3.DC power supply cannot be regulated

[faceu]

Hello,
A DC power supply, a high-speed DC motor with an emerald engraving machine can adjust the speed through a potentiometer.
The current fault: It can not adjust the speed, the multimeter detects, the DC output is 42 volts, and the output voltage of the potentiometer remains unchanged.
So I suspected that the LM723 was broken, then I changed one, and the fault remained.