Need help with my Prizm II

[Prospector]

Just picked up a like new Prizm II detector. Everything appears to be working except for one feature. When I press the all metal button I get a constant hum regardless of whether it is over a target or not. If I pass a coin under the coil the tone doesn't change. When I press the all metal button again, it goes back to discriminate mode. I have already tried new batteries and it did not help. The tone should either increase or decrease in all metal when over a target. What could be the problem?

 

[Dan-Pa.]

Call Whites main hdqs. as you may have picked up a defective unit and they are great to work with....