Did you try putting it in manual mode and tweaking the controls? (I'd assume yes of course after such a time). Can you hear any of the effects coming out of the output? check both phones out and main out? does it appear to do anything to the sound even if minor while switching effects or through the sound bank (even if the banks aren't working right).
Obviously it could be hard to diagnose your sound output, whether it's completely dead, while the rest of the synth is in the usual 'battery broken state'. If you knew someone else with a polysix you could borrow their main (repaired/ok) middle PCB board, swap it in and see if you have any sound.
The other thing could be dead voice chips on the right board. Do the diodes light up on that board (cycling through the voices) as you press a key repeatedly? You could/should check the voltage/continuity from the output jacks (inside) and trace them back to the components to make sure they are connected ok.
And yes the CPU can be damaged by the leak, I think it's a bit rarer but depends on how bad and how far the corrosion went. Have you taken it out, checked the pins for damage? reseat it and try again? and if everything is still acting erratically of course triple check all your repairs for shorts (sorry that sounds obvious). Also check the volume pot is working (with a multimeter).
maybe someone else will know more specifically about 'no sound' but it sounds to me like a number of issues caused from the damaged board that still need repairing and a thorough check of the audio path from the boards to the output.
To:
PolySix@yahoogroups.comFrom:
mack_sass@yahoo.comDate: Sat, 14 Nov 2009 01:03:12 +0000
Subject: [PolySix] Battery problems
I have been working off and on at fixing my battery issue for the past few months...replacing parts etc.
I have not been able to get ANY sound from the poly six at all. In addition to this, the controls are unresponsive and the lights are erratically on or off. I suspect possible problems with the 8048 CPU. I have heard conflicting reports on this, but with batter leak damage, shouldn't I expect to heard SOMETHING when pressing keys?
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