Mmats DHC 1400.1 outputs (should be easy)

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Crazylogix

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Joined 2018
Have a Mmats class D amp with the HIP4080 from a friend. He had tried replacing the blown FB4710 fets with china-ebay fets (probably fakes). This is my first HIP4080 based amp I have worked on. So I pulled the fets, power supply came on good, and it appeared to even have good drive from the HIP4080, but I ordered a new one anyway, and 8 new IRFB4710 fets (I think that's my mistake, should have ordered IRF3710Z fets). Tested all the diodes and resistors for the drive circuit, everything looked good. Got in the new parts and installed all fets and new chip. Amp powered up just fine and played (super low volume test, super short time, so now heating of fets). Reinstalled in heatsink (had to do some work, a fet had shorted to the sink, all fixed). hooked amp up and played at mid to low volume, everything was fine for about three minutes, then I heard a distorted pop, not sure if any sound came from the amp, but it sure did from the speaker, now the amp plays but with a bad rough distortion that almost sounds like the speaker dragging or partly blown. Tested with another woofer, the same (both woofers checked out fine on another amp). Also now with no input signal at idle get a rough hum out of the speaker. I am assuming that the HIP4080 and the B4710 fets did not play nice like i have read in lots of threads (yea I have read them now, just not before I ordered the parts). Wanting to confirm this is how they parts like to act when they don't work together right. New 4080 and 3710Z will be here tomorrow. Anything else i should check first before installing the new parts and trying it again?
 

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Crazylogix

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Swapped out for another new 4080, and used IRF3710Z fets, still have the hum with no signal applied. Found a triangle wave on pins 5,6 and 7 (the output) of U2 LM837 that might be it, but cannot trace the signal back, my o-scope is not so good and I am struggling to find the cause of the hum and or oscillation that should not be there.
 
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Crazylogix

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Joined 2018
Testing, had to make a shorting RCA connector. When install the shorting RCA connector in either jack, the level of the hum goes down considerably but its still there. At 2ms and500mv setting AC coupled. the waves look crazy on my scope.
 

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Crazylogix

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Holy poop, I had only done one of them at a time. I did both and the hum went away! You can tell me what it is, but in the mean time I am looking for something that is between the power ground and the input ground, like a diode, resistor or cap. At least that is what i think is probably wrong, alot like getting noise on rockford amps when someone has touched the power wire to the rca shield.
 
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