<p>The capacitors are mainly to block DC bias from effecting the mixing op-amp. Essentially, you want all the signals to "ride" on the 4.5 V supply to the non-inverting terminal of the op-amp. A reversed polarity cap seems to allow AC (audio signal) current to flow, but only for loud signals it would seem. At the shop we have flipped the C5 cap and been able to use the input channel perfectly it would seem. We will try to use a guitar input level or microphone tonight and see how that is working. As for advice on a malfunctioning unit, check for cold joints, and if all else fails, brute force check all the related components and connections with your trusty multi-meter. Place special attention to checking for continuity (the beep mode) where things should be connected, and make sure there is no continuity or a short where there shouldn't be.
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