Ma Audio HK 4000d works but some fets are burnt

I have a Ma Audio HK 4000d that one day the amp just had no power but was still playing. It burnt up 9 fets as far as I can see but I don't know why because it was ran a 3ohms to my subs so it wasn't putting that much of a strain to the amp. The fets are IRFP064n. I am wondering if I just need to replace these or if there is more components that would need replacing as well? Also there was one screw that was rusted???? No clue how since as far as I know it has never been wet but the door seals on my truck are crap and when it rains moisture gets in but not straight water. I will use my phones camera to try and get a picture of it. I don't have my other cameras with me so this will have to do.

[IMGDEAD]http://scrub50187.com/amp_1.jpg[/IMGDEAD]
[IMGDEAD]http://scrub50187.com/amp_2.jpg[/IMGDEAD]
[IMGDEAD]http://scrub50187.com/amp_3.jpg[/IMGDEAD]

Any help is much appreciated.
 
It appears that FETs in both supplies have failed. This means you will have to replace all 24 of the IRFP064s.

It's possible that you have one or more open/damaged gate resistors. You'd need to check them after removing the FETs. The drive circuit may be damaged. You can check the drive voltage after removing the FETs to determine if there is any damage.

You also need to check for shorted output transistors. If you read anything near 0 ohms between any of the legs of any individual output transistor, it's likely defective and would need to be pulled to be check out of the board.

If you have shorted outputs, it's possible that the driver board was also damaged. I don't have enough experience with those driver boards to know what to check on them.

If you can still edit your message, use links to the photos instead of having them show up on the page.
 
Even if the FETs check OK with a multimeter, they may not be 100%. If a weak/injured FET causes the supply to fail again, it will require that all of the FETs in that supply be replaced again. It may also cause significant damage to the board. You were lucky this time.

If all of the burned FETs are along one side, that means both supplies have failed.

While you have the amp apart, gently push the output transistors from side to side to see if any legs are broken (at the board or at the body of the transistor). The power supplies are isolated by diodes and it's unlikely that both would have failed without excessive current draw from the audio amplifier section.
 
I just got 10 new fets in and I can see 9 are burnt. You said if all the fest are on 1 side that both sides of the amp are bad??? The amp still comes on and plays but if I turn it up it starts drain the battery fast. Another thing that I am still skeptical about is the fact that I was running the amp at 3 ohms and as far as I know has never gotten hot. Also the rusted screw, I never got the amp wet and even if my windows were down and it was pouring rain, the amp wouldn't get wet, maybe condensation, but then why isn't anything else rusted??? The amp is supposed to have overload and short circuit protection on it but obviously it had failed. One other thing, what kind of solder do you recomend? Since these are parts that tend to get hot I figure that it probrably uses a different solder.
 
It's very rare that 1/2 of the FETs in a power supply fail and the other half survive 100% intact. If you replace the half that show signs of damage and the original transistors fail (because they're weak/damaged), the new transistors will fail.

There are many reasons that the amp is drawing excessive current. It's possible that there's a fault in the audio section that's causing excessive current draw.

It's also possible that there's something wrong with the drive circuit for the power supply transistors and the power supply transistors aren't being driven properly.

Have you checked the remaining power supply FETs to make sure they're not damaged?

Have you checked the output transistors?

Have you checked all of the gate resistors for the burned FETs?

The FETs actually don't run hot. Any good solder will work. If you need solder, Kester 44 - 0.04" diameter is a good choice.

After you replace the transistors, clamp the transistors to the sink and insert a 15 amp fuse in the power line to prevent damage if there are other problems.