I have a Bally Space Invaders that I'm currently restoring and I'm in the process of cleaning up the displays.
All 5 displays are the -21 six digit displays.
I have replaced the pins on all displays and reflowed all solder points as well. Of the 5 displays, 4 of them work perfectly.
One however is causing me some problems.
When I connect this specific display, the game will begin to boot but then after the 3rd or 4th MPU blink, the MPU will sometimes lock up and the 5v fuse will blow.
If I disconnect just this display, it will boot correctly every time.
I have replaced the decoder chip (socketed now) and have the same issue. In the few instances where it does boot all the way, the displays do not work correctly.
Generally display 1 and 3 will show all 7's, display 2 will show only 1 digit that is very bright and display 4 is blank (this is with the bad display as the credit/ball display).
I'm kinda out of ideas at this point as this is my first time working on display boards.
Any ideas?

I've swapped it into other locations.
No change, same display causes the issues no matter its location

Thanks for the idea. I've already done this, but I can check it again.
Also, sometimes the game will correctly boot (thought displays wont work correctly), when this happens the 5v will hold for a while before it blows the fuse. I would assume the 5v would go very fast if the 170v mixed with it?

I'll do another review of the pins for solder bridges tonight. It's always possible I missed one somewhere.

Good point.
I'll have to check the board tonight and see if I can find any ground shorts as that makes a lot of sense.
Thanks for the help Shawn!

I can check the display pin resistances tonight and see what I get compared to the other displays.
I'll throw a spreadsheet together and see if i can find anything that stands out.
Hopefully I can find some random super small solder bridge or something.

I don't think the MPU was effected as it works fine with a spare Stern Da-100 display I installed instead of the bad Bally display.
Either way, I'll check it tonight to make sure its not damaged.
Thanks for the ideas.

Definitely a possibility, I would just prefer to keep the original parts if I can. I know the glass is good and would prefer for them to match. I know this is picky of me, but I would love it if all the displays matched too. I also like the sense of accomplishment I get from fixing things. This machine is full of that kind of stuff as it was saved from the trash in the first place.

Ok, I grabbed the bad display and 2 working displays and tested the resistance from the GND, +5v and HV test points to the header pins. The working displays were fairly similar, however the bad display seems to have some differences.
Capture (resized).JPG
Anything with a "-" my multi-meter just showed nothing. I'm assuming its just not connected or too large to measure. Everything else is in ohms.
Anyone have any ideas based on this?

Good News. The issue is fixed.
Checked all resistances (as seen above).
I went between every pin with a knife and cleaned it out with a brush afterwords.
Checked all the solder points on the solder side and found nothing of note.
Cleaned the bottom of the board with 99% isopropyl alcohol and and a toothbrush.
replaced the IC again just in case.
Tested the display resistances again and they had drastically changed from what i posted above. It basically matched the two working displays.
Put it in the game and it worked!

Not sure exactly what fixed it, but my guess is there was a solder bridge somewhere that i was unable to see.

Thanks everyone for the help!