Service Sheet 804A gives a schematic of the internals of BR107, Sheet 804 shows the control box MCR1 & MCR2 and a basic circuit diagram, along with a test procedure.
My understanding is that the frame of the unit is electrically isolated from its mounting feet. If the headlight dims when engine revved, this would make me think that the battery voltage is being drained somewhere else as a consequence of the rising dynamo voltage closing the cut out contacts. So a visual check, look for dirty or broken connections, incorrect wiring to the terminal block, F goes to field on dynamo, D is dynamo output, E is the earth/ground and Terminal A is the main feed to the loom so check that these have not been mis-wired.
Simple continuity tests should show any fault in the basic electrical set up of the unit. I've never messed with the adjustment, they either work reasonably or not at all.
Swarfy.
