Voltage regulators on logic board? (+ diagnostic points definition)

Hi everybody.

My iMac is not starting up. I have a technician guide for a previous model (mid 2010), and following the instructions it could have been a faulty PSU. But it wasn't, unless the new (used) one I purchased has the same exact problem.

Of the 4 diagnostic LEDs on the logic board, only the first lights up. When I push the start button (which was tested and behaves correctly), the fans spins for about half a second, and that is it.

So I tried with the test points on the logic board. Voltage regulator in iMac 27" - screen blackout. meyer gives the definitions for the test points and the expected values. Since mayer doesn't give the points order, I assumed it is the same like in the 2010 iMac manual (I believe it is also a standard for numbering the pins of the integrated circuits): the ground pin, which has a square mark on the board, is number one, and it is at the right end of the upper row of 7 points; beside number 1 there is number two, and below that, on the other row of 7 points, there is number 8, so that number 14 is at the lower left corner, diagonally opposite to number 1.

My test with the multimeter gives a strange result (check with meyer's info). All pins give 0V but:

• pin 4: -0.00V (but LED 1 is on!)
• pin 6: 0.06 V
• pin 11: 11.90 V (but LED 2 is off!)
• pin 13: 3.93 V (the display is not connected)
• pin 14: 0.08 V

Does anybody knows if the test points have changed purpose between 2010 and 2011 iMacs? Or can anybody find a meaning in the funny values I found on my iMac?

In the description of the second test LED, the repair manual for the 2010 27" iMac says: "Indicates that the main logic board has detected proper power from the power supply when the computer is turned on and that logic board voltage regulators are functioning properly"

So: where are the voltage regulators the manual refers to: on the PSU board or on the logic board?

If they are on the PSU... as I wrote before, I already replaced it, so it looks bad. There is a tiny hope that the power harness between PSU and logic board is defective (tomorrow I will take down the logic board and test it), otherwise I will have to change the logic board (and I will have to find a cheap one, otherwise it will not happen...).

If they are on the logic board: can anybody identify them? I am not a solder magician, but maybe it is possible to replace them for little money. And if it is not possible... it looks again bad...

And the behaviour of the fans when attempting to start the machine, does it suggest anything to anybody?

Thanks to everybody.

Yassan

The power harness is fine... Logic boards on eBay from 100 €, not so bad...

July 19th

Also the new used logic board doesn't solve the problem...

I will give also the logic board back (for the graphics card is too late) and sell all the working parts.

Thanks to arbaman for the contribution.

Bye.

Yassan