820-00134: Not powering up. LED 1 blinking

Have this iMac 27” (haven’t done much work on these) that isn’t powering up. Led 1 is blinking, so Swapped power supply with same outcome. Led1 is solid with cpu removed, so replaced the cpu with the same outcome. Note, when the power button is pressed, led1 blinks a lot faster. Any suggestions on where to start? Yep
 
Ordered the transistors. While I wait, checked a few readings, is it normal for PP1V0_S5_REG to measure at 22 ohms and drop down to almost zero when i place a hot soldering iron on top of U1100
 
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