Ma Audio M700EX Power Transistor massive failure ! - diyAudio
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Old 9th July 2012, 05:17 AM   #1
dsrq is offline dsrq  Canada
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Default Ma Audio M700EX Power Transistor massive failure !

Hi ! I would let you know that ive learned english by my self and its far to be perfect , so excuse for my poor english !

I have installed a brand new MA Audio amplifier 2 ch m700ex amplifer on 2x MTX Audio TR55 subwoofers DVC at 2ohms each. 2 Week ago im was coming back from the drive-in movie theather ( Ciné-Park ) and all my Power Transistor Blew up and took on fire ,no fuse blew ( 2x25A ). So i have ordered 10x fairchild fqp50n06 transistor from ebay and a package of gate resistor , replaced all the transistor and resistors , applied thermal paste to each Mosfet, ive made sur that every holding plate was tight enought and tested the amplifier ! Everting was fine until today !

I drove about one and a half hour and it started to smell wierd from the trunk , i stopped , toutched the amplifer , it was pretty hot, i tough that the smell was coming from the thermal paste that was cooking a little bit due to the heat. About 20 mins later i turned the key off , open the trunk to see if the amplifier was fine , a lot of smoke came out from the amp again followed by fire again ! But this time , transistor seems to have melted down instead of blowing up ! I am very disappointed , can some one give me some advise on this ! Or know what can be de cause of all this !

Test that i did :
Check Voltage at the fuse block 14.45 vdc with engine running.
There is 3 feet of 4awg wire between the amp and both of the 1200cca batteries in the trunk. Same thing for the ground.
Check resistance from the subs 1.9ohm and 2.0 ohm
no short or any damaged wiring.

Thx a lot !
Sorry for my poor english again !

Bruno

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Last edited by dsrq; 9th July 2012 at 07:49 AM. Reason: image wasnt working
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Old 9th July 2012, 05:32 AM   #2
dsrq is offline dsrq  Canada
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Old 9th July 2012, 08:52 PM   #3
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It's possible that a defective driver transistor caused the failure. Before reinstalling the new FETs, confirm that you have ~5v DC on the gate solder pad for each of the power supply FETs.

If the drivers are OK, the transformer could have an intermittent short.

Ebay is not generally a good place to buy semiconductors. There are a few legitimate sellers but there are more than a few sellers selling dodgy parts. Buy from Digi-key, mouser, future electronics (in Canada) or arrow electronics in the future.
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Old 10th July 2012, 03:16 AM   #4
dsrq is offline dsrq  Canada
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Join Date: Feb 2005
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I have powered the amplifier on a 12v battery , without any power FETs on the pcb , i slowly reach 4.8vdc at the gate so i guess that is fine.

The Transformer is visualy good ! I have about 450ohm of resistance between the primary and secondary windings but the transformer still soldered in place. And it too small to Meg-ohm the windings.

So i think that the amp will go to te garbage and simply buy an MB quart or a Soundstream

Thx for the help!
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Old 10th July 2012, 08:52 AM   #5
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i think it should go to about 5v within about 2 seconds. i noticed you have what looks like a fluke87. while you have it on dc voltage, and reading the 4.8, press the "hz" button on the lower right. i don't remember which comes first, but you can press it again and have the 2 readings. if the first is hz, it should be steady at about 30k, 50k, or 60k usually. then it should display duty cycle in the range of 40-50% or 50-60%, depending. 50% is full duty cycle. i am not sure if these readings will transfer through from the i.c. with a bad driver/drivers, but if they do not, then you know there is a problem. was it able to play full power after you replaced the fets? did you check for shorting of the outputs and drivers?
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Old 10th July 2012, 03:43 PM   #6
dsrq is offline dsrq  Canada
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Join Date: Feb 2005
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Yes that's right its a fluke 87 ! I did a full check of each outputs and drivers, everything is ok ! Both of the 2-ch was giving full power and the bass was good and clear like it should be so this is why ive jut hit the limits of my knowledge about car amplifiers !

The amp was doing is job perfectly for about an hour, and started to overheat slowly until the fet start to overheat and melt down, the amp was still working even if its was overheating and smoking .

I really appreciate for the help and information !

At least ive learn something out of this ! I will never buy cheap amplifiers again !
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Old 10th July 2012, 10:48 PM   #7
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If you repair it again, clamp all transistors to the heatsink and insert a single 15 amp fuse in the B+ line. While the amp is at idle, twist the power transformer to see if the fuse blows. If it does, the transformer likely has an intermittent short. Finding and repairing the short can make the amp reliable.
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