Debugging the LEDs

A friend of mine had installed some LED lighting -- eight strips of them if I recall correctly. Two of the strips were working fine but the remainder were a lot dimmer and had a flicker to them.Why?I went over with a complete overkill of test gear. A couple of multimeters (an Agilent and a Fluke) (why two? to measure voltage and current at the same time, of course) and a bench power supply to double-check the LED strips if we got to that point. Each strip draws a nominal 5A at 12V so I had a spool of thick wire to minimize the voltage drop.From all my reading, watching, and everything else, I know the first thing to check is voltages. Sure enough, V+ to ground was reading somewhere around 9V. This is a 12V power supply that's rated at 15A driving three strings of LEDs so it should be at it's specced 12V. That's a problem. We checked the other one and it had the same issue. Even driving one string of LEDs the voltage was too low. With no load it did read a proper 12V.At this point I should have been more suspicious, but I wasn't. My bad. The chances of two units having the same issue is pretty rare.I checked the 10A supply that was driving the two strings and sure enough that was at 12V -- bright lights.We pulled the supply off the ceiling about to tear into it.Then I noticed a sticker.Yup. RTFM.Hidden under the shielding (but still kind of accessible) is a switch.It was set to 220V.<facepalm/>Flip the switch and all is well. A proper 12V is being supplied to the load and all is happy.I couldn't get a picture of the switch... it's way to hard to see. I can tell you that it does feel mushy though.I packed up all the gear. All the gear I didn't use.All that got used in anger was the Agilent U1273AX meter. The rest enviously looked on.So, yes, the thing to do is check voltages. Both at the terminal -- and the switch. (All debugging comes down to verifying assumptions. If the assumptions were right, you wouldn't be debugging, right?)