as that would probably work due to it being digitally controlled,
Just for my satisfaction I am going to replace the capacitor on the board, it doesn't look faulty but you can never tell with them