Rockford BD1500 old design

I have this amp in for repair and its giving me issues. It came in stuck in protect and had shorted outputs in one bank.


I removed all the output FET's in the shorted bank. I changed the protection IC. I powered up with the FET's out of circuit, it powered up fine came out of protect and idled perfect.


Installed all the new FET's. Powered up and Q103 and Q26 exploded. I replaced those 2 and powered up again and both gate resistors immediately burned in half. I replaced the gate resistors and powered through a limiter and the resistors almost immediately burn up. Q103 and Q26 still test good.


All readings read normal. Any ideas why these two have issues? Could FB3 and FB16 have anything to do with this issue?
 

Perry Babin

Member
Paid Member
2003-11-20 11:01 am
Louisiana
www.bcae1.com
The ferrite beads don't fail unless they open.

Desolder the red/black wires on the rectifier end and touch them together to make sure that the caps are discharged.

Short pins 2 and 6 of U17 and look at the drive signals.

The gate resistors will not burn unless the FETs have failed. Viewing the drive signals as described above will allow you to see if there is high voltage or something else killing the FETs.

Confirm that there is no way for the gate circuit to short to the drain circuit.
 
Q103 and Q26 failed again. They shorted from gate to drain instead of gate to source.


I replaced them and the gate resistors and powered up through a limiter. So far everything looks ok.


I had this happen a few times before and I narrowed it down to certain batches of the IRF3415 and IRF6215 are more sensitive to heat when tinning and soldering to the MESHA board.


I used a different batch and they did not fail from installation.
 
I can't remember Perry. I have seen it on 800a2, 1000a2 and some other between the FET legs and the heat sink.


I do remember working on a Rockford amp with kapton under a switch.


I have definitely seen traces damaged from switch removal, and over soldered pins that end up touching the case of the switch.


I tend to leave the up just a bit so they will still work with the slot in the heat sink, but not rest on the traces.