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Hi. I need some help repairing my amp
Hi. I need some help repairing my amp
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Old 21st April 2010, 08:45 PM   #1
rf2009 is offline rf2009  United States
diyAudio Member
 
Join Date: Apr 2010
Default Hi. I need some help repairing my amp

I have a Rockford fosgate 400.4 amp that doesnt seem to power on, the red led light doesnt light up when I connect 12v to amp. (with the REM also connected) I checked the MOSFETs on both sides for ohm resistance and got different readings. Also I'm not really sure, which side is the power side, and the output side?
Click the image to open in full size.
I'm not really sure if the amp was worked on before, but I noticed the MOSFETs are different, like some are IRF540, IRF9540, 5NO5HD, U1620, 337T, is this normal?
For the MOSFET readings this is what I got, as viewed from the picture, left side:
.56
.56
.56
.56
4.00> (keeps rising)
.00
.73
.73

Right side:
.56
.56
.00
.00
.24
.00
.86
.86

Thanks for any help!
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Old 21st April 2010, 10:03 PM   #2
brandes.cm is offline brandes.cm  United States
diyAudio Member
 
Join Date: Sep 2009
Location: Wisconsin
schematic of the 400a4, which is close to the same is located here at the bottom of the page:
https://sites.google.com/site/brandescm/home/projects

Anything reading near 0 on the fets isn't good. The 540's and 9540's are the outputs and the 5n05 are the supply.
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Old 21st April 2010, 10:59 PM   #3
Perry Babin is offline Perry Babin  United States
diyAudio Member
 
Join Date: Nov 2003
Location: Louisiana
There are 4 power supply FETs (2/side) near the terminal blocks.

Next to those are the rectifiers (1/side).

The regulators are the next in line (1/side).

The rest (IRF540 and 9540) are the outputs.

It appears that several of those have failed and that at least one of the large green 0.1 ohm source resistors has failed (discolored). After removing the outputs, you'll have to remove the 0.1 ohm resistors and check the protection circuit resistors as well as the driver components.

If you've never worked with the MEHSA insulators, read post #29 of THIS thread. It's for the larger transistors but it's the same basic procedure.
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