Hi: 

I recently bought a vintage 1987 Marshall Fifty Split Reverb (model 5215) from the local used gear shop.  I REALLY like this amp a lot, but one issue is that the gain only works on "10".  In other words, from 0-9 the gain knob makes no appreciable change to the sound, and on 10, it gets totally distorted.  I would love to have a range of gain options.  Does anyone know what I could do?

When I bought it, the tech at the shop removed the chassis and shpritzed deoxit in all of the pots.

Thanks for any help you can provide!