So, you're telling me I need to take the PSU out of its case and check ESR on the major caps? 
Well, I did ask for advice and suggestions.
For accurate ESR reading wouldn't I have to desolder the caps? Or can I get away with just putting a multimeter across them and watching the resistance climb and fall?
BTW my Sony PSU is similar, but not identical to the one pictured in the other post. Some of the caps in that one are placed differently, and the ramp-up circuit seems to use discrete transistors rather than an IC, so not sure I can use the pictures for exact reference.
In testing since my original post, I have had the unit boot up a few times with two different logic boards. Sometimes it will cold boot properly using the reset button - ie. not immediately on AC power. Other times it boots immediately on AC power. It will restart properly from MacOs software instruction, but does not recognise the keyboard or cabinet back button hardware resets. To me, that still points to the the soft power system and mostly likely that part of it in the PSU, unless both of my logic boards have the same fault in them. It could relate to the availability of current in particular caps at particular times in the boot sequence, so I will try to check the PSU caps. The Sony PSU seems to have about 15 of different values
Rick
Well, I did ask for advice and suggestions.
For accurate ESR reading wouldn't I have to desolder the caps? Or can I get away with just putting a multimeter across them and watching the resistance climb and fall?
BTW my Sony PSU is similar, but not identical to the one pictured in the other post. Some of the caps in that one are placed differently, and the ramp-up circuit seems to use discrete transistors rather than an IC, so not sure I can use the pictures for exact reference.
In testing since my original post, I have had the unit boot up a few times with two different logic boards. Sometimes it will cold boot properly using the reset button - ie. not immediately on AC power. Other times it boots immediately on AC power. It will restart properly from MacOs software instruction, but does not recognise the keyboard or cabinet back button hardware resets. To me, that still points to the the soft power system and mostly likely that part of it in the PSU, unless both of my logic boards have the same fault in them. It could relate to the availability of current in particular caps at particular times in the boot sequence, so I will try to check the PSU caps. The Sony PSU seems to have about 15 of different values
Rick