How to use a multimeter to test Power supply?

My iMac does not start up. I followed the light test to see if the problem is a bad logicboard. But I´m not sure, since the second light shows quickly and goes when I press the power button in the logicboard. So to be sure I want to test the power supply. I have a multimeter taht I´ve been using only to measure batteries and now want to try with the power supply. The problem is that I don´t know how to set it and use it. Sorry for my english

Thank you

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Hi, in mine it is not if the motherboard or the PSU because if I plug in only the fans work, the capacitor are good and are the good Y.

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It is easy to set up and measure the voltage. The power supply is AC resource. You can follow my step to do;

1. Turn to AC mode position on the meter. If you have a multimeter has auto detection function, you don't need to do this step.

2. First, use two probe plug in two holes of the power supply.

3. The monitor will show a value of AC voltage. If the source does not have power. It has not value.

That's it .

Read more function in here.

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The site here gives the voltages your power supply should be showing

http://jimwarholic.com/2008/11/apple-ima...

Your multimeter will want to be set to something similar to those you use for batteries as the PSU outputs 3v-12v DC. On my multimeter I use the 20v DC setting which is the same as I'd use for batteries, but it does vary meter to meter.

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Per Step 12 of this Apple Knowledge Base article: http://support.apple.com/kb/HT2173,

Your power supply has failed (and this is unfortunately a common issue, due to counterfeit capacitors sourced by Apple and many other major manufacturers at the time). No multimeter necessary.

The power supply is available here at iFixit, and you've already done the hardest part!

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Where is the power supply here in iFixit? I only see 2 optical drives

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iMac G5 20" 1.8 GHz Power Supply

or

iMac G5 20" 2 GHz Power Supply

they are basically the same, one has the ALS (Ambient Light Sensor) module, the other doesn't. Your original post doesn't specify which speed G5...

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Thanks david, is the ALS one.

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I have seen many G5 iMacs fail due to both PSU issues as well as capacitor problems. There are two types of caps on the logic board and (like most caps) when they die, they bulge and leak electrolyte. Before checking the PSU, make sure that all of the capacitors look good. They /are/ replaceable if they're bad and you're handy with a soldering iron.

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you will need to conect the black lead of the volt meter to the ground cable pin(Ususaly Black) and one to the colored lead you want to test, but this is very hard to do, because the power supply has to be tricked into thinking it's on, so you need a 470OHM resister across the standby lead(ususally green) and to a ground lead, than test the voltages, the yellow should be around 12V, the Orange around 3.3V, and the red around 5V, the blue will be -12V, but the colors may be different, but that is the standard DC color coding.

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While using a resistor might be technically correct, I've never had a problem with just shorting the GREEN wire to GROUND. I have multiple PSUs for bench testing that have been hardwired this way and have been functioning for years.

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Another power supply component that recently failed on me is the power mosfets located on the bottom of the PS circuit card. There are about 8 ea. of them , 2 for each voltage from 3.3 to 12vdc. Mine had a part number of PHP101NQ. I replaced 3 on my power supply. Found a good sub with a little higher amp rating at Newark.com

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If you want to learn how to use the multimeter effectively, you are certainly at the right place. A multimeter is a three-in-one electrical measuring device. It measures electric currents (amperes), resistance (ohmmeters) and voltages (volts). Using a multimeter in making various measurements is a simple routine provided you are conversant with electrical terms and multimeter features. However, using a multimeter to measure voltage is not the same as how you will use it to measure resistance or the current measurement. Each measurement has its specific settings. Read more

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