JL 1000/1 Board Hole

Am I really going to try repairing this? lol
It’s possible, but time consuming.
 

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This is more just to see if I can still do it, I was a solder tech active duty but we rarely get to use the skills they give us. Usually if a board has a hole in it we condemn it, but I know how to carry out the repair so I’ll see if I can do anything with it. If not I suppose I can hook a linear supply up to it and run it in the house lol.
 
When I disassembled it I saw evidence of moisture intrusion between the power supply FETs and the heatsink, which caused a white chalky substance to build up on the board (aluminum or tin oxide I suppose). I suspect that over time in someone's damp trunk an electrical path was formed between the positive potential present on the FETs and ground (heatsink), which then caused the FETs to short. This then led to a cascading failure which eventually caused the +12v and ground sides of the foil on either side of the board to come into contact with each other which led to a massive current surge and that lit the board on fire. The guy I got it from claims it was fused, but based on the damage I'm seeing there's simply no way it could have been. This is the worst burn up I've personally ever seen, these amps are generally repairable but this one is a whole nother story. It will be a challenge, for sure!

When I repair slash series amps now, I put a strip of teflon tape on the case and heatsink in the area of the FETs for exactly this reason. Hopefully it keeps this from happening on units I repair.
 
I can't say why this board failed but in general, a bad connection at a jumper will cause the board to heat. A small intense area of heat at the bad connection causes the board to carbonize and become conductive. From that point, the board will burn if it has two points with a voltage across them. The burning can continue when voltage is present until the connections open. The current needed for this can be insignificant since the carbonized board is essentially a resistor.

Other than the insulators behind the FETs, I've never seen any points that needed additional insulation in a JL amp.
 
The only jumpers in the area are the buss bars that carry secondary voltage to the rectifiers, but this failure definitely began at the PS FETs not the bars. It's also possible that one FET in the group simply failed and took the other 5 along for the ride, happens all the time. The power supply on the 1000 is different than all the other slash amps, it uses and H-bridge. I think that's why I only ever see 6 FETs out of 24 bad in them, but of course since the originals are NLA I'll have to replace them all if I can get the board fixed.

There is good news though- the output stage is still fine. Believe it or not the amp still boots up OK with the main supply disabled, so I jumpered 12v to the filter choke to see what it would do and got clear audio out of it. So as long as I can fix the damage, chances are good that it will live again.
 
Returning to this beat up board.

FET technology has come a long way since these were made, and it dawned on me that I might not have to repair the board at all. Given that the original IRF3707’s are NLA anyway I set about searching for something better to replace them, and since the board is damaged and I can only use 12 of the original FET locations anymore I came up with these:

https://www.mouser.com/datasheet/2/916/PSMN4R2-60PL-1600441.pdf

They’re roughly twice as powerful as the originals (necessary due to only being able to use half as many), really good specs on them as far as RDS and they’re logic level so they should work with the factory drive circuit. Higher capacitance, but there will be less of them so may be a non issue.


Because of the damaged board there would only be 6 of them on either side of the transformer, 12 total. Or put another way, 3 of them for each of the 4 legs of the H-bridge. My only thought is can the remaining foil on he board carry the full amount of current, or will it overheat and fail again? Cost will only be $30 to find out...

Thoughts?
 
I know, but some of the other ones I looked at were more expensive. These are the best compromise between power handling and price, so I think I'll grab 12 of these and see what happens. I traced the gate paths back to the driver transformers, I'll need to run flying gate leads since the original traces got incinerated lol.

This is the other one I looked at:

https://www.mouser.com/datasheet/2/916/PSMN1R9-40PL-1320630.pdf