Audio Systems: Faulty Technics amp, clock signal, preamp stage

Question
Hi, first of all let me thank you for your time.
I have a Technics SU-X520D amplifier which I rescued from the dump. It appears to work correctly (all the buttons work, inside the power supply is functioning at all the right voltages) but only one input works.

It has six inputs, selected by buttons on the front panel and a seventh input (Aux) which is put through no matter what the selector is on - I assume this is how it's supposed to work.

However, the Aux input is the only input which works. The other inputs come out very quietly, which I think is crosstalk since it's only audible when the volume is on maximum.

I have replaced the capacitors in the preamp stage and input selector board and that hasn't fixed it. Do you have any idea what could be wrong with it? I have quite a bit of experience in electronics and audio, do you know what the problem is?

Thanks for you time,
Tim  

Answer
there's probably a lsic (large scale integrated circuit) which actually handles the switching. without a schematic, it's awful hard to determine if it's getting the input signals from the buttons.you will probably need access to a scope to verify the data, strobe and clock signal lines to the processors. a schematic will run you about $30.00 or so.   good luck...