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If the power supply is ok it will start up without the main board connected - Obviously without the main connected the LED will not come on.
If the power supply does not start then you either have a bad power supply or one of your sustain boards is shorted and that is causing the power supply to shut down.
Apply power with only AC in connected to the power supply. If it is ok it will start up and you can measure the voltages. If all the voltages are where they should be then your power supply is probably ok.
Connect the power to the sustain boards one at a time and repeat the test.
EXT_AUTO_GEN is correct. Short these points (I just poke a piece of wire in the holes) and the controller will generate various test patterns.
It is the main chip under the heat sink. I remove the heat sink, apply liquid flux around the edges and heat it with the heat gun for a few minutes. Then I let it cool for a while and hook it up again. Success rate so far is 7 out of 10. If they start up I leave them running all day/night. All the ones that were still working the next day went out to customers and didn't come back. Those that failed did so pretty quickly.
That said, there is no point heating the main board if you can't get it to power up and make test patterns.
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