Quoted from Flippermatt:1, swapped the security chip to a new one
What is a security chip on system 7?
Quoted from Flippermatt:1 Game will not boot.
2 All G.I lamps work
3 Displays are dead
4 Sound board OK. All sounds check out and are playing out with the test button.
Sound board working has nothing to do with the mpu board - it's a separate mpu all on its own. GI working also has nothing to do with that.
Quoted from Flippermatt:The digital error display seems to be partly broken.
Assuming it is showing either a 5 or 9, these both indicate memory protect failure - coin door was closed. #5 also indicates possible bad IC20, the flipper rom which is the operating system for the game. If that's bad, it's never going to work.
A usual culprit is bad rom sockets. You need some more diagnostic tools though before continuing to shotgun various parts of the board hoping it will work. Vid's bulletproofing guide and pinwiki will help here as well as the pincoder eproms, and a logic probe/scope. You have to work methodically from the start of clock signals, etc. into the 6808 and out from there. There's no "do this, it fixes that" for system 7.