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Model A1311 / Mid 2011 / 2.5 & 2.7 GHz Core i5 or 2.8 GHz Core i7 Processor

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PSU probably dead, please help diagnose if logic board is working.

Hello everyone,

my iMac has died this morning and caused short circuit in my home's power installation. First, I thought that a power surge has blown out the PSU in the iMac, but it turns out it was the other way round. Upon inspection of the iMac's PSU with multimeter I discovered that the 250V 6.3A fuse in the PSU was blown, and, thinking that the power surge was the cause - I've soldered new fuse and put the iMac together. When I plugged the power cable to the wall outlet (iMac turned off) - fuses in my home power installation had blown again, and this indicates that the iMac caused the first power surge as well.

I have taken the iMac apart again - the new fuse in the PSU doesn't seem to be blown (I get a reading of few mOhms in it) and voltage reading between fuse legs is 240V, but the computer is still not starting. Connecting it to wall outlet doesn't cause short circuits anymore. I have checked voltages on the PSU output - there is no 12V line. There are two pins that read 1.85V between them and GND pin.

I have checked most diodes and capacitors and they seem to be working fine when desoldered from the PSU, but when they are connected to the circuit - some of them seem read short circuits on my multimeter.

Is there any way to check if it's the PSU that has shorted out in the first place? Is it possible that something has blown up on the logic board and caused a short that would damage the PSU as well?

Can I somehow check for shorts by testing pins on the cable that goes from PSU to the logic board? Does someone know the correct pinout of this cable?

I would appreciate any help!

Answer this question I have this problem too

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Mateusz Hajdziony, explain this "doesn't seem to be blown (I get a reading of few mOhms in it) and voltage reading between fuse legs is 240V" a bit better. You are getting 240 V exactly where on the fuse? You will not have to worry about measuring resistance of fuse, only check for continuity. Do you have continuity? How did you check the capacitors? What type of multimeter did you use? Are you using a capacitance meter for the measuring of the components ones they are on the logic board? What part number is your PSU?

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I changed this to an answer because it will probably be the definitive response to this question.

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Hello, thanks for your response. Yes, there is continuity (the fuse is good). Sorry for the confusion!

I don't have a capacitance meter but I'm measuring resistance and you can clearly seen the capacitor being charged with current from the multimeter and then their resistance raising. I know it's not a definitive method of measuring if caps are OK, but that's the only I have and I can at least check if a capacitor has caused a short circuit in the first place.

The PSU is ADP-200DFB. It would be great if I could repair it myself, but I'm pretty sure I'll have to buy a new one. I just want to make sure that the logic board is not shorted - I don't want to blow up new PSU.

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UPDATE: I have checked the main PSU fuse again and it's dead now. That is strange because I have not connected the PSU to any power source since the last time I have measured that it's working.

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Please check and measure off board of component mosfet transistor (20n60xx or equivalent) and diode hyperfast on primary scheme psu. Browse googling how to test this mosfet transistor with multimeter.

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Remember, when you connect psu to AC,make sure if all component on psu board must completed, soldering again all componen that has removed from ps board. Normal or not all component must on it place.

Sorry if my english not good.

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Mateusz Hajdziony will be eternally grateful.
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