HCT family of chips do not like over voltage. they almost always go bang when that happens.

Usually this means the blanking signal is floating on the driver board. Either the interconnect is bad (p37 if i remember right). or the track gets easily cut on the driver board (tracks across right near the interconnect where a flat blade driver may scratch).

besides continuity from MPU timer chip to the driver board I'd test blanking with power on.

During reset delay it will be LOW. as soon as displays turn on it goes high and stays high... everywhere including driver board. Check at a driver 7408 chip