Thanks Torpedochief. I opened up the transmitter and see that the left stick up and down is a two position switch.

I do have one interesting observation, if I turn off the transmitter the firing stops. If I turn it back on, the firing 8/10 times resumes uncontrolled. A couple of times I turned the transmitter back on and the gun stopped firing. I drove the tank around. The gun started firing uncontrollably again as soon as I pushed the stick up. It would not stop again until I turned off the transmiter.

Should the left stick, stick in the up position and not return to zero? Not that I believe that to be the problem, as I opened the transmitter and the switch looks fine.