Has anyone ever done this?
Is it difficult to repair?

I checked out the circuit board and there's some black gunk near the capacitors.

 

I've personally never repaired master switch before, but I figure if you're pretty good at circuit soldering, then I don't see a reason why removing and resoldering devices would be so bad? It sounds like you've already taken a look at the circuit board for the switches.

 

There's a plethora of things that can go wrong so it might take some trial and error, the one I fixed awhile back had a burnt trace which i bridged. Replacing caps can't hurt, sounds like you have some leaky ones but don't be surprised if it doesn't fix the problem. Inspect traces with a magnifying glass if it still doesn't work, Niles doesn't make the highest quality circuit boards in the world.