From a BR Training Booklet:

AC203 Depot Maintenance

1.	Remove pulley belts
2.	Disconnect battery
3.	Remove fan cover
4.	Remove side inspection cover
5.	Remove flexible conduit cover
6.	Disconnect cables, mark as necessary
7.	Remove brush box assembly - check brush length / condition (0.31in / 7.9mm)
8.	Rotate rotor, visually examine slip rings for defects
9.	Refit brush assembly connect model 8 AVO across F and A terminals rotate alternator motor through one revolution check field resistance Field resistance should be 10 - 15 ohms If high clean slip rings If low alternator to be changed
10.	Replace cables (ensuring correct connections)
11.	Refit conduit cover ensuring cover joint is good
12.	Refit side inspection cover ensuring cover joint is good
13.	Replace fan cover
14.	Replace belts
15.	Reconnect battery
16.	Check output with tong tester

AC203 Fault Finding

1.	Check for missing or slack driving belts
2.	Service as per depot exam
3.	With battery disconnected check fast fuse in regulator box
4.	Check F and A cable connections are in their correct positions from regulator to alternator
5.	Where fitted, disconnect auxiliary supply leads F1 or F2, from the regulator F/A terminal but leave F+A lead in situ
6.	With battery connected connect a proven 24V 2.8W test lamp to regulator between terminals B+A and F+A, lamp should light
7.	Disconnect F2 lead from regulator. Lamp should go out, if not, the regulator requires changing
The above tests determine whether the regulator or alternator field is defective
8.	Start the engine and clip F- to B- and run at fast idle only. Output should not be noted from alternator. If no output detected then the alternator stator or main diode network is defective and the alternator requires changing