2014 iMac 27"

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Problem Solution
Single beep on boot The single beep on boot is caused by a shorted capacitor on the headphone jack. Disconnect the headphone jack and test to see if beep is gone. Remove headphone jack (it is double sided taped on) and test caps for continuity, remove or replace shorted cap. I do not know the value of the cap, nor did I photograph it, but there are only 5 caps on the board and the faulty one was slightly discoloured.
LED #1 powers on, when power button is pressed it blinks only without life I had this case and it was missing PM_SLP_S4_L. A bad resistor was acting weird (below average) was replaced and issue was fixed. It was R1215 powering PCH_DSWVRMEN.

Can also occur with a dead or below threshold CMOS battery based on the schematics it should be 3.3v.
RTC Power Supply A1419.png