I have seen this - most likely the wire from the battery compartment is rotten - the last one I replaced was black all the way along. This is due to old leakage wicking along the strands of wire. You may be lucky and all it needs is re-soldering. If it needs replacing, it's a front off job, not that difficult, but be prepared to completely destroy the leatherette (it is stuck on REALLY well!),
If it happens when you focus, the other possible cause is that the wipers on the lens barrel are dirty - take the bottom plate off and clean them with some alcohol / Ronsonol.
If you hunt around, here and the net, you will find photos to illustrate both.
Repair manual should still be at Kyphoto
Hope that helps!