Diagnostic LED 1 blinks once

so i’ve recently been given a Mid 2010 27” iMac that wasn’t powering on. Initially i thought i must most probably be the Power supply that had croaked it and needed to be replaced because when i plug in the power the diagnostic LED 1 flashes once then once again but not as bright that second time, as if it was loosing power. No other LEDs are lit.

I have replaced the PSU with a known working unit but still getting the same thing.

im hoping its not the logic board that has an issue or a short on it,

is there any chance it would be the jack/EMI unit where the power goes into the imac?