make sure lamp strobe 1 is reaching speech board U6 Pin 8. THere is a 20k series resistor on speech board. P8 should be pulsing i think at or around 120hz. If U6 P8 and P9 both pulse but P10 is always low U6 may be bad.
I am not 100% sure what to expect from the speech CPU and i don't have one on hand right now to check, but i'd expect the /BUSY (u6 p9) to only be low when the processor is playing speech and sit high when not doing speech, but maybe it pulses and then stays low when speech plays.
If the Q pins on U5 and U4 never pulse then I'd think the clock pin is never pulsing which is also done by U6 with lamp strobe 2 and PD2 / PD3 input.
Double check the input connectors, Lamp strobes and PA and PD inputs, otherwise U6 might be a good shot at being the problem. You can get the datasheet and work through the truth table for U6.