I'm trying to diagnose a faulty current limiting circuit in a piece of lab equipment from back in the 80s. The fault is too much current.
The tail end of the circuit is below. Q3 represents a complex control/feedback circuit, and R3 is the test load. For the purposes of testing I've tied Q3 base low. Thus the emitter-base voltage of Q1 is 0V.
My understanding is this should shut down the darlington, but I'm seeing a base-emitter voltage on the darlington of approximately 2v.
Does this suggest Q1 or Q2 is faulty? Is there any way of testing the darlington without pulling it out of circuit?