Assuming you've done the basics like resetting the SMC/PRAM, probing the power supply could be the next thing to try. I'm having trouble tracking down a pinout diagram for your mode, but it looks like the late 2009 - mid 2011 27" iMacs all use [product|IF174-000|the same power supply|new_window=true], which means that [post|211857|this post|new_window=true] referencing the mid-2011 model should help with the pinout.
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Assuming you've done the basics like resetting the SMC/PRAM, probing the power supply could be the next thing to try. I'm having trouble tracking down a pinout diagram for your model, but it looks like the late 2009 - mid 2011 27" iMacs all use [product|IF174-000|the same power supply|new_window=true], which means that [post|211857|this post|new_window=true] referencing the mid-2011 model should help with the pinout.
It may be hard to probe the power supply rails with the computer running, but I would specifically look out rails that are below or above their specified value (ex: 12V rail reading 11V).
A few other questions that may help debug:
* Are any capacitors on the logic board leaky or buldging?
* Does this machine have its original CPU or has it been upgraded?
* Can you remove / swap around some memory, or install the original memory the computer shipped with? Sometimes these machines are finicky with aftermarket RAM
Assuming you've done the basics like resetting the SMC/PRAM, probing the power supply could be the next thing to try. I'm having trouble tracking down a pinout diagram for your mode, but it looks like the late 2009 - mid 2011 27" iMacs all use [product|IF174-000|the same power supply|new_window=true], which means that [post|211857|this post|new_window=true] referencing the mid-2011 model should help with the pinout.
It may be hard to probe the power supply rails with the computer running, but I would specifically look out rails that are below or above their specified value (ex: 12V rail reading 11V).
A few other questions that may help debug:
* Are any capacitors on the logic board leaky or buldging?
* Does this machine have its original CPU or has it been upgraded?
* Can you remove / swap around some memory, or install the original memory the computer shipped with? Sometimes these machines are finicky with aftermarket RAM