What is the age and size of the alternator?
I have had experience on a 1973 7KW unit coupled to an SR2 engine as a Start-o-matic
If it is this sort of age, I know my way round it pretty well.
In the mean time:
If it has not been run for some time, make sure the commutator is clean and that the brushes are OK. Same goes for the slip rings and brushes.
Check that the field resistors are intact; a common problem is the brass tap clamp breaking, causing the resistor to go open circuit
Apart from an open/short in the windings, there is not much to go wrong.
BTW, for those in the group who remember my request for help some 9 months ago, the gen set is fully restored and working. It has worked for me twice in the last 6 months during two 6 hour power cuts, so I am very pleased.
It cost me £310 to restore which included a rebore, new pistons, valves, guides, refurbishment of the injectors, gasket set and odds and sods. The alternator needed a new rectifier in the voltage control circuit together with replacement of the 3 wire wound resistors.
I built a modern Automatic Mains Fail unit to control the set and it is now wired into the house supply. A much needed piece of kit in the Petworth area.
James
James