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Old 3rd August 2012, 08:18 PM   #1
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Default MTX Thunder 7801 problem

I have a MTX thunder 7801 it would not power up and was pulling about 40amps. I found some corrosion on the bottom near the outputs, when I checked the fets I found several shorted. I removed all 10 fets and applied power and the service light was on and the lights on the preamp board. I measured +82v and -82v on the power rails as 15.5v on the regulator. My question is besides replacing all the output fets (IRFP240's) is there anything else I should check before I install them and power it up.
Thanks
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Old 4th August 2012, 08:13 AM   #2
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Do not apply power to this amp without having all of the heatsink mounted semiconductors tightly clamped down.

Confirm that you have both positive and negative regulated voltage on the power supply pins of all of the op-amps.

With the black probe on pin 5 and the red probe on pin 8, confirm that you have between 15 and 20v on both opto-couplers.
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Old 11th August 2012, 05:03 AM   #3
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The Op Amps are blown. The one just below the 3120's I can tell let out the magic smoke. FET 192 is shorted as are transistors Q108/109/111, resistors R147/148 are open (smoked as well).
I need the the part numbers for the OP amps, transistors Q108/109/111 and resistors 147/148 and due to the damage on the rectifier side as well as the output portion any suggestions on what to check besides the obvious?
I would also like to know the best way to isolate the power supply from everything so I can ensure that it is operational before I connect the output fets
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Old 11th August 2012, 09:36 PM   #4
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It's likely that most of the op-amps have failed if the regulator transistor (FET191) shorted.

There are likely to be a lot of open 10 ohm resistors. These will be near the op-amps. They feed the power supply pins of the op-amps.

Removing the rectifiers will isolate the power supply. It doesn't generally cause problems on other amps but I've never done it on one of these amps. Try it at your own risk.
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Old 11th August 2012, 10:30 PM   #5
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Fet 191 does not appear to be shorted. however Fet 192 is. Do you happen to have the part numbers for the ones I cannot read and asked for in previous reply?
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Old 12th August 2012, 11:49 AM   #6
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I think they are both IRF540s but I'm not sure and you need to confirm what they. Take some of the white heatsink compound and wipe it onto the face of the transistor. Then wipe off as much as possible. That should allow you to read the numbers.
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Old 12th August 2012, 05:08 PM   #7
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Thanks Perry, could you tell me the part numbers for Q108/109/111, resistors R147/148 their blown and unreadable and the op amps as well if you know please.
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Old 12th August 2012, 05:18 PM   #8
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The attached photo is from a 1501 but I think they are the same.

You can get the part numbers by looking up the markings on the following site. If you can't read them, they're 2L, 1D and H5.
The SMD Code Book
Attached Images
File Type: jpg IMG_0806x.jpg (267.0 KB, 48 views)
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Old 12th August 2012, 11:55 PM   #9
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I think I found all the part no.'s except for the resistors. Any idea on those R147/148?
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Old 12th August 2012, 11:57 PM   #10
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Oh and the Op Amps under the 3120's the tops have burn holes and I cannot read them.
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