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?
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?
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
Here is the bad fets, can only read the p/n on one
Here are the good fets.
Also the PCB is labeled PC-5445-B / 2004 i guess its an older revision
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
Here is the bad fets, can only read the p/n on one
Here are the good fets.
Also the PCB is labeled PC-5445-B / 2004 i guess its an older revision
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.
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.
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
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
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.
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.
Use 10minutemail.
As for the gate capacitance, with a higher cap. the gate drive circuitry needs to supply more current to charge the capacitor. If the capacitance is too high for the drive circuitry then the gate won't charge fully and the mosfet will run hot.
What is driving the gates of the mosfets? Try to find an IC on there that preforms that task, and see what the current rating of it is. DYK what frequency those fets switch at?
As for the gate capacitance, with a higher cap. the gate drive circuitry needs to supply more current to charge the capacitor. If the capacitance is too high for the drive circuitry then the gate won't charge fully and the mosfet will run hot.
What is driving the gates of the mosfets? Try to find an IC on there that preforms that task, and see what the current rating of it is. DYK what frequency those fets switch at?
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