Boston PV600 subwoofer blowing MOSFET

[abundant2]

Hi there- A newby to the site.
I have the Boston PV600 subwoofer and it keeps blowing the MOSFET (sometimes the fuse as well if both MOSFET shorts out).

Replaced the MOSFET (complementary pair IRF640 and IRF9540) and will work for mayby 8 hrs or so and blows again.
Have checked the gate resistors etc and they are all ok.

Any thoughts what I should do - sorry I do not have the schematic and appreciate it if anyone out there has it.

thanks a million for any help here.

 

[hyedenny]

Does it sound normal when it's working? Are the MOSFETs overheating before they blow?
Check your power supply, and check for shorted electrolytic caps.
That's my 2 cents without any other info!

 

[crutschow]

How hot the MOSFETs get is a critical thing to determine.

 

[abundant2]

When operating, the mosfet are inside the unit. Will try and test it when it is opened up and see how hot it gets.
Also was thinking of parallel 2 mosfets to share the load. Anyone has tried this?

 

[abundant2]

Another option i am thinking of trying is to bring the mosfet to mount it on the outside of the box so it gets better cooling (rather than sealed in the subwoofer).

 

[hyedenny]

That's treating the symptom rather than the cause. And neither is likely to work anyway!

 

[tomizett]

Yea, agreed - it should work as designed. There must be something a bit subtle wrong.
Does it fail only when it's being driven, or just switched on idle?
I presume these transistors are located in a class-D output stage - am I right? As always, pictures, schematics or traces of circuits will always be helpful.

How hot the MOSFETs get is a critical thing to determine.

This will help us work out what's going wrong. Just be careful of high voltages (and temperatures) while you're probing around.

 

[unclejed613]

i can't find a schematic for it, but my best guess is it's an analog amplifier and not a class D amp. check carefully by pushing the woofer cone in to see if the voice coil is rubbing somewhere. class D amps don't usually have complementary output devices, so i'm fairly sure this is an analog sub amp. a picture of the amplifier PCB would be helpful.

 

[tomizett]

class D amps don't usually have complementary output devices

True, although I've seen some that do.
Good advice on checking the driver, certainly.

If the OP's still here, is there any chance of a picture? I think they may have given up on this....