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Old 25th September 2010, 06:05 AM   #1
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Default MTX Thunder 81001 fets

Hi guys, new to the forum and looking for some help with a MTX Thunder 81001. The output filter caps were blown and one of the output fets is bad. The other ones seem good in circuit but I`ll pull them to double check.

Can i replace one of the fets or do i have to replace them all to get them to match? Also 4 resistors got hot if someone could confirm the value of r490-494.

Do these work of a split +- rail voltage or a single supply. This is the first class D I`ve seen.

Any idea what causes a failure like this?
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Old 25th September 2010, 06:02 PM   #2
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MTX thunder 81001 board mvp171-c identify 4 burned resistors

Any FETs that are in parallel with the failed FET need to be replaced. The replacement parts should be from the same batch of parts (same date code).

The amp has positive and negative rails.
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Old 27th September 2010, 12:12 AM   #3
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Thanks for the response. Can i just order 6 of the IRFP240's from Digikey and expect to get the same date code? If not a source would be great. Anything special about the caps in these?
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Old 27th September 2010, 02:18 AM   #4
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FYI, You can also email MTX for the values of Resistors and Caps, etc. They will not give you a copy of the schematics, but as long as you know the number on the board, they will give you the proper info.
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Old 27th September 2010, 09:09 AM   #5
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Ask the salesperson if you can get all with the same date code. If they can't guarantee it, or them in multiples of 10 (order extras) and you are more likely to get transistors from the same batch.
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Old 6th October 2010, 09:08 PM   #6
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Installed my parts today, 6 fets (same date code) 4 10k resistors and the 8 output filter caps. The amp is running and the short lite is out.

How hot should this be running with no input. I have an 8 ohm speaker connected but no input signal and after about 40min the fans are on and its very hot. Measured input current 4.6A at idle. Also got 180khz 1vpp on the output. Is this normal or do I have other faults?
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Old 6th October 2010, 09:57 PM   #7
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Let it cool down completely. Power it up and monitor the temperature of the heatsink. Does one particular area (behind the outputs, behind the power supply FETs...) tend to neat up more quickly?

What's the DC voltage measured directly across the positive and negative speaker terminals?
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Old 7th October 2010, 12:33 AM   #8
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When I first power it up there is some varying dc on the output but it settles down after about 10 secs and ends up at 30mv dc. The heat is coming from the bank of 4 to220's behind the rca inputs. I pulled the bar off them and the IRF540 seems to be generating the most heat when i touched it directly.

Thanks for the help with this btw.
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Old 7th October 2010, 02:25 AM   #9
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Also just wanted to add the 2 large inductors in the output filter are get hot fast too.
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Old 7th October 2010, 02:36 AM   #10
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The inductors operate at relatively high temperatures. Even at idle, they'll get too hot to hold your hand on continuously.

The two regulators will get hot very quickly. Do not operate the amp without having them clamped. If they fail, they can cause extensive damage. They can fail within seconds if not clamped tightly to the heatsink.

When the fans were running, were they running at full speed? You can determine if they were at full speed by measuring the voltage at the white 2-pin connector for each one. If they were not running at full speed, it's likely that the temperature wasn't a problem.
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