Just in case you're working straight from the manual...

WARNING: Schematic Mis-Prints 4049 vs 4050.
On some system11 schematics (such as Earthshaker) chip U8, U9, and U10 the schematics says these are 4050 chips on the display controller board. But this is *wrong*! They are actually 4049 chips (hex inverter buffer). If 4050 (non-inverting hex buffer) chips are put in by mistake it can cause damage and the displays will not work!