So if you unplug everything except J17, will the board get past that initial LED in the boot sequence?

Can you post a picture of the board?

So if you press the CPU button on the side of the board when powered on, how many LED flashes do you get?

Before you did any work, did the diagnostic or blanking LED come on at all?

Can you swap the U15 and U24 (6802)? I am not familiar with the brand of U15 and like the Hitachi CPU, it might require a jumper change.

And pressing the CPU button results in no flashing LED?

Normally, a PIA failure will keep the blanking LED on. Seems you may have a problem between the CPU, RAM, and game ROMs being both LEDs are off. Do you have a logic probe to check signals?

Any output out of U20 pin 2?

Do you know the EEPROM chips are good for sure? A bad burn will stop it from booting. If you're sure...

I'd check the PIA chips with the board off. Set the multimeter to diode test and put the red lead on the leg of pin one (should have a white dot next to it), then run the black lead down the chip. You should get .4 to .6 reading. If it's 0, that's a short and the chip is probably bad.

I find that strange that you see a low on U20 pin 2 but yet the blanking LED is not lit solid.

Can you probe the U25 and report the pulses and highs/lows you see?

So, bad timing crystal? I don't know what could cause the pulses to accelerate except that.

Ok. I would test to be sure U25 pin 4 and 19 traces are good to the game ROMs. Test from the top part of the pin near the body of the RAM to the pin near the body of both ROMs in case there is a damaged traced under the RAM.

!!B MPU RAM to ROM trace test (resized).png

I was hoping that was where the issue would have been. I think you now need to test lines that go from the RAM/ROMs back to the CPU chip as it seems you're missing a part of a signal.

Did this ever get solved Arnie?
I have a Whirlwind that is having a very similar issue and thought this would be a good post to resurrect.
I have it on the bench with only J17 hooked up for power. Just the 5V led is all that lights up. The reset lines are high but like Arnie, I have a stuck low blanking signal on U20. I also have a working game that I swapped U15,U26 and U27 out of. Those 3 from the dead board are working in the machine and the dead board is still the same so I can rule out the processor and ROMs. I have not done anything else so far and am looking to maybe send it to Chris Hibler but his website says he is 60 days out so thought I would try my luck a little longer.
The machine this came out of had a shorted BR1 on the power supply which I repaired but still would not boot so I pulled it out and put it on the bench. It looks clean with no acid damage and the only thing I can see is a previous repair where U17 and Q6 have been changed. Those are in the solenoid driver circuit so I don't think this is a problem. I am looking in the area of the 555 timer at U43. The blanking signal comes out of there at pin 3 and 7 and should be high. When I probe U43 pin2 trigger on my working board the signal is strobing but on the dead board it is showing high steady 3.3 volts. That line is tied to U51 pin 4 and U44 pin 21 and I am wondering if one of those could have failed or if there is something else in that trigger circuit like Q50 or C58 that I should focus on. Looking for some suggestions to try or maybe somewhere else reasonable to ship to with a shorter turnaround. Thanks.

Some more notes and ramblings -
I tried piggybacking another 555 on U43 - no help.
From there, I downloaded the test rom and was able to get it to run as instructed here - https://www.pinwiki.com/wiki/index.php?title=Leon_Borre_Williams_System_11_Repair
I set it up exactly as described. All the PIA outputs toggled as they should. The blanking signal
flashes high very briefly as the program is running and the k1 relay toggles in sync with my probe. The document gets kind of confusing beyond the PIA testing as far as checking the address lines and such so I figured I would stop here. Still open for thoughts and suggestions.