If you are living in pretty central Tokyo and need bicycle repairs (not motor bike), you can't do better than go to a place operated by 3 or 4 ojisan on Meiji Dori about 1/3 of the way from Ebisu going towards Shibuya on the right hand side. They are usually there Monday to Saturday except for holidays and just have a place on the street, not a shop. I go past there a lot and often drop in to ask them to adjust my gears or replace brake pads, but today had what I thought was a catastrophic problem. The rear derailleur gear mechanism had got stuck in the spokes of the rear wheel and the bike was completely unusable. I walked it to them and the guy I usually talk to there fixed it completely within about 20/30 minutes, and wouldn't take a single yen (I bought him a cold drink instead). Of course if a repair needs parts, they will charge, but basically it's a free service for one and all. Can't get cheap(o)er than that! They don't have a huge store of spare parts, but have the basics such as brake pads, spokes, pedals etc etc.