From a BR training booklet:

AC8 Depot Maintenance

1.	Release adjustment and remove pulley belts
2.	Remove junction box cover at alternator
3.	Disconnect rotor field circuits cables
4.	Remove inspection cover
5.	Visually examine condition and length of brushes and check for free movement of brushes in brush box holder (less than 0.75in / 19mm)
6.	Rotate alternator visually checking slip rings for grooves and burns
7.	Connect AVO to field circuit rotate rotor through one revolution and measure field resistance. Field resistance should be 16-20 ohms If high clean slip rings If low alternator to be changed Check bearings
8.	Reconnect rotor field cables, replace cover
9.	Replace inspection cover ensuring cover joint is in good condition
10.	Replace belts on pulley and tension correctly
11.	Check output with tong tester

Fault Finding AC8 / RUG 11A / 14A

1.	Check for missing or slack belts
2.	Service as per depot exam
3.	Remove AC connection covers, start both engines, run at idling speed, use a proven 24 volt x 2.8W test lamp, connect between terminals F+ and F- to check rotor field circuit supply. Lamp should light, if no light RUG unit should be changed (no self exciting circuit)
4.	Connect test lamp between AC outputs, if lamp lights but no charging output RUG defective (main rectifier defective)