How would someone come to such a diagnosis?

I have 6RA2632 that's giving me a similar problem, except it's a V96 led and it's constantly lit. Actually if I turn the main disconnect on at the machine after it's been sitting a while and everything is completely cold it won't light, but as soon as I turn the drives on it triggers.

If I leave the mains on, but the controller off that transformer gets really warm. 

I've gone through everything I can think of to verify supply.voltage before and after "drives enabled" is triggered and I see nothing missing. I really am suspecting that transformer but I can't find any data on it to diagnose.

------------------------------------------------------------------------------------------
Split from 6ra2675: V79 phase loss/ reduction led..