Under certain conditions (fully charged LiIon battery, unit switched off and back on in less than 30 seconds), as the internal voltages ramp up, the FET Q108 can be turned on for a few milliseconds and get destructed by the remaining charge in C145. This can happen to 4000 and 4500 detectors, if it happens to you then most likely there will be a burned spot on the bottom of the circuit board. The transistor is a BF820 , you can replace the transistor but you can blow it again by the same method as switching the detector on and off too quickly.
If you have an older 4000 or 4500 we can carry out the factory mod that will stop this from happening. It involves increasing the emitter voltage by adding smd diodes and one resistor change to increase the current through the transistor to overcome the voltage drop of the series diodes.
Note... The transistors used in the gpx are made by zetex, now called diodes. The transistors are not normal types but low saturation voltage types, they are rated at 20 volts at 3 amps.
