Desolder and Test the Field Effect Transistors (MOSFET) in your amplifier

Testing the MOSFET's from an amplifier is a lot easier than you would think. Naturally the only time you would do this is if you suspect a blown MOSFET (IE. you've smoked the amp, its made a bad smell or isnt making power anymore). The MOSFET's in this tutorial were kindly supplied by a blown Earthquake Power House D2000.

Equipment you'll need:
- 35-55W soldering iron with a pointed tip
- Desoldering wick/braid (I recommend Servisol or Goot Wick brand braids)
- Digital multi-meter with continuity function

Desoldering the FET



1. First things first, remove the FET's from the PCB ! You cant test them in-circuit.
I recommend using a 35-55W soldering iron, with a 2mm tip. Make sure the tip is clean, and the iron is up to temperature before you start.
Use a desoldering braid/wick. Never use a desolder pump or bulb, these generate high static and can surge (hence blow) otherwise goods FET's with static.
The methods simple enough, lay the wick of the solder you wish to remove, then apply heat to the solder through the braid. As the solder melts, it will be drawn into the braid, simple ! As the braid gets full (you'll see it change from copper to silver in colour) move to a clean section of braiding. Continue until no more solder is drawn out into the braide, then flip the PCB over and do the top side.
The more technically inclined readers will notice I'm actually removing the recitifier diode in this shot, the process is the same for any PCB mounted electronic componant.

2. Once the FET's are removed its time to start testing. Get out your trusty digital multimeter and turn it to diode test/continuity mode. You know, the one which beeps when you touch the probes together.
First you need to Arm the FET. Place the -ve electrode on the center pin (drain) and the +ve on the left pin. You should get no signal (Open Link indicated on the meters display), the FET's now armed.
  

Arm the FET in continuity mode


3. Now its time to test the FET's ability to pass power. With the -ve on the Drain (center pin), put the +ve onto the Source pin (right pin). You should get a very low reading, or a closed circuit (the DMM will beep).

Testing the FET has

4.a. Now test the FET's ability to switch off. First touch all three pins with your finger tip....

Discharge the FET by touching all 3 pins with your finger

4.b. ...and re-probe the Drain (-ve on center pin) to Source (+ve on left pin). The meter should show a high reading.

Test continuity to check the FET has disarmed


But how do I know if its not working ? A bad MOSFET will give these results:


Displaying continuity between Drain & Arm, pins 1 & 2. Step 2 test failed.

Continuity between Pin 1 & 2, Bad FET

Displaying open circuit between Drain & Source after being armed, pin 2 & 3. Step 3 test failed.

No continuity between Pin 2 & 3 AFTER arming. Bad FET

Displays continuity between Arm & Source, pins 1 & 3. Basic function failure.

Continuity between Pin 1 & 3, Bad FET

Inspection reveals damaged FET's without testing. Burnt out FET still on PCB (bottom).

Physical damage (burnt) FET, lower FET

Burnt out FET, missing pin (this one caught fire !)

Overloaded FET's physically burn, this ones case is burnt and is missing a pin

 

These testing methods apply to any Field Effect Transistor from any electrical appliance, not just car audio amplifiers. Examples include home amplifiers, coffee makers, microwave ovens, etc.