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From the schematic, it looks like the CPU supplies the +5v for the 50/60 Hz signal (likely via a pull-up resistor on pin16). 16TR4 then pulls down pin16 in sync with the line voltage. Not seeing a signal could be that the CPU is not supplying the voltage to pin16 or that 16TR4 or 16D5 is bad.
What happens on pin16 when you power-up?
My best guess is that the 50/60Hz signal would NOT be needed to initialize. The only thing that I could think of it being used for is for the timer functions (which aren’t critical for operation). Then again, there could be a few lines of code that check for the signal changing and fault the processor if it isn’t as expected. I would expect the display to indicate a fault if this happened.
What is insidious about trying to debug this are the possible “chicken or the egg” situations that could be happening and not knowing if one is dealing with chickens or eggs ;-).
What does you display do when you power up?
Glitch