GP detectors are starting to fail and how to stop it.

[woody]

The soft start or current inrush protection mosfet has burned a hole straight throng the circuit board.

This is starting to happen on older detectors, the reason could be just thermal cycling or metal migration causing the failure.

The small part that burns out is the Fairchild 455 mosfet, it is pictured on top of the white transformer core, the replacement is a 40 amp mosfet as shown with its 3 legs soldered to where the small
device was located.  The lower the on resistance as in the more amps it handles the better.

The FET that i used is a 100 volt rated 40 amp N channel mosfet but any high current mosfet will do the job, I left the legs on the set long as to make the connections easier to see.



This circuit board is not repairable as the power was left connected to the detector after it failed. The message here is to remove power from the detector if it stops working or more damage can be caused
making the detector unrepairable.

[ronjeanosborne]

Hi Peter, What wood be cost to have you replace these parts  before this happen to my GP Extreme, Ron.

[woody]

Hi Ron, it could be done for say $50 turn around. You could get the local computer tech to do it, I would of said TV repairman but they just about do not exist anymore.

[ronjeanosborne]

Thanks peter, I will Be in touch soon, Ron.

[woody]

I should add that anyone wanting to replace the mosfet should use a low gate threshold voltage type, as these detectors use a 6 volt battery a standard mosfet will not work, it may work but it will get hot as standard mosfets really require at least 10 volts on the gate connection to turn the device on hard in order to get the lowest on resistance. The fet that i used in the picture is a logic gate drive mosfet, it means that the 0.04 ohm specified on resistance can be obtained with only 5 volts on the gate connection, even though the mosfet is rated to 10 volts on the gate it still turns on hard at voltages as low as 2 volts.  If you want to put in a replacement 223 package device then this part will work well. http://au.rs-online.com/web/p/mosfet-transistors/8303304/

I do not trust using the smaller device mounted directly on the board as seen in the picture it can destroy the circuit board. Use a stand up 220 package mosfet for safety.  Like i used.

[ronjeanosborne]

Hi Peter, You say 6v battery, the detector i sent you i use a li-ion7.4v battery with a coiltek regulator 6.4/7.3 v, hope this is ok,Ron.

[woody]

Hi Ron, no problem at all with any battery system as long as the voltage falls between 6 volts to 7.4 volts.
By having a less resistive fet installed you will get a little bit longer battery life and the overall efficiency is increased slightly, the detector was express posted back to you on thursday, gave it a good going over and it should be like new.

[ronjeanosborne]

Thanks Peter, We are going bush next thursday let you know how we go,  Ron.