This might save you some shipping expense: Try to set the internal Ground Balance pot for your ground. Usually that is all that is needed. Here is a past post by Monte on how to do it, might take you 5 minutes.
I just got a used Cibola yesterday and I set the GB today and it improved the air test from 6 1/2 inches to 12+ inches on a clad dime.
Power Balancing in the motion Discriminate mode.
It is much easier with a model that has an external GB control, but if you're careful you can use a small screwdriver and adjust the internal trimmer as follows. Just remember, the trimmer is a delicate and touchy little control so only move it a tiny bit at a time.
With the Discriminate level at minimum and Sensitivity at maximum, bob the coil toward-and-away from the ground in the Discriminate mode. Do not lower and raise it slowly, but make a quick down and up bobbing motion.
If it is silent, you want to adjust the GB control toward the negative side. Do this while bobbing the coil and stop as soon as you get a 'beep' on the coil's uplift.
Now that you have a too negative GB, keep bobbing the coil from about 6" down to