Dead iMac, PSU or Logicboard?

I was called on to diagnose this dead iMac (21.5 inch, EMC 2389). Owner said she was typing something in Excel, and the whole thing just died. She had the hard drive removed so she could get her data off it. She brings it to me and I remove the LCD.

I free up the power supply and all components look OK. I plug it in and LED #1 lights up. I press the switch and there is no response, and LED #1 goes dead. Other LEDS remain dead. So, what do you guys think? PSU or logicboard?

Here's an update: I measured the voltages at the PSU output to the logicboard. With the iMac on standby (just plugged in to AC), pin 4 has 12.12 V (with respect to ground) and pin 6 has 4.0 v. After I press the power button (and diagnostic LED #1 goes dead), pin 4 still has 12.12 V and pin 6 now has 3.6 V. All other pins (12 in number) read 0 or near 0 V whether at standby or after I've pressed the power button. Sounds like PSU problem?

There's a label next to the black power output wiring connector covered by while glue. I have picked off the white glue, and I see 4 labels (12 or 12.1 volts+current rating) that tell what the proper voltage outputs should be. But none show which pins they should read from. So I guess the PSU is bad? Bummer.

Thanks, oldturkey, here are my voltage readings:

With the set on standby, pin 4 reads 12.12 volts and pin 10 shows 4.02 volts. All other pins at the test points read 0. Only LED 1 is on.

after I press the power button, LED 1turns off, the other 3 remain off. Fans don't even twitch. The whole thing is still dead. Now, I get 12 volts from pin 4 and 3.8 volts from pin 10. I still get 0 volts from all other pins.