How do I go about fixing a Schumacher 10/40/100/200 charger for 12v and 6v auto batteries?

The cooling fan hums when unit is supposed to charge, so that has power. Which components do I check first and how?

Can't find any references to this unit. Is it fairly old ? Heavy ? Checked any fuses you can see ? Have you got a voltmeter to start poking about ?