Mystery power issue with iMac

elliero · Oct 9, 2015

I have a really strange problem and I was wondering if anyone has come across it before or had any theories as to the cause.

My iMac will not turn on in my flat, or my building, but will elsewhere. I get no response whatsoever - no chimes, beeps, whirring, lights - nothing.

History
I had been having a few troubles with the mac since my friend unplugged it while it was booted up (end of August) and two days later my flatmate switched off the entire power supply without warning me to change a lightbulb (don't ask). After these two incidents, I sometimes had to leave it plugged in for 20-30 mins before it would power up. But it always eventually did and it was working OK generally.

I decided to upgrade the RAM to help speed it up. Once I'd seated the new RAM it wouldn't turn on. I spent hours trying this in various permutations but it just refused to turn on, even if I returned to the original RAM. Thinking I'd messed up the seating, I took it to a repair shop. It turned on straight away and they tested the new RAM to make sure it was OK (c. 6 hours and 3 passes of testing).

I took it back home, it wouldn't switch on. I took it back to the shop, it turned on. Back to mine, no joy.

Attempted solutions so far
I've tried different three power cords (including the one that was used in the repair shop), every socket in the flat and two different ones at the reception area of my building. I've left it plugged in overnight. I've reset the SMC. I've checked that the power supply is definitely connected. I've switched off all other sockets on the circuit. I've done it direct into the sockets and via an extension cord.

I would have thought there was a clear electrical issue, but we happened to have had an electrician in testing the circuits in the flat, on the day the Mac was in the repair shop. He said the voltage supply from the sockets is all standard and correct. I can't work it out for the life of me. If the Mac didn't work elsewhere I'd wave goodbye to it. But it just doesn't seem to work in this building!

In summary, this is an iMac that doesn't appear to be broken, using electrical circuits that appear to be working fine, but it still doesn't work! If anyone has any ideas I'd be really appreciative!

Darkwood · Oct 9, 2015

Hi welcome to the forum

ifindoubtdont · Oct 9, 2015

This might sound like a weird response, but I get something similar with one of my mobiles, in that in some parts of the house nothing and in other parts fine, outside never a problem. I call it the "Faraday Cage Syndrome". It turns out that some of the RSJ's in the house are blocking signals. Have you tried plugging in to an extension lead and taking it outside. If it works you may have found your problem. Granted it doesn't answer the question of why it worked before and not now.

Darkwood · Oct 9, 2015

I had it with a light fitting switching a laptop off whenever you operated it... it would seem RFI plays a part and induces a signal into computers delicate circuits and mimics a shutdown command, this could be piggy backing in the power supply or just picked up from the location.

Lucien Nunes · Oct 9, 2015

I sometimes had to leave it plugged in for 20-30 mins before it would power up.

it was working OK generally

Classic symptoms of a failing power supply unit in the computer, which often manifests as unreliable starting but the computer itself works fine when it does. The PSU might work at some voltages but not others - your flat could be at the low end of normal and the other locations at the high end, which would make no difference to a fully working machine but is the last straw for yours.

The PSU is a separate module within the machine that can easily be exchanged. It is just possible that another hardware problem elsewhere in the computer is affecting the PSU operation, but seems unlikely.

shanky887614 · Oct 10, 2015

Lucien Nunes said:
Classic symptoms of a failing power supply unit in the computer, which often manifests as unreliable starting but the computer itself works fine when it does. The PSU might work at some voltages but not others - your flat could be at the low end of normal and the other locations at the high end, which would make no difference to a fully working machine but is the last straw for yours.

The PSU is a separate module within the machine that can easily be exchanged. It is just possible that another hardware problem elsewhere in the computer is affecting the PSU operation, but seems unlikely.

i think your right.

a lot of old power supplies had two voltage ratings for arguments sake lets say

230 and 120

now there used to be a switch to select between them, now a lot of power supplies can take any voltage from say 90-250v and there quite happy.

have a look to see if anything is bulging in the psu then put your nose to the psu and give it a sniff, if you can smell tcp/similar its the psu failing.

have you tried take the psu out and shorting the pins to turn it on? the poweron pin is one of the ones in the 24pin connector (big one on motherboard)