First Step: Thou shalt check voltages.

[Tinkerer]

Can it get any more simple than the title? When you first start troubleshooting, check the voltages!

This should be stickied.

[mcinque]

True.

[pickle9000]

Before power up.

Eyes, nose first.

See or smell smoke, anything melted?

[Kremmen]

+1
Of course you mean checking to be aware of the voltages present to avoid dying too much or worse, causing additional damage to the D.U.R.
It also serves as a good starting point for the actual troubleshooting as demonstrated in my latest repair effort a few days ago. It was a hefty 230VAC / 24 VDC 1 kW half bridge switcher that had just quit in action.  Initial check quickly showed main 325 V DC bus present. OK, so fingers under tight control. Next check, the PFC driver gets no aux voltage. The boot resistor divider seemed to be OK but there was something strange about it. So chasing further around the board i realized there is actually a separate small flyback converter to provide the aux voltages for the PFC and bridge controllers. Only it was not doing any flying back. Took me a while (no schematic, you see) to locate the flyback input point to the main bus, but finally i found the 2.7 ohm 1206 resistor that taps to the bus to feed the flyback transformer. Only the chip looked a bit suspicious. Sure enough when lifting it from the board, it was open circuit and the microscope showed that the metallization at one terminal was burnt through. Replacing that with a 1/4 W regular TH part restored life right away and that issue won't repeat.

[at2marty]

Before power up.

Eyes, nose first.

See or smell smoke, anything melted?

That was my first thought.  Before I start any kind of voltage checks I like to give as thorough of an inspection that I can.  I also re-seat connectors and/or clean contacts as part of a first step.

[David Hess]

Check the voltages *and* the ripple.

[G0HZU]

I think the generic faultfinding procedure can be enhanced as follows:

Step 1/ Is it really faulty? It could be fully functional but suffering from operator error/misunderstanding.
Step 2/  If it is faulty then try and get as much fault history as possible from previous users
Step 3/ Get some behavioural experience using the equipment under the fault conditions (assuming it isn't on fire)
Step 4/ Read the manual for any technical description and schematics and follow any initial 'operator' troubleshooting guides
Step 5/ Start to take it apart and keep a brief note/log/photo of each part of the process
Step 6/ Look and sniff for anything obvious or loose
Step 7/ Follow the flowcharts in the troubleshooting guide or make up your own if there isn't one.