I might be way off the mark here but maybe it one of the chips on the cpu board for the lamp matrix. U19 or u20

If you put 50v onto the matrix they will blow.
Like a fuse.
Can easily happen.......... I learnt the hard way!

Wait. Scratch that.
That would take out the switch matrix not the lamp matrix!

Start with the obvious stuff. Check all the fuses with a meter. If all the lamp matrix is out then I'd start with power. It could also be related to communication between the cpu board and the power board.

How have you ruled out the cpu board as perfect? Nothing posted here so far has ruled the cpu board out unless you have put your cpu into another game and seen the inserts working. Toy can swap your cpu board with getaway or Addams or tz.

Have you looked at Pinwiki? WPC documentation is like a pirate's treasure chest of gold there. It looks like most of the lamp matrix operation is handled by the driver board, so focus on that (and not the CPU). I understand bad ribbon cables are common as well, so maybe swap them from another working game to rule that out as well.

Give this section a good read: http://pinwiki.com/wiki/index.php?title=Williams_WPC#Controlled_Lamp_Problems

As markmon said, the first thing to check is if you have voltage at the controlled lamps. Check from either lead on the lamp to ground.

If that's good then I would swap the cable from the cpu to the pdb with another game.

If the problem is with the pdb it will be a logic issue so you will need a logic probe.

P.S. - When you say you checked the fuses, did you do that visually or with a meter. They can look good visually but be blown.

C7
https://pinside.com/pinball/forum/topic/dr-who-no-inserts-lights-still-struggling#post-1538665

You won't get a credit dot for non-switch matrix related errors. The CPU has no way to know lamps are out.