IC301 pin 2 must be pulsed or there will be a fault.
Actual function side of the circuit has too high voltages, so transformer T603 is attenuating.
Pulsing must be high enough to be able to trig pin B, so close to 4V.
If no pulses are present the situation is clear, that is the cause, maybe not the first.
Above relay and FET circuits are static LC time changers and not necessary, pulsing must always be.
Capacitors C639 and C640 are the center, both ends must have a galvanic connection to the end, supply voltage and ground.
At least one capacitor must also be present, more or less, here those FET circuit capacitors can act as alternatives, if activated.
H. DEF DOWN LED driver 74HC123 (IC301) is a retriggable monostable.
When its pin 4 goes down pin 13 must go up, that is a no protection situation and LED is off, since TR301 is also off.
This can happen only after monostable is activated and RC timed (pins 14 and 15) time has not passed.
Start of this is a rising edge of B AND NOT A if NOT NOT CLEAR, since the connection is as it is only B (pin 2) is changing.
So when B goes up Q goes also up and stays there until time out or retriggering by new rising edge of B, and so TR303 base is not pulled down and PROT is not pulled up.
Here those boxed transistors are inverters and double diodes are switches.
R301 before B is a pull down, so if nothing happens Q stays down and it's always a fault.
D301 is overvoltage protection and C301 a DC filter.
A bit left of IC301 is IC604.
There FBP OUT TP606 is similar to H.DEF DOWN but not inverted and activated by pin 1 NOT A.
So main feedback FlyBackPulse TP606 pulsing means that indicating monostable and at least partial T602 are operational.
All down from TR609 is MUTE, it can also stop the party.