OK, did some tracing.
Again, that resistor is taking the bulk rectified DC from the input, and then branches and feeds the 555 timer, and goes up towards the output transistors and appears to also be feeding the light?
So, one possible failure cause is load shorted- on the output.
Or, for whatever reason, the voltage coming in is high, and/or, the 555 or another component is failed and drawing current.
The Zener diode is reading 12.3V across when powered on a good board and that is the voltage regulation past the 150 Ohm resistor.
Again, quick tracing and measuring, AC power comes in, gets rectified by the 4 diodes. The larger 35V 470uF bulk cap charges up to whatever the peak voltage AC you were feeding into the board. Then the 150 Ohm resistor takes that high voltage, and jumps over to the 12.3V bus and that is regulated or kept at the max voltage by the large Zener diode.
If the 150 Ohm resistor burns and fails, it's because somewhere, somehow the load is shorted. Again, could be the 555, CD4093BE, maybe one of the smaller capacitors. Again, the 150 Ohm is part of the power supply regulation part of the circuit and if something fails, then it pulls higher current until the resistor smoke like a fuse.
