So let’s get this out of the way first. I know this PC is very old now. I got it for free from a friend and with my “more than beginner” electronics repair knowlege, I would like to learn further and if possible fix this computer.
Diagnosing:
Plugging in the AC cable, it remains OFF. Upon pressing the power button, I can hear fans, HDD and disc reader come to life and remain like that, fans stay at a low speed like during normal operation. No chime, nothing.
After seeing this, I decided to swap RAM, try all the slots with just one stick and swap in between 2 ram sticks for each slot at a time. Nothing. So I knew it was time to take it apart, after removing the screen and finding diagnostic LEDs, after plugging in AC cable, LED1 is green, which is a good sign. After pressing the power button, LED1 stays on but all other LEDs are off. Fans spinning, now going to MAX speed after 15 seconds as it should when unplugging temp sensor on the screen, and everything remains as mentioned before. The fact fans go full speed tells me that motherboard works, but main components aren’t getting proper power to work. This is where I got stuck as I don’t want to disassemble the iMac further before doing some more research. It does react to the power button, shuts down after holding it down and all.
I wasn’t able to find this exact problem anywhere on the internet, so I am asking for your help. From my research it appears as if the power supply switches on but the motherboard does not power on properly, maybe faulty voltage regulators, PSU not sending proper voltage, who knows.
My friend gave it to me thinking the GPU is dead but I don’t think so from the diagnosis to this point. Maybe both, GPU and motherboard are faulty, I am lost as it could be anything with these machines. It seems that the PSU works by the fact that all accessory parts get power but it may just be “lying” to me.
Can anyone provide guidance on triage or repair for this vintage of Mac and described symptoms?