Power On, fans spin up, power light on 1 second then off

I recently acquired a Power Mac G4 1.25 MDD 2003 era machine. Top-notch cosmetics and I had plans for good Mac times here. But, instead my "new" MDD is suffering from this problem: 1) Attach power cable & DVI connection to Samsung LCD display & USB connection to Apple brand full size keyboard and the optical mouse; 2) Touch the power button above the mirror drive doors; 3) the power button illuminates for just 1 second and then immediately goes back off; 4) Fans spin up - low hum at first, and then the full "wind tunnel" effect; 5) no image appears on the LCD display, and the G4 does not respond to any keyboard commands; 6) I have let this G4 run for several minutes, but no change in the power light status and no responsiveness / output. I have reset the PMU, and did perform a full upgrade of RAM to 2GB OWC 333 MHz and dropped in a fresh PRAM battery. I have reseated the NVIDIA AGP graphics card. Looks like a trip to the 1 local Apple Specialist in a large Midwest metro is on tap.

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Dave Weaver, looks like a bad processor or logic board. here is what apple says:"

Computer begins to power up, the fan and hard drive are spinning, the power

LED is lit (or the LED goes off when button is no longer pressed), but there is

no startup chime or video

1. Reset the logic board. Refer to “Resetting the PMU on the Logic Board” in this

chapter.

2. Verify all PCI and AGP cards are seated properly.

3. Reseat the processor module.

4. Replace the processor module. Refer to "Before Replacing the Processor" in this

chapter.

5. Replace the logic board.

Update (09/25/2015)

Let's see if this will help you out a bit more

Power Supply Verification

To power on, the computer’s logic board requires a “trickle” power of +5V. If the system fails to power on, first reset the PMU. Then follow the procedure outlined below to determine whether the problem is related to the power supply.

Note:

To verify the power supply, you need a volt meter. When connecting the volt meter leads to specific pins, make sure the power supply remains securely plugged into its connector on the logic board.

The following figure shows the pins and voltage on the computer’s power supply connector. (The color noted represents the color of the wire attached to the pin.)

Block Image

1. Plug a known-good power cord into the computer. Do not turn on the computer.

2. Connect the black lead of the volt meter to pin 12 of the power supply connector; connect the red lead of the volt meter to pin 14. The volt meter should measure approximately +25V. If you do not get a reading of +25V, recheck the volt meter connections and measure the voltage again. If voltage is still not present, replace the power supply. If you do measure +25V on pin 14, the power supply is likely OK. Go to the next step for further verification.

3. Check the trickle voltage by connecting the black lead of the volt meter to pin 12 of the power supply connector; connect the red lead of the volt meter to pin 1. The volt meter should measure approximately +5V. If you do not get a reading of +5V, recheck the volt meter connections and measure the voltage again. If voltage is still not present, replace the power supply. If you do measure +5V on pin 1, the power supply is likely OK. Go to the next step for further verification.

4. Start up the computer by pressing the power button on the front panel. If the computer starts up normally, the power supply is OK. If the computer does not start up normally, go to the next step.

5. Check to see if the power supply fan is spinning. If the fan is not spinning, replace the power supply. If the fan is spinning, go to the next step.

6. Connect the black lead of the volt meter to pin 12 of the power supply connector; connect the red lead of the volt meter to pin 24. The volt meter should measure approximately +12V. If you do not get a reading of +12V, recheck the volt meter connections and measure the voltage again. If voltage is still not present, replace the power supply. If you do measure +12V on pin 24, the power supply is likely OK. Go to the next step for further verification.

7. Connect the black lead of the volt meter to pin 12 of the power supply connector; connect the red lead to pin 6. The volt meter should measure approximately +3.3V. If you do not get a reading of +3.3V, recheck the volt meter connections and measure the voltage again. If voltage is still not present, replace the power supply. If you do measure +3.3V on pin 6, the power supply is likely OK. Go to the next step for further verification.

8. Connect the black lead of the volt meter to pin 12 of the power supply connector; connect the red lead to pin 3. The volt meter should measure approximately +5V. If you do not get a reading of +5V, recheck the volt meter connections and measure the voltage again. If voltage is still not present, replace the power supply. If you do measure +5V on pin 3, the power supply is likely OK. Go to the next step for further verification.

9. Connect the black lead of the volt meter to pin 12 of the power supply connector; connect the red lead to pin 10. The volt meter should measure approximately +12V. If you do not get a reading of +12V, recheck the volt meter connections and measure the voltage again. If voltage is still not present, replace the power supply. If you do measure +12V on pin 10, the power supply is OK and the testing is complete. You have verified that the power supply is not the cause of the “no apparent power” symptom..

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Comments:

Thanks for the very helpful response ! Symptoms match EXACTLY my problem with this MDD 1.25 GHz = power button does not remain lit after touch, no startup chime and no video output. So, it's off to the Apple Specialist. Best regards, DW

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Best of luck and let us know how it turns out.

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Thanks again for the super helpful response here. I carted my 'new' 1.25 MDD 400FireWire tower off to the 1 independent local Apple Specialist.

The young Tech told me:

" Well, I'm pretty sure it's the Power Supply. "

" And, then in the next breath -- BUT it COULD BE the logic board. "

" I've only worked on a couple of these MDD's and I won't be much help with diagnostics here. "

I have actually tracked down a certified used MDD 1.25 logic board and separate daughtercard processor assembly. I'd really like to check condition of the Power Supply before springing for the board and processor assembly.

So, my next step is scouting out any tips on checking G4 MDD power supply operation. Hmm, more searching on iFixit :-) :-)

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I have this problem too and have verified it is the power supply.

What is the part number for the power supply and where can I get one?

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Dave Weaver will be eternally grateful.
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