After some more diagnosing I've concluded that there is something shorting out the secondary 115V side. That makes sense with the voltage rapidly dropping and the sound the power supply makes when it cuts into protection.
I measure 20,7 ohms over the 115V filter capacitor, it is not supposed to be that low. I have checked the filter capacitor C635 with an ESR meter and the over voltage protection diode D601 with a multimeter, both are okay.
At 115V and 20,7 ohms, the power draw would be 5,5A or 632.5 watts, definitely not right.
I realized that his has an identical chassis to my Sharp C-3700, so I've been using that as a reference if I find something suspicious.
I have systematically measured most of the 115V circuit and followed wherever the resistance gets lower, and I am at 9,5 ohms at the lowest around the horizontal output transistor Q602.
Removed the transistor and tested it, no issue there.
I have also tested C618 that sits right by the HOT and is referenced to the ground.
Here is a screenshot with my measurements, the numbers show resistance to ground measured over the component that references ground, mostly capacitors:
I have hit a dead end, can't seem to find any resistance lower than 9,5 ohms to the ground.
I have tested most of the components that are referencing ground in the area with the lowest resistance.
What am I missing? Could it be an internal short in the flyback? I hope not.