Aleph 2 fuse burned and killed mosfet!

[Aerosol]

Hi there,

Please pardon me for this triteness but I really need your help!!

I am building another set of Aleph 2 using BrianGT's layout and found that the fuse burned 5 seconds after I powered on the amp.

I've checked my PSU without main board connected and got (+50V)-0-(-50V) with happy fuse. Can I conclude that my PSU is good?? The PSU was using the Aleph 4 design.

When it blew out the fuse, I found both bias and output group have one MOSFET destroyed by the current. It seems that it just picked one MOSFET randomly from each group everytime when I replaced the bad one.

No burned sign shown in the main board.

Any suggestion I can do to kick the bad guy out of my circuit??
Thanks!!

 

[swede]

Hi,

You are lucky you didn't blow more than one. Well, one reason for not blowing more of them is that a "blown" FET usually transforms to a short circuit, letting all the current flow through it (the broken one).

To mee it sounds like you either have FET's with too low voltage specs, or WAY TOO MUCH bias. (short circuit somewhere close to R18?, R19?; cold solder at R19?; solder smeared on the board, shorting something?)

Do you have any pictures on your board?

//magnus

 

[wuffwaff]

Aerosol,

does it blow at once or does it take a few minutes?
I once had a bad heat transfer from one fet to heatsink (screw too long ) so it got too hot and died.

william

 

[Aerosol]

The fuse burned in 5-10 seconds and the mosfet already dead at that time.

I will re-check every solder point later today to make sure I have no cold solder on my baord. I used 3% silver solder and set my solder temp to 300C (~570F) which might be a potential problem becuase this this my first to use high silver content solder.


I will also get a picture and post it later.

Thanks guys for your time and help!!!

-Victor