MOSFET failure usually open or closed

[cbdb]

I have a power amp circuit with mosfet output transistors. The amp was damaged by dropping it. The amp blows fuses and the power transformer is fine. Do I check these transistors or do they normally fail open circuit?

 

[KatieandDad]

My experience with Mos-Fets is they are difficult to test with just a meter. Its better to put them in a simple test circuit and vary the gate voltage to see if they behave as they should.

Different types of Mos-Fets require different biasing but basically changing the bias voltage should change the current through the device.

There should be virtually ZERO gate current.

 

[KatieandDad]

If an amplifier has been dropped, the first call would be to carefully check all connections for broken solder joints.

 

[Mooly]

Failure mode is almost always a near or total short for common vertical power fets.

Use a bulb tester and fault find in the normal way.

 

[KatieandDad]

If you can achieve zero D-S current with G bias then they might be OK

If you can identify the MOS-FETs then we can be more specific as to how to test them.

 

[cbdb]

Failure mode is almost always a near or total short for common vertical power fets.

Use a bulb tester and fault find in the normal way.

Thanks. Will check the fets.

 

[SemperFi]

The FETs dont fail from a drop. More likely u have a short somewhere in the voltage supply circuit.

 

[RJM1]

You probably have a cracked circuit board or trace. Mosfets usually fail shorted from over voltage, over heating ( excess current ) or excessive gate voltage. I have rarely seen them measure open unless their plastic covering had exploded due to the initial short.

 

[bobo1on1]

I've seen a shorted electrolytic capacitor in a dropped active speaker, the problem could be anywhere, the output stage is not something that I would suspect to get damage from a fall.