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Old 28th August 2012, 12:26 AM   #1
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Default MTX Thunder 302 blowing output transistors

I bought this amp knowing that it may have a problem. When I first tested it, it would not power up. I checked the output transistors and some were blown. There were some IRF540's & some SSP45N20A's in it (one channel had the 540's ad the other had the SSP45's). I replaced all 8 with IRF540z's. It also appeared that one, if not both of the filter caps had vented, so I replaced them too. This amp had been repaired before by a CC tech (the repair tag is still on the bottom).

The amp played fine at low power, but blew the fuses under a little bit of strain. I opened the amp up and two of the 540z's had blown. I have checked the resistors associated with them and all are fine.

This is a two channel amp and there are 4 outputs per channel:
FET315, FET317, FET314, & FET316 on one channel, then
FET215, FET217, FET214, & FET216 on the other.
The FET215 and FET 216 are now bad and all of the others are fine.
With the four 200 series outputs out, it will again turn on.

I may be wrong here, but I believe that 215 & 217 work together as well as 214 & 216 working together to equal the high and low of one channel. This would mean that one high and one low each shorted. Does this sound correct?

What did I miss the first time? Where should I look from here?

I do not own a scope, but if necessary, may have access to one. I am fairly new, but have repaired a couple of amps, some with guidance (help), some without.

Any help is greatly appreciated.

Last edited by playzwtrux; 28th August 2012 at 01:20 AM.
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Old 28th August 2012, 01:38 AM   #2
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When you said some caps vented, you have to clean up the mess as this stuff is very corrosive and Conductive. Just a thought.
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Old 28th August 2012, 01:39 AM   #3
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It's common for one of each of the output transistors to fail. One fails and causes one of the opposing transistors to fail as the amp tries to compensate for the DC output caused by the first transistor that failed.

If it uses the white sil-pads, could one of them been defective and allowed the transistor to short to the sink?

Did you re-adjust the bias setting after replacing the transistors?

Did you check the gate resistors for the output transistors?
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Old 28th August 2012, 01:53 AM   #4
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90scaraudio - they did not leak any fluid on the board. they were wrapped in black plastic and it looked like there was a relief at the crown - I did clean under them when they were off.

Perry - it does use the white sil-pads, I will recheck them

I did check the gate resistors and they appear fine

I did not adjust the bias. I will go back and read the tutorial on it. Can you adjust it without the use of a scope?
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Old 28th August 2012, 02:10 AM   #5
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This is from a different post...very helpful...

Originally Posted by Perry Babin

As was suggested, you need to bias the transistors. If you have a power supply with an amp meter, set it as is shown in the following demo.
http://www.bcae1.com/temp/ausettingbias.swf

If you don't have an amp meter on the power supply, set the bias by using the DC voltage across the emitter resistors. Set the bias so that you have approximately 0.001v across the emitter resistors at idle.
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Old 1st October 2012, 11:01 PM   #6
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Is adjusting the bias as difficult in reality as it is in my mind? It appears that there is very little room for error when adjusting them. It is like they are too low and then quickly transition to too high. Maybe it is just the unknown of not having done it before that bothers me most.

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Old 1st October 2012, 11:25 PM   #7
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I've never adjusted the bias on that particular model but sometimes they're a bit touchy (as is shown in the demo). It's not critical. As long as you have it adjusted where it just starts to pass current or just a bit before that point, it's OK.
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