Some more details about what the displays are doing incorrectly would help.
But judging by the fact your displays should probably be showing zero digits instead of fours in your picture, then you have exactly the same problem as here:

https://pinside.com/pinball/forum/topic/scoring-displays-only-showing-4-5-6-7#post-5420181

Did you repin the connector on the MPU? The first digit should always be 0. Looks like you are off by a pin on the digit select wire.

So with your multi-meter set in low resistance mode, do you measure zero ohm continuity from pin 17 at any display connector, to the MPU board on the top leg of resistor R82 (which is near the bottom of the MPU J1 connector)?

This looks to me like the digit select pins are off by one and need to be shifted over.

Where did you measure this HL pulse? Because below that you said that D2 at the BCD segments of MPU connector measured High.

Nope. U20 is responsible for selecting display digits. It doesn't control the display BCD data.

The BCD data comes from U10 PIA "port A" pins - these same "port A" signals also share control features with the lamp driver board and the switch matrix.
First thing to do is disconnect both J2 and J3 (switch harnesses) from the MPU board, and also J4 from the lamp driver board. This isolates the U10 "port A" pin solely to the displays.
Does the problem go away?

Since that BCD display signal also shares function with the lamp driver board and game switches, are all the lamps and switches behaving correctly?

Grab your multi-meter and set it to resistance mode. With the machine off, black meter lead on ground, use your red meter lead to measure the resistance on both the upper and lower legs of resistors R80, R81, R82 and R83 (next to the lower end of the J1 connector).

Sounds like you haven't set the resistance range on your meter properly. I measure between 5k and 15k resistance on various boards depending on the brand of PIA chip used. So if you haven't got an auto-ranging meter set it to 20k resistance range and if no luck, then set it on 200k resistance range.

BTW, you should start your own tech thread and post clear pictures of your MPU board.

Ok, you can see there is a different resistance reading on the problem signal at R82. The problem looks like it's on the board side of that signal, not the MPU J1 connector side.
There are three capacitors on that signal which one might be internally shorting, or the U10 chip itself may have a fault on that signal pin.
That signal goes to the MPU board dip switch block S9 to S16 so the next least invasive test is to make sure all those switches are in the off position to isolate the switches from that signal and redo the resistance checks on the lower legs of resistors R80-83. See if the reading on R82 changes to match the other resistors.
If not pull out the 6821 chip at U10 and again redo the resistance checks on the lower legs of resistors R80-83. If R82 reading is still vastly different the problem will be on one of the capacitors on that signal either C47 or C53. Unsolder one leg of each of these capacitors one at a time to see if the R82 reading changes. Try C53 first then C47.

Please grab your multi-meter and set it to DC voltage.
Measure the voltages at the following chip pins on the lamp driver board during attract mode and report back:
U1 pin 1
U1 pin 2
U1 pin 3
U1 pin 21
U1 pin 22
U1 pin 23
U2 pin 23
U3 pin 23
U4 pin 23

The following image shows how to find pin 1 on the chips. Look for the notch on the end and/or a dot at a corner pin. Pins are numbered anti-clockwise.

4514_PinNumbering.gif

Not sure how but capacitors C47 and C53 have no correlation to the start button.
Has the resistance reading on the lower leg of R82 changed from before?

Ignore that. When I mentioned switching off dip switches S9 to S16, that disconnected any other capacitors.

What happens if you swap U10 and U11? (they are the same spec part)

Find resistors R35 to R38 (they're under the red S33 switch at the top of the board). What do you measure across them? (i.e. meter leads on each leg). R38 is on the PA6 signal, if it measures a lower than the other three resistors, unsolder one leg and lift that leg from the board. Remeasure the resistance across that R38 resistor and report back. Those four resistors are 3.3k ohm resistors.

While you have the board removed from the game take it outdoors in daylight but out of direct sunlight and take some high resolution pictures front and back. When you attach the images here, make sure you select "Original size (no rescaling)" - I need to see as much detail as possible.

Umm, I just reread post #17 and realised I missed something. Resistor R82 is 1.2k ohms spec. But according to your R82 readings, yours measured 3.24k (4.65k - 1.41k). Your R80, 81 and R83 (which I didn't quote) gave good 1.2k readings.

Please confirm the reading across R82 (place meter leads on each leg).

Hook up your logic probe. In attract mode, what does it indicate on the PA6 signal originating at U10 pin 8, then the bottom leg of resistor R38, bottom leg of R82, top leg of R82 and top leg of C47?

That tells me the U10 socket has loose receptors. Lately I have been using this trick to check socket receptors. Using a new faulty chip, I bend a single leg outwards 90 degrees and try placing the bent leg in each location of a socket looking for receptors that don't grab the leg. I've already found a number of bad sockets with one or more loose receptors doing this.

Well, that's already out of spec and note that measurement was in circuit. That resistor is a 5% tolerance part, so at 3.3k spec, a reading above 3.465k is out of spec. I suggest desoldering the lower leg and lifting it out of the board to measure the resistance across it out of circuit.

It's a 3.3k ohm resistor. The other resistors are reading lower because there's other components in circuit putting parallel resistance across them which this signal is missing somewhere.

Do the logic probe checks first before checking the socket receptors.

I didn't mention it above because I was hoping the voltage readings would confirm but I suspect you have a suspect connection on the "Lamp Strobe #1" signal from the MPU board J1 pin 11 to the lamp driver board at J4 pin 13 (Orange-White wire). Note the pin numbers on the lamp driver board are upside down - pin 1 is at the bottom and counts upwards.
An open circuit on that signal can cause a bunch of lamps to come on when they shouldn't. If the problem comes back, re-terminate the crimp terminals with new ones on both ends.

Cheers. The logic probe readings show that you've got activity on U10 pin 8 which is good. There's two traces running from that pin, one goes to the dip switch S9-S16 block then up to the bottom leg of R38, the other trace runs to the top leg of C47. Both of these showed no activity with the signal sitting high (because R38 is a pull-up resistor).

You have a bad pin receptor connection at that U10 socket on pin 8.

You can try the pin receptor test I previously mentioned - do all locations on that socket.

If you don't want to change the socket right now, pull U10 out. Very carefully pry off the plastic part of the U10 socket without damaging the PCB - this will expose all the pin receptors. Compare the one at pin 8 to the others. Carefully bend the receptor back into shape (and any other suspect ones) and then put the socket cover back on including the U10 chip and try it in the game.

See the example pictures here - if you zoom in on the third picture look at the receptor at pin 4 which you can see is squashed in compared to the other receptors:
https://pinside.com/pinball/forum/topic/as-2518-35-continuously-rebooting#post-5400784