No chime, no startup, no screen but fans and power-LED is on

I have a nice iMac G5 (iSight) on my desk but it doesn't run...

The system does the following:

After pluging in the power-cord diagnostic LED 1 lights on.

After switching it on the following occurs:

1. NO (!!!) start-chime
2. Diagnostic LED 3 flashes very shortly (10ms)
3. Diagnostic LED 2 flashes very shortly after LED 3
4. the right fan runs normal
5. the Power-LED (which is see through the case) is on
6. the optical drive goes in begin state
7. the HD sprins up and goes in begin state

The system turns exactly after 1 minute all fans in full blow mode...

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What did I have checked?

1. the main powersupply:

- 12V is OK and stable

- 3,3V is OK and stable

2. the other voltages are produced on a combo-board, with the inverter for teh TFT.

I measured the following voltages and they correspond to the printed voltages on the mainboard:

• 3,3V all

• 3,3V run

• 5V all

• 5V run

• 12V all

• 12V run

But there are two lines in the plug on position 1 and 8 (white and gray cables) there is no voltage and they are not GND!

Does anyone can say, what these two cables to have been supposed?

I have numbered the cables of the plug in this way:

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| 01 | 02 | 03  | 04  | 05  | 06 | 07 |

_________________________

| 08 | 09 | 10 | 11 | 12 | 13 | 14 |

_________________________

When you lay down the open iMac with the top towards you and take a look to the cables then the white and the gray cable should be at the left side...

Btw. I've just changed all the ELCOs on the mainboard with Low-ESR-Elcos, but it is the same problem with the iMac!!!

Best regards

Georg