Have you changed the belts on your Sony Reel to Reel deck and it still slows down after playing a while or need a boost to get going? This is usually a very easy fix. This problem is not limited to Sony Reel to Reel decks but it is where you will see it most.
The decks use an AC motor and it has an external motor run capacitor to help it maintain accurate speed. These capacitors are as old as the unit and almost always need to be replaced. They are very easy to replace. Just take out one screw that holds the old capacitor in place, screw the new one back in the same position. Then just solder a couple of wires to the new capacitor. The capacitor is easy to access with just the back panel off of the unit.
Symptoms of a bad motor run capacitor are running slower the longer the unit is on. The motor will also run very hot (oiling the motor while you are in there is always a good idea), and sometime the motor won't start up unless you give the shaft a little spin.
We have the capacitors (1.5uf 250VAC in most units) available on our Misc. Parts page. It will be the best $8.00 you spend on refurbishing your deck.
The large rubber idler that most units have is also a source of problem with speed. Cleaning them helps but rarely completely fixes the problem. The best is to send the old idler to Terry at this web site to have it rebuilt:
www.terrysrubberrollers.com