Hi all.
I have an ongoing issue with a 2010 A1311 21.5" iMac. It starts up, chimes, lights up and boots to the OS, however the screen starts to lightly flicker, steadily getting worse before switching off altogether. Measuring the voltages reveals that the 12v rail drops to around 10.5v once the Mac is switched on.
Until yesterday, I wasn't sure if the problem lay with the PSU, the backlight inverter or the logic board.
Yesterday, however, I borrowed a good PSU and the iMac works fine.
Hence my opening a new thread within this troublshooting section.
Searching online, I could find only one other person with this problem, on the iFixit site. He then managed to trace the fault back to a shorted Mosfet on the output of his PSU and resolved the problem.
This is where I am looking for some advice.
It seems reasonable to assume considering the identical symptoms and model iMacs that the problem should be similar, or at least related.
It has become clear to me that I don't have particularly good diagnosis skils when it comes to good or bad mosfets. Mosfets are not something I've much experience with but this is how we learn.
The fella in iFixit had an easily identifiable short. The PSU here doesn't.
I have pulled three of the five fets on the board. All three read a mosfets with a Chinese T7 component tester. Two of them switch on and off fine, when using the DMM in diode mode. One doesn't (although it does show as N channel Fet with the T7).
So my first question is, "which test is more reliable? Whether the Fet turns on and off with a DMM, or the Chinese tester?"
The Fet in question is a P15N60C3 and Drain/Source either shows 'open circuit' or the reading steadily climbs from just above zero (maybe 20) rising up to the 'OL'
Datasheet is here;
https://pdf1.alldatasheet.com/datash...PI15N60C3.html
I have an ongoing issue with a 2010 A1311 21.5" iMac. It starts up, chimes, lights up and boots to the OS, however the screen starts to lightly flicker, steadily getting worse before switching off altogether. Measuring the voltages reveals that the 12v rail drops to around 10.5v once the Mac is switched on.
Until yesterday, I wasn't sure if the problem lay with the PSU, the backlight inverter or the logic board.
Yesterday, however, I borrowed a good PSU and the iMac works fine.
Hence my opening a new thread within this troublshooting section.
Searching online, I could find only one other person with this problem, on the iFixit site. He then managed to trace the fault back to a shorted Mosfet on the output of his PSU and resolved the problem.
This is where I am looking for some advice.
It seems reasonable to assume considering the identical symptoms and model iMacs that the problem should be similar, or at least related.
It has become clear to me that I don't have particularly good diagnosis skils when it comes to good or bad mosfets. Mosfets are not something I've much experience with but this is how we learn.
The fella in iFixit had an easily identifiable short. The PSU here doesn't.
I have pulled three of the five fets on the board. All three read a mosfets with a Chinese T7 component tester. Two of them switch on and off fine, when using the DMM in diode mode. One doesn't (although it does show as N channel Fet with the T7).
So my first question is, "which test is more reliable? Whether the Fet turns on and off with a DMM, or the Chinese tester?"
The Fet in question is a P15N60C3 and Drain/Source either shows 'open circuit' or the reading steadily climbs from just above zero (maybe 20) rising up to the 'OL'
Datasheet is here;
https://pdf1.alldatasheet.com/datash...PI15N60C3.html
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