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Old 11th December 2006, 12:48 PM   #1
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Default Help needed in identifying MOSFETS

Hi All

I was given this amplifier to repair and discovered why the protection light remained on. Turned out to be a bank of Power MOSFETS in the amp section that had shorted.

The problem is that I have no way of identifying these parts as their numbers were ground of, probably by the manufacturer. I have taken photos of the amp and would be most grateful if anyone is able to assist. The amp is called American Amps model AA200.4
PS. I do not have a schematic for this amp either which makes it even worse.
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Old 11th December 2006, 12:50 PM   #2
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Another one
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Old 11th December 2006, 12:52 PM   #3
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Old 11th December 2006, 07:14 PM   #4
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Hello, do you know which transistor is defect or what is defect in the amplifier. When you know which transistor are destroyed, please write the numbers and letters from this transistor in the forum. I have the ECA VRT 2005 disk. Perhaps I can help you.

Regards jörg
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Old 12th December 2006, 07:28 AM   #5
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Hi Jörg

The MOSFETS in the amplifier section for 2 channels are shorted. The reason why I posted here is because there are no markings on these MOSFETS as they have been taken off by an abrasive method.
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Old 12th December 2006, 01:08 PM   #6
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Default Do not think they are mosfets

Hi,
Archimeds, it seems to me that they are not mosfets. Probably they are BJT transistors. The parts may be 2SA1695/C4468 complements. I do not know your amplifier model nor I have shematic of it. But, many of this kind of amplifiers uses the BJT transistors, i wrote above. If u can not find original replacements, U can use TIP35/36 pairs by replacing the driver BJTs. But, at first you should make a basic measurement, to be sure if they are mosfets or BJTs. Please make a measurement with your multimeter's rectifier mode. Measure the output Transistors' 1 to 3 and 1 to 2 legs(from left to right). If you see ~0.6V we can be sure that they are BJTs. If my assumption is true U can use original replacements (1695/4468). Otherwise if they are really mosfets you should need a real help for the shematic of amplifier.

Good luck
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Old 12th December 2006, 01:17 PM   #7
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Hi Whortless

Thanks for the reply and advice.

I have already done those tests by removing a transistor from the other bank(that is working) and testing it to see if it is a BJT or MOSFET.

I do not measure .6V on from 1(B) to 2(C) or 1(B) to 3(E). The absence of the bias transistor on the heatsink as well as the small size of the driver transistors indicates to me that this is a MOSFET amp. It looks like some type of Fairchild Complementary MOSFET.
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Old 13th December 2006, 01:52 AM   #8
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Although anything is possible, I really doubt that the amp is using FETs in the output stage.

There are many amps that don't use bias transistors. They operate as class B amps with no bias current.

Do you not get any readings from pin 1 to pin 2 (on both ohms and diode check) no matter the lead placement?
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Old 13th December 2006, 07:37 AM   #9
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Hi Perry

Nice to hear from you.

I have tried swapping leads around (PNP,NPN) to take measurements but they did not show. I took out the transistor and tried measuring the HFE on the multimeter and this did not help either.

I will have another go later on this evening and will post my findings.

thanks
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Old 13th December 2006, 07:27 PM   #10
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Hi,
It is interesting. Two weeks ago I repaired one of my friend's amplifier, named Grunberg(4*200W). The inside of the amplifier was very similar to yours and it was using the BJT pairs i told before. I replaced rear outputs' BJTs with A1943/C5200 pairs to use it in a stable 4ohm bridge application and the amplifier is wery well now. But, this is not the solution of your problem. There are many choises in TO247 pacage to use as output devices. SJ/SK or IRFP*/IRFP9*. If they are labeled as Fairchild, please check the Fairchild's web site for power mosfets. Last week I bought 4 IRFP150 labeled as Intersil product and have look the complementary pair but they do not producing IRFP9150. Therefore, I think you can decide which mosfets could be your amplifier's output transistors(probably they are IRFP240/9240 but, they can not be work in 2ohm in such kind of an amplifier).
On the other hand your outputs may be darlington complementary pairs. To check this please measure the C-E legs in reverse polarity. Although some darlingtons include diodes but, mosfets always include body diodes.

Regards
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