Been a while since I've done a tutorial for you guys, so here's my latest one ! Testing the MOSFET's from an amplifier. The only time you would do this is if you suspect a blown FET (IE. you've smoked an amp). The FET's in this tutorial were kindly supplied by a blown Earthquake D2.

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

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. 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.
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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.
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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).
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4.a. Now test the FET's ability to switch off. First touch all three pins with your finger tip....
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4.b. ...and re-probe the Drain (-ve on center pin) to Source (+ve on left pin). The meter should show a high reading.
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But how do I know if its not working ? A blown FET will have the following conditions in these steps:

2. Show continuity, which is give a low reading on the display, or generate a continuity BEEP from the DMM.
3. Show an open circuit, which is a high reading on the display, or show Open Circuit warning on the DMM.
4. Show the same reading generated in step 3, or show a closed circuit (generate a BEEP from the DMM)

Or has continuity between Source & Arm pins.


Examples of bad FET's:

Displaying continuity between Drain & Arm pins (Step 2 failed)
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Displaying open circuit between Drain & Source after being armed (Step 3 failed)
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Displays continuity between Arm & Source pins
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Burnt out FET still on PCB (bottom FET is definitely blown)
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Burnt out FET, missing pin (this one caught fire !)
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Hope that helps some of you with an interest in playing with amp guts. Always remember, I only provide the info and if you do this, do it at your own risk.