REQUEST FOR COMMUNITY HELP HERE

Upon searching, I've found several posts about various system 11 MPU boot fails but they all seem to fizzle with no resolution. So here goes with my first post looking for community help.

I recently restored my Whirlwind, including repainting backbox and inside cabinet. This means I removed all cables and electronics from both.

Overall, the boards are very clean and appear to me to be unmolested. They were working fine before the tear-down. Installed is an X-Pin power supply board, all others are original Willams with matching serial numbers.

Upon reinstallation, and power-up:

All GI came, single bong from the speaker, 5V LED came solid, diagnostic LED blinked for about 60 seconds and blanking remained off.

After a little Pinside sleuthing, I unplugged all MPU cables except 1J17 and began testing voltages. Note that with the MPU board isolated, if I power on after a long time off, the diagnostic LED does not blink. If I cycle power (say after 2-3 sec), I get the diagnostic blinking.

Here are my readings:
GND to Ground Strap = 5Ω
TP2 = 5.13V
+12V = 12.05V
-12V = -12.65V

U15 pin 8 = 5.11V
U26 pin 28 = ~1.9-2.6V <-- this is bouncing around a tad
U27 pin 28 = ~1.9-2.6V <-- this is bouncing around a tad
U10 pin 20 = 5.12V
U38 pin 20 = 5.12V
U41 pin 20 = 5.12V
U42 pin 20 = 5.12V
U51 pin 20 = 5.10V
U54 pin 20 = 5.10V

I've got at my disposal a multimeter and logic probe (though honestly never used the probe but I not scared to learn).

Attached are photos of the board. Let me know if the above readings tell us anything or what my next step might be.

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No. on power cycle it blinks 60 times then goes off and stays off. As of now, pressing SW2 produces the same results.

Resolved. While pressing on socketed chips to see if there was a possible physical connection issue, I could get the blanking LED to come on sporadically.

Decided to remove the MPU from the game to remove and reseat the chips and get some photos of the back side.

When I flipped the board over, a staple fell onto my table. Looking back at the photos reveals it was shorting pins 2 and 3 on U16 and possibly to a leg of a capacitor as well.

Decided to put the board back in the head without reseating anything and it booted up, blanking LED and all. Glad it didn't let any of the smoke out.

Can anyone chime in with the function of U16 and speculate if it's failure would cause my symptoms.

Moral of the story... dismiss the unexpected at your own peril.

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