# Poping fets on QSA1500D?



## blu3rav3n (Aug 20, 2014)

Ok so im new to the forum as far as posting but been looking around trying to find out if I missed something when I replaced a few IRFZ44N's in my amp. 

Back story: I took the amp apart to do some cleaning seeing as its an older amp, lost grip of a screw and it landed on one of the fets causing a spark, followed by smoke, etc...

I de-soldered the original and re-soldered a new one in. Plugged it in and Pop, Pop, Pop.. 3 more of em blow further done the line. Replaced those plugged it in and 5 seconds later pop. Same fet I replaced the first time blew again. Looked through the forums and read about bad gate resistors. Checked the board and from the looks of it cant find one that looks bad. Heres just a quick pic I took the second time of the board. Ill take a clearer picture of the board later today with everything for you guys to see and maybe give me an idea what to check next. 


Thanks for any help in advance.


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## blu3rav3n (Aug 20, 2014)

OK so a few more pics of the board. Hopefully I am posting this in the correct section. If not little help would be much appreciated.


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## shawnk (Jan 11, 2010)

After you replaced the fets, did you keep the insulators underneath them? Are they (the insulators) still in good shape? Be sure that you do not have continuity on the drain (metal tab) of the fet to the chassis. The insulators are there for this very reason.

You may also have shorted driver transistors. You need to check those as well (Q10 & Q9 appear to be the drivers)

You should also have a fuse inline while testing (10 amp should be ok). To take it a step further, place a 2ohm (at least 10w) resistor inline with the power cable. This will drop the supply voltage a bit, but it will also help with troubleshooting. If you continue to have problems ie... dead shorts, the resistor will limit current flow and will hopefully prevent the fets from frying over and over again.... hint: the resistor will get very hot if you still have shorts in the amp :surprised:


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## vwdave (Jun 12, 2013)

What are you using for a power supply? How many volts? IIRC those FETs don't like being powered under 12 volts.

I've had a similar problem and it turned out to be a bad PWM (credit to ShawnK for that suggestion). Maybe you messed up the PWM by creating the short with the stray screw.


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## blu3rav3n (Aug 20, 2014)

Thanks guys for the replies. 

Shawnk, when I took it apart the first time I check all of the insulators before putting the board back in place and reapplied thermal paste, check the fets when they in place and rechecked after tightening the bracket down on them. 

How would I check the driver transistors? I got my ohm meter but im gonna guess need the info of the transistor to measure against and check right? I think I have an idea on your whole testing with the 2ohm resistor, im more techy then the average newby but on this level of troubleshooting im still new so thanks in advance for explaining things . 


vwdave, As far as a power supply. I lack as far as testing goes. I hook it right back up in the car to test. You mention PWM ? youll have to explain if you could.


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## shawnk (Jan 11, 2010)

You can check if the the drivers are shorted with a continuity test between any of the three legs. It would be best if you had an oscilloscope to see if the square wave from the pwm (pulse width modulator) is being driven into and out of the drivers.


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## blu3rav3n (Aug 20, 2014)

Ok checked the drivers and nothings shorted(my multimeter didnt make a peep except from when I hit a resistor somewhere else on the board nearby), I dont have an oscilloscope but from alot of the videos I watched on youtube I know at least what your talking about. 

I read in a few places that when I replace the mosfets, I should replace all of them. Think this is necessary ? Also with the idea you gave for testing, how could I then test it (and locate where the short might be) with it hooked up that way?

Sorry for the ton of questions, but Im also trying to do a bunch of research on the side to get a better understanding on what to look for and how to test it. Id say since this amp is over 6 years old id just go get a new one, but funds are so limited right now(and for awhile till after wedding and car payments) I gotta hope I can fix this on the cheap.


Thanks again,


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