The voltage is generated by spinning a big magnet inside the fixed coils. The magnet is actually another set of coils with dc current running through them. Somewhere there is a stabiliser that alters the DC current to keep the output voltage constant. The DC current is normally fed to the spinning coil by means of brushes on a pair of slip rings or such like.
So one good thing to check is that the brushes are good and move freely and the slip rings are in a good state. After that the voltage stabilizer must be looked at. There are many sorts, but there will be some sort of detector that checks the output voltage(s), the signal from this will then alter the DC current throught the armature coil. So just checking connections and relay contacts may find the fault. FWIW low output voltage = low DC current!
Frank