u15 is the most common chip to go bad on the original MPU, besides AMI CPU/PIA/RAM. There is a 150 ohm pullup resistor on one output. I think the 7437 original TTL type with no letters is an OK sub too.... better than the LS flavor for sure.

Double check you are not counting the 'flicker' as a flash. If so no 4th flash means U10 6821 PIA has a problem or the CPU cannot control it properly. I see you have AMI brand PIA chips installed in that board. That brand is notorious for failing seemingly 10x more often than HD6821, EF6821, F6821 or MC6821 brands of PIA. AMI brand RAMs, CPUs, and PIAs are all so unreliable I would not use them. Trash canned so many bad ones it is ridiculous. So consider replacing both U10 and U11, but start with just U10 and see where you are.