Start over. I presume you pulled the CPU to add the external battery holder, and thusly disconnected all the connectors.

You've lost the 5V for the game to boot properly is my guess. Did you install a diode on that battery holder?

Follow this thread and start testing fuses and your 5V supply. I would remove all the connectors not needed for boot (solenoids, lamps, switches, sound, displays) and concentrate on getting the mpu to boot. In that process double check that you have all connectors installed in the right places and the ribbon cables oriented correctly.

https://pinside.com/pinball/forum/topic/high-speed-williams-1986-wont-start-up

Also since you messed with the BRs, get your DMM out and verify the correct voltages after the bridges.

The BR for the 5V regulated supply is on the power supply board, not one of the two you modified, so sorry about that, but not likely your issue for boot issues. I didn't have HS schematics in front of me.

As Chris suggested check your voltage at TP1 on the power supply. If you don't have 5V there, move back to TP3 and see if you have 12V. If you don't, check your fuses F5 and F6 on the power supply. If they are good, switch your meter to AC, put one lead on F5 fuse and one lead on F6 fuse and see if you have AC voltage. Check with leads first on one side of fuses then move to other side of fuses. That will also double check fuse continuity.

Report back findings.

Something isn't right. TP3 is probably just labeled 12V on the board by C9 cap. There are three test points there. 12, gnd and-12. Put you black lead on gnd, dc volts on your meter, and measure 5V (tp1), 12V and -12V with your red lead and report. You should not have nearly 10V at your regulated 5V supply. Need to rule out if the 9V was a measurement error.

Yes....thats tp 3 and 4. The middle one should be your gnd reference tp2. With black lead on middle tp2, tp3 should be 12v and tp4 -12v....keeping the black lead in the middle pin, should have 5V on Tp1.

Your 12v off the BR appears fine but between the power supply and MPU you are losing 5V. You need to check TP1 on the POWER Supply to see if the 5V regulated circuit is working. If you have 5V on the power supply but not on the MPU then it could be a cabling connector issue, or a broken trace on the MPU.

TP1 is marked as 1 on the left leg of the ceramic resistor below the upper connector next to the two blue caps.

You can also pull the connector 1j17 on the MPU and with your voltage meter on DC check pins 1,2,3 (all gnds) to pins 4 5.6 (5v) on the connector you pulled off. This is the 5v connector from the power supply.

So black lead on pin 1, red lead on pin 4 should have 5V. Since pins 1,2,3 are tied together and 4,5,6 are tied together, any combo should yield 5V. This would be measured on the connector with the black and grey wires...not on the header on the mpu.

.498 or 4.98? And you got that on the left leg test point of the ceramic resistor on the power supply?

If you only got .498, pull connectors 3J5 and 3J6 on the power supply and measure again. Want to make sure something isn't shorting the 5v supply.

Yes, red lead there, black lead on tp2 (center test point). Meter on DC volts, game powered up, connectors 5 and 6 pulled.

If in this condition you still have 12vdc on tp3, but nothing on tp1, your 5V regulator circuit isn't working.

As a sanity check you can put red lead directly on the ceramic resistor leg next to the test pad. You can also put red lead on the brown disc capacitor to the right of the resistor (below the axial capacitor)....you can get to the solder pads with your meter lead on that disc capacitor...one side will be 0vdc, the other side 5Vdc. Not sure which side is which without inspecting traces but you can check them both. All these measurements are keeping black lead on tp2.

Lastly, when game has been on carefully touch the metal housing of the regulator that's sitting in the center of the giant heat sink. See if it's getting hot. Touch it quickly, don't linger, so you don't burn yourself. It should at least be warm if not hot when operating correctly.

Your 5v regulating section is bad. DO NOT plug the MPU connectors back in until you get this resolved. 10V can do damage to components on the MPU board.

You might try reading through this thread.

https://pinside.com/pinball/forum/topic/system-11-dead-mpu-amp-high-power-supply

He had the same problem. Dont know what your level of comfort is with proceeding with board repairs and deeper troubleshooting. You might see if Chris can repair this PS for you. It's really odd though that it coincided with your other work.

No problem....Chris is the best, so you'll be in good hands.