update - 90909090 means the monitor is crashing (I believe)...meaning possibly RAM is bad OR TTL chip or CAP or.... whatever in circuit that supports these two.
All but 2102 U5 and U16 Probe pin 11 - steady LO, steady HI, pulse visible
2102 U5 and U16 Probe pin 11 - no/weak LO, steady HI, pulse visible.
swapped RAM, same issue. means problem is with the board.