Where to get PPI A300 mosfets? rfp25n05 - diyAudio
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Old 18th July 2006, 12:59 AM   #1
Jexx is offline Jexx  United States
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Default Where to get PPI A300 mosfets? rfp25n05

Hi,

Just bought this A300 dead. It seems that I just need to replace the power supply mosfets. However, I have had no luck in finding a parts distributor that will sell just four. Attached is a pic of the blown transistors.

http://img.photobucket.com/albums/v3...1/ppiblown.jpg

http://img.photobucket.com/albums/v3...1/DSCN2326.jpg

^ That's the whole board.
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Old 18th July 2006, 01:16 AM   #2
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Those are 25 amp FETs. You can do much better. The IRFZ44N can handle twice the current and should work well with what appears to be 100 ohm gate resistors.

http://www.mouser.com/search/Product...ualkey56050000

If the link doesn't work, it's mouser.com. There is no minimum so you can buy what you need. I would, however, suggest that you buy some extras in case something causes the new ones to fail.

When testing this amp, it's important that the bottom cover be in place so that it applies pressure to the transistors. If you operate it without the cover, the transistors may overheat and fail in a matter of seconds.
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Old 18th July 2006, 02:13 AM   #3
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As Perry says.

I purchased these from here:
https://www.alliedelec.com/Search/Pr...&IND=T&SEARCH=

They were $0.94 each and no minimum. The shipping however was $7. I would get at least 8 just to make you have enough. You will also need to remove all traces of the heat transfer pad and replace with white grease. I hope you have better luck than me. My amp was toasted the same way, but mine has other problems not related to the power supply.
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Old 18th July 2006, 02:30 AM   #4
Jexx is offline Jexx  United States
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Stunning! I have exactly four extra IRFZ44N mosfets in a bag...I'll pop them in and give them a try. Thanks guys!
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Old 18th July 2006, 02:33 AM   #5
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They have mica insulators. They can be cleaned and reused. With the mica insulators, you must use heatsink compound also.

If you used grease alone (without the insulator), the transistors would short to the sink causing the FETs to fail or causing the fuse to blow.

The soft sil-pad insulators can be used dry.
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Old 18th July 2006, 03:37 AM   #6
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STOP!!!!!!!!!!!!!!!

Don't just throw the new fets in without doing some tests first.
You have to check the drive transistors out of circuit and make sure they arn't shorted or more commonly open junctions, it you miss this step all your fets will go up in smoke.

The reason you have to remove them from circuit is if they have a open junction you'll still get a reading from it in circuit cause you'll be testing the PWM chip (SG3525 or TL494).

I'm guessing your going to clean the board up and test the gate resistors? (you will woun't you?).

Also if you can help it, try not to use a battery for your power supply, a nice 3-5 amp regulated psu with current limiting is much better.

GO!!!!!(lol) (after you have checked what was listed)
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Old 18th July 2006, 03:55 AM   #7
Jexx is offline Jexx  United States
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It's working!!!

And yes, I did do quite a bit of testing before I threw the new mosfets back in! I don't want to have to wait another three days for more IRFZ44Ns! haha.

In addition, I used my Tenma psu that only puts out 3 amps max. i turned up the current slowly until the amp had enough current to turn on (~1amp).

Thanks again everyone!!
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