Re: Optiplex 755 only turns on through power supply button, no video output.
In this situation, I think I would replace the motherboard. I would say, though, we haven't tried a different power cord. If you have one, I would try connecting the alternative power cord to the system, just to rule out that the power cord is causing this. Also, one other thing, are all the USB ports undamaged? I've seen damaged USB ports causing this issue.
Assuming that it's not the power cord and that there are no damaged USB ports, I think we can say that the motherboard has developed a fault here. Would you like me to find out the cost of a replacement motherboard for you? Please let me know.
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