I have an Epiphone Les Paul Standard and when the the switch is on treble or in the middle it works fine, but as soon as it is switched to rhythm the sound becomes all muffled on the amp and a lot quieter. I'm guessing its something to do with the wiring but I have unscrewed the back of the switch and there doesn't look like there are any loose connections or anything. Any ideas?
There are only two possibilities...
The rhythm pick-up has gone open circuit; would need to be replaced, or more likely the changeover switch has dirty contacts; can be squirted with switch cleaner and moved rapidly from side to side to clean the contacts. That's assuming you can get to the contacts at the back of the switch.
You could also do a bit of maintenance at the same time and clean the volume and tone controls as well.
Hope that helps.
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