Hi JD,
The brushes are the input to the system to "assist excite" the rotor/armature - There is a feedback from one of the field windings to the AVR that gets rectified and pushed back into the rotor to get the output voltage to spec. It defs looks like the AVR is at fault/suspect here, juice the brushes with about 12VDC nominal and check your output terminal voltages, if they are LOWER than rated, you are fairly safe to continue. Then check any additional inputs into the AVR, there are usually at least one, sometimes two or even three pairs of wires coming back to it. If there are no voltages coming in to the AVR, shut down the unit and disconnect the AVR completely and try again. Still no voltages, then you might have lost or burnt one of the windings. If there are voltages present, above 20V nominally that is, then there is pretty much a certainty that the AVR itself is fried, a common occurrence....
In my limited experience with these creatures, the voltages to/from the AVR/Connectors are
(a) a "driver" output from the windings going into the AVR that gets regulated and fed back to the brushes to maintain output voltage
(b) a "sensing" voltage that is of fairly low current to power the AVR and used to "sense" the output of the main terminals for a control of the output voltage
(c) The regulated output from the AVR which goes to the brushes derived largely from (a)
Note: some AVR's have 2, 4 or 6 wires input as well as the 2 wires out to the brushes...
A brief way I understand them:
2 wire input AVR - a+b above are combined, still has c
4 wire input AVR - as above (most common around my neck of the woods)
6 wire input AVR - as above but sensing and supply are split, still has c - ie 2 sensing wires, 2 supply wires to power AVR, 2 "power wires" to be rectified, regulated and fed back to the brushes via (c)
This is all excluding the 2 or 4 or 6 "output" wires that go to the outside world to run your electrical doo-dahs....
A pertinent question though... with the AVR disconnected, the output wires disconnected and the unit not spinning ie just with the genhead electrically isolated from the rest of the world, have you checked for continuity of the windings and also that none of the windings are shorted to ground? (all winding resistances should be fairly low, in the region of less than 1000 ohms and all windings should have a resistance of greater than 100 000 ohms when measured between coil and chassis.)
If not - time for a re-wind!
Hope this helps
Cheers
Ed