Ronald Dodd, a member of the HRCA who has had many memorable layouts which he has shown on UTube, has just posted 3 videos on servicing and repairing HD locos. They really are excellent and hands on so you should be able to use them easily to repair any problems that your locos may have. They are

Cleaning and lubricating

http://www.youtube.com/watch?feature=player_detailpage&v=hYyIBWWrsBo

Adjusting brushes and pick ups

http://www.youtube.com/watch?feature=player_detailpage&list=UUBsdZ5pGeFUfk9kyf8XCDww&v=B87x3AqGGGI

Fault finding and non runners

http://www.youtube.com/watch?feature=player_detailpage&list=UUBsdZ5pGeFUfk9kyf8XCDww&v=EN5nlPIObNc

Many thanks to Ronald for posting his videos – very useful indeed