Had a crazy fault today, the detector works without the coil connected but shuts down when a coil is connected, LCD screen stays on and the negative 15 volt rail that supplies the output Fets hits 25 volts and shuts down the negative supply. Hmmm thought it might be in the voltage comparator circuit around the TLE2022 , voltage reference or divider resistors gone high? No, I made the detector force itself on without the coil and removed the negative tx rail capacitor as it was getting over voltages. then hmmm a burning smell, the second blocking Fet to the AD797 went up in smoke. Great so I removed it and the input gets as these possibly had gone short and allowed tx voltage to take out the second Fet. Then hmmm more burning smell , the ADG333 was growing a brown spot in the middle so I put in a new ADG333 and fired the detector up. Well it worked, there are probably some parts that were stressed out as the ADG333 supplies switching voltages to the damping resistor control Fet, the input protection Fets, the GAas Fet clamps and TX drive. Why a failure in the ADG333 caused the shutdown was strange as all waveforms were correct before it smoked itself. So if you get this fault and the detector shuts down it would be wise to think the ADG333 is the problem as trying to find a shutdown problem like this could be a real bastard fault to track out. I did have another customers detector that did a similar fault and came up with the dreaded coil over current problem, it was a sneaky fault as it was a loading problem that only showed when the coil was connected, it turned out to be the FMMT718 pop bipolar that is in part of the 5 volts power supply. These transistors are a low loss switching transistor and if the VCE rises due to thermal issues the detector drops out the +5 volt rail. If you test the transistor it will test fine but the detector will shut down, change the 718 and all is good again. Anyway it is interesting working on the Minelab detectors, sometimes frustrating but mainly straight forward.
