Technics SA-600


I bought a Technics SA-600 last week, and it was sounding quite
crappy when I first turned it on.
I then cleaned all the switches and pots with a contact cleaner,
and now it works perfectly, EXCEPT for the AUX input...

So I was wondering, how is it possible that the TAPE1 and
TAPE2 inputs both sound cristal clear, while the AUX input
is REALLY bad (almost no volume, cracking and humming) ???
Don't these channels go through exactly the same circuits
(except for the tape/source switch) ?
Could it be that the tape/source switch works perfect in 2 out of 3 positions,
and almost doesn't work when in the 3rd position ?

Any advice would be welcome !