Dead (or dying) amp. Want to fix. - diyAudio
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Old 10th October 2006, 09:29 PM   #1
vageta is offline vageta  United States
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Question Dead (or dying) amp. Want to fix.

I have a Tsunami 4440db 4x75 amplifier that recently took a crap on me and would like to see if it would be possible to fix. Basically I was just listening to music one day and my front stage got quiet. Went to check the amp and noticed it had popped it's 2 30amp fuses. Went to replace them and they began popping the instant the fuses were inserted, even without the amp being turned no via remote wire. Now for some reason it won't even pop the fuses, it just will start smoking one of the mosfets the instant power is applied.

So I opened up the amp and noticed that the first mosfet in the path was broken physically. One of the prongs was completely seperated from the rest. Thinking I had found the problem I went to my local electronics store and bought a replacement part, replaced it on the board and after firing it up it did the same thing. I left the top of the amp off while firing it up and I could see the mosfet just burning the instant 12v power was hooked up.

It seems like some sort of short somewhere but I can't seem to pinpoint it. I have soldering skills from previous jobs though I can't say I'm an expert on electronics. I have just been probing around with my DMM but so far nothing has struck me as obviously wrong.

Is there anything specific I should look for based on my explanations above. Where to start, how to know if I've fixed the short/open circuit? I can supply pics of the board and parts if needed.
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Old 11th October 2006, 04:52 AM   #2
djQUAN is offline djQUAN  Philippines
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in most cases. you should replace all the mosfets. not just one that looks blown. others may look fine but they will show up with shorted pins.
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Old 11th October 2006, 10:09 PM   #3
vageta is offline vageta  United States
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Ok so I pulled all 8 mosfets from the amp and tested them individually and found that half of them were indeed shorted even though they showed no signs of it. I've tested all the areas on the mboard around the fets and everything measures out consistent and I'm about to reinstall them. Before I actually attempt to test it is there anything else I should pay close attention to?

I'm curious if the original mosfet that physically broke and shorted is the problem that caused the amp to fail, or if something else happened which made the mosftet(and the others) fail. I'd really hate to burn out the mosfets again after replacing them.

I will be putting a headlight between the 12v power line for protection and was thinking about installing much smaller fuses in the amp itself as they haven't been blowing lately. I guess the fets are burning up at much less current than takes to pop the 2 30a fuses.

So any recommendations on other areas to check before I attempt this?
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Old 12th October 2006, 09:12 AM   #4
djQUAN is offline djQUAN  Philippines
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try and read other threads about car amp problems here. this is a very common problem and it has been dealt with many times.

check the output transistors of the amplifier section to see if there are also shorted/open transistors.
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Old 12th October 2006, 09:30 AM   #5
peufeu is offline peufeu  France
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next time you power it up, put a high wattage 4 ohms resistor in the 12V power line, or use a current limiting bench supply, so that it will just heat up instead of frying

if you use a transformer from the mains you can put a lightbulb inseries also.

also usually when output devices burn they take the rest of the active devices (drivers etc) with them, so be sure to check them out.
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Old 12th October 2006, 09:41 AM   #6
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If the first leg broke loose, that can easily cause the amp to fail.

If you haven't replaced the transistors yet, power up the amp and measure the DC voltage on pins 9 and 10 of the TLx94 IC. If you don't know which pins are 9 and 10, go to ti.com and download the datasheet.

With the black meter lead on the ground terminal of the amp, the meter should read ~1/2 of B+ on pins 9 and 10 and also on each FET gate (pin 1 of the FETs). Since the FETs are not in the board, you can measure the voltage on each gate resistor (on the 'FET' end of the resistor).

After you replace the FETs, clamp the transistors to the sink BEFORE applying power. If there is a problem, the FETs have a much better chance of surviving if they're clamped down. If they're not clamped down and there is a problem, the FETs can fail in seconds... Even if you have a current limiter. If they're clamped down, the current limiter will virtually eliminate any chance of damaging the FETs.
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