MOSFETS Smoking, Help needed. - diyAudio
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Old 13th July 2006, 06:28 AM   #1
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Join Date: Jul 2006
Question MOSFETS Smoking, Help needed.

Amp is made by Thump, it is a model T4006. I would call it generic but it worked very well.

Problems,

1.) Subs would buzz and act wierd if I connected both the + and - terminals to my phono plugs. I found that if I did not hook up the - completely it would work well. I can live with this problem.

2.) Burning smell from trunk, found the the remote wire was melted and shorted to the aluminum casing of amp, I am not sure if an internal malfunction caused the remote wire to melt or if the remote wire rubbed thru and caused the failure of the internals. After rewiring the amp would power up then all of a sudden smoke started coming out of the inside.

See attachment for a pic of the MOSFETS. I am not sure if they are part of the power section or sound section, the one in red is one that is smoking. Any ideas on where I should begin? I replaced the other 3 that were burnt, but they did not burn out yet. I am not equipped with much other than some soldering tools and a DVOM. I only have very basic electronic knowledge. Any help will be much appriciated.

http://community.webshots.com/photo/...44425697zAdIWk

http://community.webshots.com/photo/...44425697qMuLLN
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Old 13th July 2006, 11:49 AM   #2
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Join Date: Nov 2003
Location: Louisiana
Those are the power supply transistors.

If you replaced all 4 transistors in that area and one failed, there could be several problems.

It looks like there's a dark area near the power supply transistors directly across from the ones you replaced. Did you check those? If you read a very low resistance or a direct short between the first and second pins, the transistors are defective. If they are defective and share the same drivers as the side you replaced, that could have caused the new ones to fail.
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Old 13th July 2006, 04:59 PM   #3
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Excellent observation Perry. I found 3 that were melted and was an idiot and only ordered 3. The rest of them will be here today I I will be replacing all of them just to be safe. I just did not want to simply slap the new ones in and have them fail a second time. In your guide, I am not sure when I should have the amp powered up to test them and when not to have it powered up.

I was curious what the root cause is of these failing. I see these fail in lots of electronics.

Thanks for the promt reply.
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Old 13th July 2006, 10:07 PM   #4
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Location: Louisiana
When checking virtually anything on ohms or diode-check, the amp should not have power applied. The voltages that are present when the amp has power applied to it will skew the readings.

The power supply transistors can fail from any of the following:

Shorted output transistors
Shorted transformer
Shorted rectifiers
Improper speaker load
Improper fusing (especially if the power connections are reversed)
Low voltage if the amp is driven hard
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Old 14th July 2006, 08:09 AM   #5
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Well, I am probably going to give up at this point. I soldered all new 8 mosfet in and powered it up. Everything worked and no smoke this time. I was all excited but when I put it in my car all I get is crackeling out one speaker and not a thing out the other. Like I said, it powers up just fine but does not produce usable sound.

Is this something that can be diagnosed fairly easy or am I better off pitching it?
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Old 14th July 2006, 01:36 PM   #6
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Join Date: Nov 2003
Location: Louisiana
It could be that one supply is not operating. If the preamp's regulated power supply is supplied by the inoperative supply, that could be the problem.

With the amp powered up, measure the voltage on the center legs of the output transistors. Do this on both channels. You should read positive 35-50 volts on half of the ouptus and negative 35-50 volts on the other half of the outputs. All outputs must have either positive or negative voltage on the center legs.
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Old 22nd July 2006, 08:48 PM   #7
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Ok, I found that my RCA input block was not soldered very well from the factory. It was not salvagable so I unsoldered it and just soldered some wires directly to the PCB. I did indeed pay close attention to the polarity. Everything works just fine now, but my next question is why can't I connect my negative input leads? If I do, it makes the subs sound blown, if I leave just the + connected it sounds perfect. Any idea's?
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