hmmm...doesn't make sense that a digimod would affect this unless you slipped somewhere with the soldering iron. The two spots that get work for the digimod are both in the digital domain where left/right are irrelavent. Here's what I'd try, in order:
Switch the cables feeding into the left and right to confirm that it's actually the UA-5 and not your source device or cables?
Does the same thing happen with the rear inputs (just use a line level signal instead of mics)?
Have you tried any other type of connectors - the ones in the front accept xlr, 1/4" mono and 1/4" stereo.
Did you try the unit before the mod?
and lastly, please don't be insulted, but just to be sure, you do realize that there is a separate front gain knob for left and right
Report back!