First post. Recently bought a Black Knight machine off of FB marketplace here in Virginia from a single machine owner. Was listed as non-working due to blowing the solenoid fuse. With further troubleshooting I am finding the MPU to be an issue as it usually locks at "0". Hoping someone can help me step thru this a bit. Have not done any soldering to printed boards with chips etc. but have soldered other things and generally know how it works. I have no supplies on hand of component parts. Just a meter, schematics, very basic soldering gear, a new fuse set for BK from Marco. I have the PDFs of the schematics but I am not a guru on using them for tracing down circuit paths.

Current state:
Driver board removed, MPU with 1J2 cable connected only. IJ2 cable while connected to board read proper voltages per schematic.

Observations:
Normally goes to 0 on the display and stays but sometime will randomly boot with a "t" or another number from the trouble code list. Nothing regular or consistent but stays on 0 90% of the time.

Voltage tested at the 1J2 cable and shows normal per schematic.
Voltage tested at the Test Points:
Expected reading per schematic - see attached image.
1 - 12.5v
2 - 1.1v
3 - 118mv
4 - 13.5mv
5 - 4.86v
6 - 2.46v
7 - 4.59v
8 - 4.95v
9 - 5v
10 - good continuity with ground

Previous owner first started having the solenoid fuse issue late last year. He send main and driver boards to a guy that repaired boards. Got boards back with some new chips and stated that boards worked when final tested after work. Previous owner reinstalled into machine and it still would blow fuse and also would not boot as it is now. I got the machine end of March. May reach out to tech that did the work unless I get some promising leads here. thanks.

Pics below are before anything was disconnected obviously.

IMG_0994 (resized).jpgIMG_0995 (resized).jpgIMG_0996 (resized).jpgIMG_0997 (resized).jpg
Capture.PNG

Reposted in this forum since I mistakenly posted in the EM section Yesterday.

Cable connections are good and not mismatched.
Change out all fuses for new ones.
What goes into rebuilding a power supply? Isn't it possible to take some readings to determine if power supply needs to be rebuilt? I am getting proper voltage at the input connector on the main board. Readings take at test points on the main board show differences on some points from what schematic shows for expected voltages.

Thank you all for the input. So the main board will not boot without the driver board attached?

Roger that. Will reinstall driver board. I did swap the main cpu with sound cpu.

I ordered a new power system 6 supply board from pinballpcb.com. Anyone using one of those? Figured I just want to have a known good board to eliminate that as a possibility.

With everything plugged in.... Still goes direct to 0. Occasionally will here a random click that sounds like a relay from the cabinet area. Might display a different number from time to time or immediate go blank. Only getting general lights at power up. On a few power ups it would immediately kick out a ball. Did that maybe 3 times.

Received the new power supply board today. Installed. It looks real nice in there but... no change in behavior. Main board goes to "0" and stays. I may get a rogue click or a 2 showing but this is only on the first couple of attempts to power up after days untouched.

Pintec, I have not tried your latest diag steps yet.

Just to close this one out... I gave the main and driver board to a local guy in the area that supports/repairs for other local arcades. He also has a BK 1980. Got the boards back this past week and installed them in my machine. He put them in his BK machine to further test them out so whatever else is wrong will be with my machine.
Installed and machine boots and plays so now I'm able to start a todo list for other issues.

No sound
Lower ball lock - does not recognize that a ball is in there. No new ball is ejected for play.
#4 player display is blank
Adds points in 5,000 intervals randomly sometimes with a flipper or even static with no movement.
Bulbs etc