I would agree that if it is working ok then not to bother with a professional service that would probably cost more than the cost of a replacement (which isn't that much if you look at ebay prices for the 3020s).
You could however do a bit of a basic service yourself by carefully blowing away any dust on the boards (unplug it first, obviously!) and using deoxit or similar to clean the switches, phono outputs, speaker connectors and volume control pot to get the metal contacts good and clean again. The upgrade I always wanted to make with my 3020 but never got around to it was to replace the silly speaker terminal 'grabbers' that I could never fit all of my 79 strand cable into, and solder on some proper binding post connectors instead.