MTX Thunder302 FET problem


2007-12-15 6:46 pm
Hey guys,

I recently blew out one of my FET's on my amp, so i purchased a replacement online, and then I soldered that sucker back in place.

I was really hoping that would fix it, but no, it was too easy.

Originally, only the FET on the far left was blown, so I replaced it with the one you see there. Now when I measure any of them with my ohmmeter its ~2-8ohms. So they're blown.

As you can see the chip on the left has 508 on it compared to the original chips 008. That mean anything?

Should I try and replace all 4 of the blown FETs or is the problem elsewhere?

Thanks for the help!
those are date codes for the build of the device. In a class D amp there should be little to no issues, unless the other fets were leaky then it will all fail again very quickly.
It looks like the 508 chip is touching the case at it tab end, now this could be a issue...

this appears to be a class AB type amp at first glance, if so then there are usually damage issues with the OK looking devices right beside the blown ones. I replaces entire sets on AB type amps, and on some D amps depending on its design....

Also in AB type amps damage likes to spread back into the driver stages, and sometimes all the way back to the Diff input of the main amp for each channel. The drivers are inexpensive so be ready to replace more to repair this amp...

Perry Babin

Paid Member
2003-11-20 11:01 am
The tab of the transistor looks like someone grabbed it with a pair of diagonal cutters. If so, there are probably burrs on the bottom that can cut through the insulator and cause the amp to fail.

I don't know what you paid for the SSP45N20A but they're generally expensive (~$15 ea.). Something less expensive may work just as well. Of course, if you've found a source where you can get them for only a few $ each, use the original replacements.

Pull all of the outputs in that channel and test them out of the board. If you can find 2 that are OK, reinstall them in the two center locations and try to power up the amp. Have all of the transistors clamped down and use a 10 amp fuse in the B+ line when powering it up.


2007-11-08 2:14 am
Check the gate resistors. Here is another thread about this part:

The left pin should go to a resistor, likely the tan colored one next to each chip. When you take the mosfets out test ohms on those resistors (they will be hooked to nothing) and see if they are the same, some people recommend you put them in new as well since they don't cost hardly anything and should ensure the mosfets all share the load equally, or make output equally if outputs. If a bad mosfet shorts to the gate it can send full current up that resistor to the driver in a power supply or it looks like in yours that is the output, so like was said it can damage the differential amplifier/etc. Or it all may be fine hard to say but checking the gate resistors is easy and a good idea.