Vintage Synth Repair

Hi all,

I recently acquired a Yamaha CS5.  It sounds great, except there's a
random crackling sound in the output signal.  I thought it might be a
problem with the keys at first, but I even hear it if I have "initial
level" turned up so that the oscillator sounds without a keypress, and
also if I have nothing but the input signal (drum machine, etc...)
turned on.  There are a few crackly pots, particularly in the amp
section, but they've been improving with use.

I've checked the output jack for bad solder joints -- it looks fine. 
Cleaned all ins/outs with contact cleaner.  Had a bit of a clean
inside the case and removed a ball of lint that was right behind the
output socket, but even this wasn't the problem.

If anyone has an idea what might be the root cause this will help me a
lot.  I'm a long way from anyone that will repair these things
(central B.C., Canada), so if it's something I can fix myself I'd like
to sort it as it's doing my head in.

P.S. the boards inside are soldered directly to the pots, so all the
components are tricky to check without unscrewing the pots from the
main panel.  There's no sign of bad solder or corrosion on the back of
the boards, however.  All in all it looks pretty clean in there, and
there are no other sound defects such as wobbles in pitch or anything.
 Also, I've tried several speakers, cords & headphones, all with the
same problem.  The power-cord is plugged into a surge protector.

Thanks,
-Josh