Hi,

I've done a number of old williams repairs in the past but it's been a while. For me, Mark's guide was by far the best resource. Here's the MPU section, it may also be worth looking at his power supply section to make sure your work didn't cause any errors. Hope this helps!

http://pinball.flippers.info/system6repairpart4.asp

Was this machine working before you pulled it apart? Go to the power supply page in that guide and go over everything just to make sure there is no issue there, a very small change in voltage to the MPU will keep it from booting up. I'd pull the connectors off the driver board and see if that has any effect as well. If there is no change and you can't make any headway with Mark's guide, PinWiki also has a bunch of information on them and you'll probably get a lot more help here:

https://pinside.com/pinball/forum/topic/system-3-9-club/page/2#post-5931125

I haven't worked on an older Williams in years so I'm afraid I've forgotten most of what I used to know! Good luck!

If they are flashing and then going out when you start it up, that's good. The 5101 chip is often the issue here. You said you replaced it with a nvram, do you have a 5101 you can put in the socket to test? I'd take a good look at your nvram socket and make sure no leads were damaged when desoldered. I'm sure you haven't done this but just in case....make sure the chip in in the right way....someone I know may have put a socket in backwards and then oriented the chip wrong because of it in the past

Have you tried the quick "off and on" that's listed in the guide? Basically once the game is on, (the led's have flashed and gone out) turn it off and back on quickly. If the game's attract lighting now comes on but not the displays, the issue is probably with the display driver. If not, I'd be back looking at the 5101/mvram chip.