Mosfet Transistor question

[Osvaldo de Banfield]

Check also all drivers. Some times all drivers are catastrophically destroyed when the power mosfet gets shortcicuited from drain to gate, more in case of high drain voltages. Is case of routine at my job repairing speed drives for AC motors.

[Mooly]

Use a bulb tester to work safely on it and save blowing transistors.

[orbanator7577]

Im going to desolder them tonight and test. It sounds like a good place to start and I will probably order 8 new mosfets tonight. I ordered 2 yesterday, but was wondering about the matched issue. Im glad someone mentioned it. In the least, I will have two spares. Unfortunatly it takes forever to ship from china.

[orbanator7577]

I replaced the suspected transistor, set the bias and the amp fired up. It does run hotter than normal, but sounds fine. I think I understand why they have to be replaced in series now. I think I am going to buy 12 matched from ebay. What is the tolorance to make them "matched"? Is 50mv alright?

[Mooly]

Do they need to be matched ?

The bias (quiescent current in FET's) will determine how hot it runs. Have you followed a specific manufacturer procedure to reset it. Is the bias stable with temperature ? Have you rechecked and reset the bias when the amp is really hot ?

[moschfet]

I think it's mostly unnecessary to match all transistors.
My advice: Make sure that the complementary types are from the same manufacturer.

[diy didi]

Replace all of them. Make sure the batch numbers on them match!! If the unit has source resistors, make sure they are matched too. Once everything is soldered in. measure across source resistors. They should all give the same voltage reading. If they are not, move mosfets around a bit to mix 'n match!!