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Old 28th February 2012, 03:04 PM   #1
m4ick is offline m4ick  United States
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Default MTX 1501D

This had a blown power supply and all of the outputs were in tact.

I replaced the 39R2 resistors and all 8 75339ps. It powers on an oscillates but FET260 (IRFB42N20D) gets hot EXTREMELY fast. Within seconds you cant touch it. In the 81000D I just fixed that part did not get that hot that fast.

I'm not familiar with this section of the amp, if I had to guess it appears to be a voltage regulator. Any idea why it would be heating up so quickly?
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Old 28th February 2012, 03:08 PM   #2
m4ick is offline m4ick  United States
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Also FET260 is not shorted and I have all 3 LEDs lit.
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Old 29th February 2012, 12:55 AM   #3
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Compare the resistance across pins 5 and 8 for each of the opto-couplers. Is one significantly lower than the other?
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Old 29th February 2012, 04:57 PM   #4
m4ick is offline m4ick  United States
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No, they both read ~990 ohms and slowly rising while charging the caps.

I did notice though that one of the caps marked 220 10A has a VERY small dent in it. It's the one right next to the mounting screw. In this amp it says "e2 220 10V". I don't know if it would affect FET260 but I thought you should know.
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Old 1st March 2012, 03:22 AM   #5
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On the opto-couplers wit hthe black probe on pin 5 and the red probe on pin 8, what is the DC voltage for each one?
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Old 1st March 2012, 12:45 PM   #6
m4ick is offline m4ick  United States
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+27v and +28.2 after muting is released. The +27 seems to drain much more quickly after powering off.
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Old 1st March 2012, 09:31 PM   #7
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27v is too much. The regulated voltage should be very near 20v. The gates of the FETs may break down with anything over 20v. This would drive about 27v to them.
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Old 2nd March 2012, 01:49 AM   #8
m4ick is offline m4ick  United States
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I have no idea where the voltage regulators are in this amp. Where should I be looking for this over-voltage?
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Old 2nd March 2012, 09:10 AM   #9
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What's the voltage across D260?
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Old 2nd March 2012, 02:05 PM   #10
m4ick is offline m4ick  United States
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26.7v
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