Looking great!

Were you able to find U19 UDN7180? Don't buy these from China, they are most certainly fakes.

Maybe someone found a bunch of tubes of them in their backstock. BUT, $8 is a steal, even when (if) you can find them in stock somewhere trustworthy they can be $15-20 each so be cautious. Hopefully they didn't source them from China. If you do buy spares, test all of them in your board to make sure they work before storing them. Maybe perform the diode test as described on PinWiki before installing them. http://pinwiki.com/wiki/index.php?title=Williams_System_9_-_11#Display_Fuse_F1_Blows

Well, that looks to be pin 9 which is ground. No telling what would happen with that left floating. Great eye to catch that missing pin.

You have a couple problems in those pictures, lets start with the top segments of display 2 locked on.

Now that you have replaced the missing 7180, lets step slightly back. Use a logic probe to test the inputs and outputs of the input buffers for those segments. Looks like a, b and f segments which come through U9 and U10. These should be pulsing and NOT stuck high or low. If they are stuck hi or lo on the input, you have a ribbon cable or CPU board issue. If the input is pulsing but the output is not, you have a bad buffer chip.

Similar test for the display 2 segments that never turn on c and e. Those input buffers are on U11 and U12

For the dead digit and the always on digit in display 1 and 2, we know the strobe signals are making it through the input gates since digit 5 has a problem on display 1 and digit 4 has a problem on display 2 (if the signal was not making it through, both displays would have a problem on the same digit). The problem is probably in the 6118 at U13 and U17. Are either of these socketed?

Cool, then there is only a potential problem with U17. Is that by chance socketed? If so, check out the solder job on that socket and if another 6118 is socketed, it would be a simple test to swap them and see if the problem follows the chip.

The hollow line represents all the lines that enters and exits the hollow lines. It usually represents a bus (address or data) but was a way to draw the schematic without drawing all the individual lines, which took a lot of time to draw (by hand) and was messy. Look for the line description as it enters and exits the hollow box to know what connects to what.

Also, the parts you thought were diodes, are actually buffers. They recondition the signal as it passes through. This is used when signals travel a great distance (the ribbon cable in this case) or multiple devices are on that same signal. Since there isn't a small circle at the point of the triangle, you know the same signal entering (hi or lo) should exit the buffer. If there was a small circle at the point, it would be called an inverter and the opposite signal at the input would be at the output. Diodes have a line at the point of the triangle.

In your case, you are looking for pulsing at the inputs and outputs of the buffers. If there is a solid hi or lo signal, you have a problem.

The prefix on those chips indicates the manufacturer. 4049 is the chip you want.

No harm playing the game with those segments stuck on, unless any of the segments are extra bright. That might cause premature failure of the digit.

Awesome!

Don't beat yourself up, it happens to all of us! Simple mistake.

I don't have access to schematics at the moment. When I get home tonight, I'll have a look and try and tell you what that may have fried, if anything.

In the mean time, remove the ribbon cable from the display and see if the CPU will boot.

The lines from U19 connect through buffers U7 and U8 on the display board ultimately connecting to U42 on the CPU board.

I suspect what that error is saying is U42 Failure. Its hard to read because some display segments are stuck on likely caused by blown buffer chips or the bad PIA itself.

Start by replacing the buffers U7 and U8 (4049) on the display board BEFORE replacing U42. The reason you want to do the buffers first is the bad buffers MIGHT blow the PIA outputs. Unlikely but better safe than sorry since they all need to be replaced anyway.

Then try U42 (6821), and you might have to replace U41 (another 6821) if the buffer chip also corrupted the other PIA.