MTX MiTEK Thunder4250D blown - diyAudio
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Old 22nd October 2009, 07:34 PM   #1
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Default MTX MiTEK Thunder4250D blown

Working on this Class-D 4250D. Had a shorted mosfet output, FET101 is the ID number. Replaced mosfet to see the speaker output pinned to one of the rails.

Q117 has a fairchild logo, marked D124 A42, no idea what type of transistor this is, but it is shorted. all resistors are in tact, no burns, no opens, checked them all out. everything A-OK here.

any ideas what the actual part number for Q117 would be? its an SOT-223 style SMD transistor by the way...

Last edited by mbates14; 22nd October 2009 at 07:37 PM.
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Old 22nd October 2009, 07:51 PM   #2
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Pzta42?
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Old 22nd October 2009, 08:22 PM   #3
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yea i figured it out. i replaced it too and it was still pegging the rail around 50v. then out of nowhere, it quit pegging the rail, but i still had rails.

Then the output mosfets exploaded again. lol. **** it. these are the worse amps made in history. i had another one before this with similar issues.
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Old 22nd October 2009, 09:42 PM   #4
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These aren't bad amps but you can't expect to repair one by replacing only a single part.

When you have a failed output, you should replace all of the ones working together (in this amp, all of them). In class AB amps, a leaking transistor (one that's defective but not shorted) will cause excessive current draw but you can generally shut the amp down before any damage is done. In class D amps, a leaking output can cause instantaneous failure of other outputs and driver components. That's why you need to replace them all.

I don't have anything on this amp but in similar amps, the following components fail:

* gate resistors (confirm that all are within tolerance)
* driver transistors (generally the smaller ones but sometimes the PZTA drivers)
* VAS transistor (Q115 and Q116?)
* High side supply transistor (Q114?)
* Low side supply transistor (Q110)
* Various other resistors (virtually always obvious when you look at them through a lighted magnifying glass).
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Old 22nd October 2009, 10:48 PM   #5
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yea theres something wrong somewhere... i been through all teh transistors, none are shorted or open. been through all the resistors, and they all appear to be within tolerance. only thing failed was that output transistor along with its driver Q117
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Old 22nd October 2009, 10:54 PM   #6
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Also, all 6 of the R220s off the FETs are blown open now also, when the outputs shorted again.
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Old 22nd October 2009, 11:01 PM   #7
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It's nearly impossible to detect leaking transistor in the board. Any of the drivers or outputs could be leaking and causing problems. You'd need to pull and check them very carefully to determine if any are leaking.
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Old 23rd October 2009, 01:15 AM   #8
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need to figure out what "R220" means. 0.22ohms?
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Old 23rd October 2009, 01:17 AM   #9
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Correct. 0.22 ohms 1% tolerance
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Old 23rd October 2009, 01:24 AM   #10
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I might have to change the caps in the output circuit, they dont look so well either, the outter wrapper has shrunken down from the top. however they ESR fine.
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