Blown JBL PX300.4 Amplifier?

That is a power supply transistor.  It probably got damaged when the tape was folded and the transistors were touching the heat sink.  I have no idea why the capacitors are exploding.  Chances are that all of the power supply transistors have been damaged.  You will have to pull the exploded transistor out of the board and check the remaining ones just as we did the others.  The Metal back of the transistor is connected to the middle leg of the transistor. 

IMPORTANT   When removing the transistors, it will be less harmful on the circuit board if you add as much solder as the joint will hold, then heat all 3 joints at once, lay your iron across the 3 joints.  Now with a pair of pliers gently remove the transistor from the board.  You are using the pliers only so you do not burn your fingers.  Do not manhandle the transistor with the pliers.  After you remove the part, now you can use a solder sucker or desoldering braid to clean the hole out.

What is the part number of the burnt transistor?   IRF-Z44, IRF 3205, or MTP50N06?   Or something else?

No I'm 100% sure it got damaged this time - it wasn't like that before, or at least got damaged to this extent.  Anyways - I have a precision soldering iron that is essentially the size of a pencil - works well for soldering on stuff like this.  But I feel like if I am going to completly remove this power transistor I might have to apply the heat to them for a long long time - would it be better to use my larger hotter soldering iron - which would reduce the amount of time I have to hold the iron on the board.

Also - this rubber thing - what is it really?  I want to replace it with fresh stuff - is it just an insulator?  One of the strips has an extra set of holes in it from the screws that secure the board to the heatsink.  The rubber strip must have moved a little when I put it all back together and me not knowing what the stuff was must not have realize it.  So the extra set of holes worries me - maybe the rectifier diodes, power supply transistors and/or output transistors are making contact with the heat sink through these extra holes.

The LED - I'm sure I can replace it with any old LED. I'll get on it asap.

The Part number of the power transistor is... I cant find it.  Maybe its written on the transistor itself.  I'll try to remove it and get back to you.

I got it out no problem.  However - getting all the solder out of the holes seems impossible.  I used the braid to essentially remove all the solder I added - but  I can't get it all out of the hole.  No biggie for now.

The part number for the transistor is found where?  Right on the transistor?  It says:

"Fancy italicized "F""    1G29

Kapton Tape? Never heard of it - just googled it.  Can it be bought locally?  I wont necessariyl completly remove the Sil-pad, but patch it up for sure.

Im having trouble getting readings for the rest of these transistors now.  I cant get anything consistent - I just dont understand how they work. 

I started with the row opposite the fried one, and I essentially got readings of 1471/460 on all 4.  Then I went to the 3 on the same side as the fried one - and I was getting pretty much */1.  So I went back to the bottom row and I was getting */1 or 1/1.  Is there a more precise method to testing these thing that wont get me frustrated?  I essentially positive probe on back plate, and then negative to the left, then to the right.  Just isnt working well.

That is a Fairchild part.  It is an N channel mosfet transistor rated at 50 amps and 80 volts.  I have never seen a 50N08, are you sure it is not an 06?   If you can not find Kapton tape without buying an entire roll of it, I can mail you a piece.  You need to check the rest of the power supply transistors to see if they are shorted.   Even if they are not shorted, I would recommend you change all 6 of them.  They all went through the same abuse.  They may have been weakened by the strain.  There are several other things you will need to check before replacing them.  It will be easier to check with the power supply transistors out of the board.

If you are going to use the Kapton tape, you need to replace the entire length of the sil-pad with the kapton tape.  There is a thickness difference that will possibly keep some transistors from being held tightly to the heat sink if you just patch the bad spots.

F1G29

It was difficult to read - and it wasnt clear.  I was scared to scrape the crud off it thinking I would scrape the letters off - but I made it much easier to read.  Its definately 06 like you said.

I just ordered a 20mm wide roll of kapton tape for $5.33 here: http://www.dealextreme.com/details.dx/sku.5442 - free shipping too.

As far as the kapton tape - I didnt mean I was going to just patch the holes.  I meant I would add a strip along both end pretty much identical to the Sil-pad just over it.  Do you think it would be better to just completly remove the Sil-pad?  Btw - thx for the offer for the tape - you've already done much much more than expected.

So now that the fried transistor is out - what do you recomend I do next?  Remove all the power output transistors?

When you say you recommend changing all 6 of them - do you mean all 8 of them?  Cuz if thats what you recommend thats what I'm doing.  You really know your stuff; who am I to disagree.

I thought there were 3 and 3.  After taking another look, there are 2 separate power supplies on that amp.  Pull the one out and check the 3 associated with it.  Check them just as we did the outputs.   Check the other 4 also.