It's time to pull out your multi-meter and measure the voltages on the test points of the rectifier board, then on the solenoid driver board in the back box.

If the voltages on the MPU board measure in spec as you say and the self test LED stays stuck on, then your MPU board is not starting up.

What happens when you press the reset switch on the MPU board?

Measure the voltage at TP5 then TP1 on the solenoid driver board and report the exact readings.

What about the voltage measurements on the solenoid driver board?

Indeed, the 11.5V at TP5 suggests you've likely got a bad connection at (and/or faulty) capacitor C23 on the solenoid driver board. It's the big blue capacitor to the left of the large square heatsink. You should be reading between 14.5V - 16.5V at TP5 (ignore the schematic saying 11.9VDC)

Measure the voltage across the C23 capacitor with the meter leads connected as per the picture below. If you get zero volts, you have a bad connection to the capacitor. Check the two red wires to it and on the back of the board are well soldered - resolder if they look bad/cracked. More likely though it will be a bad connection at the J3 connector pin 10 (red wire) on the solenoid driver board whose whole purpose is to provide ground to that C23 capacitor. You can do a ground mod on the back of the solenoid driver board to overcome this but let's see what you measure first.

If you measure 11.5V across that capacitor (less likely in this case), then that capacitor is faulty (dead).

I went into a discussion on a similar electrical issue with someone else yesterday if you want to read the nitty gritty - that post relates to the high voltage for the displays but same principal here and the voltages listed in the diagram are more relevant to your issue:
https://pinside.com/pinball/forum/topic/ball-ss-grand-slam-low-voltage-from-power-supply-module-for-displays#post-5009748

SDB_C23_MeasureMeter.jpg

Making progress..
Regarding the displays, 2 things to try:

(1). With the machine off, disconnect connectors J2 and J3 from the MPU board, J4 connector from the lamp driver board and J1 connector from the Auxilliary lamp driver board. Power on the game. Do the displays work?

(2). With the machine off, disconnect all displays except player 1 display. Power on. Does player 1 display look normal? If not, power off, disconnect player 1 display and connect the player 2 display. Power on. Does player 2 display look normal?

Exactly which connectors did you rebuild and clean?

TP2 is a little low but I would expect the displays to still come up. The voltage at TP2 can be adjusted on the solenoid driver board but we will come to that next time based on the results of the following:

What voltages do you read at the actual displays? Connect your black multi-meter lead to TP3 on a display board and measure the voltage at TP2 on the same display (should be 5 volts) and TP1 which should read the same as TP2 at the solenoid driver board.

So power connectivity to the displays is ok.
Connect the black meter lead to ground somewhere, connect the red meter lead to either TP2 on the solenoid driver board or TP1 on a display. Adjust the little blue pot on the solenoid driver board with a small flat head screw driver until you get the voltage to around 180 volts.

SDB_HV_Adjust.jpg

This is normal - happens somewhere around the fifth LED flash.

Yes, now that you have proper voltage to the displays, the issue is logic on/between the MPU board and the displays.

Have you got a logic probe?

BTW, have you performed a memory clear on the Alltek MPU board? The manual has details of how to do it - it has been known to fix unusual problems with some games.

You should replace it with a crimp style connector.

25 position housing:
https://www.greatplainselectronics.com/proddetail.asp?prod=22-01-2257

Blocking key for position 2 (only one is needed)
https://www.greatplainselectronics.com/proddetail.asp?prod=15-04-9209

Crimp terminals (100 bag), but currently out of stock.
https://www.greatplainselectronics.com/proddetail.asp?prod=08-50-0114-C

For now make sure none of those wires have a chance of shorting to another and carefully put that connector back on paying close attention to how it was originally installed.

No, the fact the displays come up for a split second is good.
Conversely, some of the digits show strange non-numeric characters which isn't so good, but hard to say if it's just the split second camera shot doing it. The chip on the display board can only show numbers, not non-numeric characters.

Do this little test for me.

At the player 2 or player 4 display (whichever is easiest to access) lift the connector about 1/8" so you have access to the pins.
Connect a jumper wire to test point TP1 (5 volts) on that display, and carefully touch the other end of the wire to the display connector at pin 15. We are forcing the display to always be enabled.

Does the display come up with (junk - mostly 8's) numbers when booted?
.

If not, this time jumper TP3 (ground) on that display to pin 10 of that display connector. We are forcing the display to not be blanked.

Does the display come up with (junk - mostly 8's) numbers when booted?

Yes that's the one. You can see pin 1 is marked on the board at the very left of the pin header counting to pin 20 marked on the very right of the connector on the board.

The fact that only the millions digit comes on with the jumper test kind of says there may be a problem with the display interrupt generator on the MPU board.
The display interrupt generator is the 8 pin chip at U1. Zooming in on your slightly grainy photo of the MPU board looks like that chip is installed upside down...
Can you post a clear picture of the U1 chip so I can get a better view?

1) You jumpered TP3 to pin 10 right? You didn't say ^^

2) What happened when you jumpered TP1 to pin 15?

3) After the MPU LED flashes 7 times on powerup, does the LED go dim or does it completely switch off?

Yes, your U1 chip is upside down.
What's even worse is it's the wrong chip!

Did you replace this chip at some point?
Did you buy the machine with the Alltek MPU board already in the game?
Or did you buy the Alltek board and it came with the wrong chip fitted?

Alltek_U1.jpg

I see.. yes U13 is missing.
To be honest I would install a new MC3423P at U13. It's job is to sense over voltage on the MPU board and to protect it by creating a short circuit to ground on the 5 volt VCC supply rail (via the SCR1 shown in the schematic below) resulting in the 1.10 amp F1 fuse blowing. This then cuts power to the board but leaves you with a blown soldered on fuse to replace after fixing the over voltage issues.

Alltek_VCC_Protection.jpg