Gecko failure/repair

Greetings,

I'm hoping someone has some wisdom on a particular problem that's
developed with one of my old style Gecko 201 stepper controllers.

Symptoms:

The motor vibrates back and forth as if one of the phases is not
working.
Replacing the stepper controller with the other axis' controller
results in proper operation (motor and wiring are correct).
The "failed" Gecko powers up and responds to step and direction
signals.

It seems as though one phase output was blown. There are no burn
marks or blown parts on the top of the board that I can see. Since
these are the open case selloff units with no warranty, I removed the
heatsink/mounting plate and examined the drive FETS. One of them next
to the mounting screws shows a slight burn mark on the leads.

It may be possible that my motor leads shorted and popped the output
for that phase since I found a loose wire on one of the pins on my
motor connector.

Questions:

Has anyone seen this behavior?
Are there any specific tests to perform?

I have the parts here to repair the unit, if it's possible. I'm
hoping someone has seen this issue before and can tell me what damage
was done so I can get a head start on troublshooting and repair.

Thanks in advance for any help.

Chris