I made the following tests:
1. Vout=10.12V, Iout=3.25A (max current, the led does not light), R23=100R, Rdrain=1R
DS0510-DS0511.jpg
2. Vout=10.12V, Iout=3.25A (max current, the led does not light), R23=100R, Rdrain=10R
DS0512.jpg
3. Vout=25.5V, Iout=3.25A (max current, the led does not light), R23=100R, Rdrain = 10R
DS0513-DS0514.jpg
4. Vout=25.5V, Iout=half of 3.25A (the pot at half, the led lights up), R23=100R, Rdrain=10R
DS0515.jpg
The oscilloscope probe was put on the output of the power supply, with the crocodile clip at GND and the other end of the probe at +. I used a 8k2 resistor instead of 4k7 for the 555 "PSU SHORTER".
Please have a look and tell me what you think...