Let's recall the circuit:



The upper part is a voltage source. With Pot R6 you define the output voltage of LM338 (compliance voltage), means the maximum voltage drop the current source can cause across the load.
Q4 provides a soft-start, means a slow turn on of LM338.

The lower part is the actual constant current source. But to work properly, inverting input of U2B must be connected to output of U2B, not to ground.
With the voltage drop across Pot R17 (from wiper to ground) you define the current of current source. The output of U2B buffers this voltage and feeds the non-inverting input of U2A with a copy of it. As the OPamp always tries to force the inverting input to show same potential as non-inverting input, by the help of suited feedback, the voltage at output of U2B also appears as voltage drop across R12. This voltage drop is caused by the current of constant current source. Output of U2A controls Q2 and by this Q3, so that the voltage drop across R12 always equals the potential at non-inverting input of U2B. Changing the setting of R17 also changes the setting of constant current source: 1V at non-inverting input of U2B sets the current of constant current source to 1A. 2V to 2A, and so forth.

Q1 can be used to shut-off the current source, when it's needed.

Kai