iMac capacitor replacement didn't help...

i recently bought a imac g5 20" 2.0 GHz. the guy told me that he took it to a local repair shop and they said it had a bad logic board. when i got it home i plugged it in and pushed the power button. the fans started racing and it beeped (no ram). when i opened the cover i immediately noticed 10 bulging 6.3v 1800 uf caps. i powered it on with the cover off and only the first two LED's were lit, no video LED. i ordered new caps for it and decided to replace all of them. i went to radioshack before the caps came in and bought a brand new desoldering and soldering irons. now i have a lot of soldering experience and i made sure i took my time to replace the caps neatly and made sure i didn't burn up the seats. now after being very excited about powering this thing up all i got was fans running, DVD drive spinning up, no video and the same two LED's on. the only thing different is that it doesn't beep, i have never got a chime out of this thing. i checked the battery and it has 2.95v so im sure thats not the problem. i tested the power supply and it checked out. i have tried a dozen different combinations of plugging and unplugging components. what is the next step in diagnostics to take, or maybe im overlooking something... please let me know!!!

iMac G5 20" 2.0 GHz, Mac OS X (10.5.8)

Posted on Aug 24, 2010 9:18 PM

Reply
4 replies

Aug 24, 2010 11:05 PM in response to Kozman16

Hey Kozman,
checked the battery and it has 2.95v

Hmm mine all read above 3.0 Volts usually 3.09 V.
i tested the power supply and it checked out.

How did you test the supply?
Did you reset the SMU? (from the Service Source)
"The SMU supports the following functions
:
• provides initial configuration of system clocks
• automatically initiates and sequences voltage and frequency slews when the OS
requests a transition to a new run level
• handles all power button and environmental user events
• controls RPM fans
• provides processor Mode Ring and thermal data store
• monitors processor temperature and power consumption
• monitors VRD10-compliant processor power supply digital interface.
The iMac G5 system employs advanced thermal and wattage management to keep
acoustic noise to a minimum. The enclosure is divided into discrete zones, each with
independently controlled fans bringing in cool air from the front of the enclosure, directing
it over system components and exhausting it out the rear. Temperature and power
consumption are monitored by the operating system which communicates with the SMU,
which in turn controls and monitors fan operation. If Mac OS X is not booted, thermal
management must be provided by the alternate development operating system.
Note:
If Mac OS X is not booted and the alternate development operating system does not
manage the fans, the fans go into an unmanaged state and run at full speed.
The SMU controls the fans and regulates the speeds to run each fan. The SMU derives
fan speed from sensors in each thermal zone. Some of these sensors monitor thermal
only, while others monitor both thermal and power.
If the SMU does not receive an update from the operating system within two minutes, it
begins to ramp up the speed of the fans to full speed.
Resetting the SMU (System Management Unit)
1. Unplug all cables from the computer, including the power cord.
2. Wait 10 seconds.
3. Plug in the power cord while simultaneously pressing and holding the power button on
the back of the computer.
4. Let go of the power button.
5. Press the power button once more to start up your iMac G5."
Here's the part about the LEDs,
"•
LED 1
indicates that the trickle voltage from the power supply has been detected
by the main logic board. This LED will remain ON whenever the iMac G5 is
connected to a working AC power source. The LED will remain on even when the
computer has been shut down or put to sleep. The LED will turn off only if the AC
power source is disconnected or the power supply is faulty.

LED 2
indicates that the main logic board has detected proper power from the
power supply when the computer is turned on. This LED will be ON when the
computer is turned on and the power supply is working correctly.

LED 3
indicates that the computer and the LCD display are communicating. This
LED will be ON when the computer is turned on and video signal is being
generated.

LED 4
lights only if the computer detects an over-temperature condition. This LED
will be OFF when the computer is turned on and running at the correct
temperature."
Richard

Aug 25, 2010 6:41 PM in response to spudnuty

thanks for all the info. richard!!! but yes i did reset the SMU and nothing. to test the power supply i jumped the power on pin to a ground. from there i measured the voltages across the P1 connector. all voltages came back normal according to the specs. i opened up the power supply and there wasn't any bad caps either. so its back to trying to figure this thing out!

Aug 25, 2010 8:55 PM in response to Kozman16

Kozman,
all voltages came back normal according to the specs

Hmm what about when it's under load. I have a moded ATX power supply that I drop in as a PSU verifier. I'm working on a couple of 20" 2.0s w/ bad power supplies and over 12 bad caps. I just moved and packed that ATX somewhere?
I have two working 17" PSUs and I'm considering dropping one in this 20" to verify the repair.
Richard

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iMac capacitor replacement didn't help...

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