Ha! Just been there. Ancient tractor with no ignition warning light at start up. Initial investigation found ample carbon left on alternator brushes so assumed the fault was elsewhere. Not so:-( The alternator slip rings were worn down to the underlying insulation leaving a narrow strip of copper either side. As a temporary fix, refacing the brushes regained contact and the system is temporarily working.
Seems odd the copper wearing faster than carbon but maybe this was a recon alternator.