Servo Dynamics SD4-1525 Shunt Regulator

I have a Yeong Chin Supermax (Bridgeport clone) circa 1989. It has servo dynamics motors and amps ( SDF-1525-12).

Lately the X axis VOL LED is tripping frequently. Otherwise the axis is running normal and turning it by hand has normal resistance.

I have a manual for the amps which says that nuisance tripping of the voltage LED may indicate the that the shunt regulator fuse may be blown. It also says that the shunt regulator fuses are optional. My manual doesn't indicate where these fuses are located. Are they on the amp board or on the chassis? Is there one per amplifier or shared? How do I know if I have them and how do I test?

If that isn't the problem, what else might be the issue?

...dirty Tach brushes can cause this. The Shunt Reg is the Unit in the lower left in this picture

Thanks!
My doesn't look quite like the picture. The unit in my chassis has space for three fuses. Only one is populated and the fuse isn't blown. I'll try cleaning the tach brushes.