Rockford T10001BD blown output fet - diyAudio
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Old 5th July 2011, 12:21 AM   #1
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Default Rockford T10001BD blown output fet

alright so i have a rockford T10001BD that has 4 of the 8 output fets blown out. I haven't really torn into this amp to see if anything else is wrong.
First thing I did is look for the fets but they're discontinued. They're Fairchild SF154 and I can't find them anywhere. I did however find some FQA28N15 from fairchild that are the same package same voltage but rated at 33A instead of 34A. The specs are very similar but i dont know how much if any varition you can get away with.
I remember reading parry posting that class D amps get picky about changeling fets on them.

Anyway; anyone have any idea what fet i could use in place of it? also should i check any gate resistors? or rather what all else should i check?
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Old 5th July 2011, 01:00 AM   #2
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Are you sure that those are the original transistors? I have photos of the 2004 and 2006 versions and both have IRFP3415s for the outputs. The power supply FETs are HUF75344Gs.
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Old 5th July 2011, 01:29 AM   #3
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i'm not sure they are the original fets; if they are not whoever replaced them did a fantastic soldering job!

i have not cleaned off the board or anything yet i just started playing with it; i need to stop by rat shack and get some pcb cleaner as this one is quite nasty. I also found that the cap by the inductor is busted, i guess it expanded when whatever happened to this amp happened. My guess is speaker shorted to ground?

Here are the psu fets
Click the image to open in full size.

Here is the bad fets, can only read the p/n on one
Click the image to open in full size.

Here are the good fets.
Click the image to open in full size.

Also the PCB is labeled PC-5445-B / 2004 i guess its an older revision
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Old 5th July 2011, 01:44 AM   #4
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The photos I have of the 2004 amp are a rev. C.

If this amp isn't a priority and you're just learning, this probably isn't a good amp to work on now. It could go easy and build your confidence or you could blow several sets of parts and ruin your confidence.
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Old 5th July 2011, 03:19 AM   #5
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i'm no stranger to soldering and electronics; I do EFI and a lot of digital stuff. I've repaired probalby 10 amps with success but honestly most of them were see the burnt chip replace it repairs and i just had good luck.

I'm still "new" in the sense that I have a good scope and i'm learning the proper steps to real troubleshooting and more on design/theory and how all the little parts work together.

Plus i have $0 invested in this amp and if it explodes in a giant fireball i'm not really out much but if it works i'll probably use it or give it to a friend of mine to upgrade his 600w amp as he's the reason I have it.


So back on topic, what parameters should i worry about when finding a counterpart to these old fets? i know you mentioned input capacitance.

the fets i mentioned were only $.80/ea i could buy 8 of em and be out < $10 even if there is a catastrophic failure.
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Old 5th July 2011, 07:26 AM   #6
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If you're going to try to repair it, I'd suggest using the IRFP3415s since those are known to work. If you get it working with those and want to try something else to see if they'll work, that would be OK. If you try something unknown and it blows several sets of them, you won't know if it's the substitute part or something else causing the problem.

When you power up the amp after replacing all known defective parts, cover the amplifier (or at least the output transistors) so that you won't be hit in the face by pieces of hot plastic if they do decide to fail. There's no clamp over them and when the drive circuit switches on, they outputs can fail without warning.

Email me if you want the schematic diagrams. You need to check everything between the output of the driver transformers and the output transistors.

babin_perry@yahoo.com
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Old 6th July 2011, 03:24 AM   #7
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email sent, any idea where ro get the IRFP3415s at?
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Old 6th July 2011, 03:26 AM   #8
timwebb is offline timwebb  United States
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Old 6th July 2011, 03:31 AM   #9
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alright; probably going to order 10-12 of em from them in the near future... i still need to clean this amp up, remove the blown fets and start poking my dmm around with my low current ps and make sure this thing is worth saving!
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Old 6th July 2011, 06:19 PM   #10
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Look for better fets than the SFH154.
Specs of that fet:
Vds = 150V
Rdson = 75mohm
Id = 34A
Gate capacitance = 3.7nF

A quick search on fairchild turned up this mosfet. It's much better:

Vds = 200V
Rdson = 30mohm
Id = 62A
Gate capacitance = 5.4nF.

Except for the gate (input) capacitance. It might work, it might not. Since fairchild allows 8 free samples 5 products a week, I'd give it a shot anyway.
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