fostex r8 keeps stopping

[tapeboy]

Hello! My Fostex R8 keeps stopping on me. If I swop the 'rollers' I think they're called - you know the big wheels left and right - it runs fine, except (!) that I lose the use of the locate function. Hence, the read out just says 00000 and I don't know where I am.
The left wheel has black and silver foil inside it and I was told if this is scratched it could be the problem. But after talking to a Fostex expert, I was told it would have to be horrendously scratched (and it isn't) to stop functioning. He said I should look at the sensor unit behind the left wheel. I looked and it 'looks' okay but I don't dare touch until I know more. Can anybody help please!!!!!??