Hi Randy,
Have you tried swapping the scales on the inputs to the readout? that would eliminate the readout itself: assuming you have, it's worth first checking the tram of the scale, put a DTI to read any deflection (in both directions) as you move along the scale, ideally there should be NO deflection, in reality a few thou" is tolerable. If there's a bow or twist in the scale it could be enough to change the alignment between the read head and the graticle on the glass, which could stop it reading the quadrature signal correctly. Similarly, if there's anything that could deflect the read head that could be affecting it.
It's even possible that the cable to the readout has gone intermittent and that dead segment is where it loses continuity, could try bypassing the cable temporarily with a piece of Ethernet stranded-core *patch* cable (although that would need a bit of soldering into the correct plug, probably opening up the read head to get to the other end).
As far as repair, if it's reading correctly for part of the range of movement it implies that the read head itself's good, I doubt the scale glass can be repaired although *gentle* cleaning may remove any dirt that's built up without damaging the graticle (which should be etched/engraved rather than printed).
Just my ha'pennorth,
Dave H. (the other one)
