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#4397516 - 12/30/17 09:43 PM Anyone know anything about electronics repair?  
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I'm going nuts. My Klipsch promedia 4.1 subwoofer/speaker combo has died. I already managed to replace a high voltage capacitor and that fixed it for a while, but now I've found one of the MOSFETs on the DC to DC converter board was sheared off above its solder joints. I must replace the MOSFET, so I bought a new one.

Easy, right?

Would be if I could figure out what the MOSFET is attached to a thick metal L shaped backing plate. I assume it is a thermal compound or a glue, as the old residue is white. Something. The old mosfet is an IRF740, the new one is a NTE 2397--an equivalent part. I can solder the new one in, but then I get to the metal backing plate and I'm stuck. What do I use to stick it to that plate?

Thermal pads?
Thermal paste?
Thermal grease?
Glue?
Will CPU thermal compound work or do I need to go to the store to get something else?

I'm going mad wearing headphones. I need my speakers back. :-|

Any pros around here have some tips?

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#4397526 - 12/30/17 11:12 PM Re: Anyone know anything about electronics repair? [Re: Mr_Blastman]  
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Ice Cold in Alex or Eating in ...


Chlanna nan con thigibh a so's gheibh sibh feoil
Sons of the hound come here and get flesh
Clan Cameron
#4397571 - 12/31/17 04:33 AM Re: Anyone know anything about electronics repair? [Re: Mr_Blastman]  
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Looks like the old mosfet was using plain old heatsink grease, and came off real easy. No pad or anything.

Anyways, looks like I have to wait another week before I get the proper part, because I read the electrical schematics wrong. I was assuming it was NTE2397 but turns out when I pulled the old mosfet out, it read U1620 which is a NTE equivalent of the 6240 part.

Lesson of the day: NEVER TRUST SCHEMATICS. ALWAYS DOUBLE CHECK THE ACTUAL PART BEFORE INSTALLING THE NEW ONE.

Or better yet... never trust your own dumb self for thinking you can read schematics when you don't know a lick of electrical engineering like my mom never knew what I did in my bedroom every night before bed...

Okay, she probably did. Yuck. They BOUGHT me the playboy subscription, what do they actually think I was gonna do? Read the articles?

TMI!

So, off to the electronics store to order the right part. Good news is I finagled the old part back on using the retaining clip and new grease so it made a tight contact, so, ghetto zipties for sound again.

#4397579 - 12/31/17 06:50 AM Re: Anyone know anything about electronics repair? [Re: Mr_Blastman]  
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"Anyone know anything about electronics repair?"

if you're repairing/replacing individual components on a board then you're already beyond most of us here or anyplace


"everything lives by a law, a central balance sustains all"
#4397589 - 12/31/17 10:14 AM Re: Anyone know anything about electronics repair? [Re: Mr_Blastman]  
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Originally Posted by Mr_Blastman
Looks like the old mosfet was using plain old heatsink grease, and came off real easy. No pad or anything.

Anyways, looks like I have to wait another week before I get the proper part, because I read the electrical schematics wrong. I was assuming it was NTE2397 but turns out when I pulled the old mosfet out, it read U1620 which is a NTE equivalent of the 6240 part.

Lesson of the day: NEVER TRUST SCHEMATICS. ALWAYS DOUBLE CHECK THE ACTUAL PART BEFORE INSTALLING THE NEW ONE.

Or better yet... never trust your own dumb self for thinking you can read schematics when you don't know a lick of electrical engineering like my mom never knew what I did in my bedroom every night before bed...

Okay, she probably did. Yuck. They BOUGHT me the playboy subscription, what do they actually think I was gonna do? Read the articles?

TMI!

So, off to the electronics store to order the right part. Good news is I finagled the old part back on using the retaining clip and new grease so it made a tight contact, so, ghetto zipties for sound again.


Has that mosfet been changed before?
And are you sure it is a transistor and not a rectifier diode?

Edit: There is quite a bit of a difference between a IRF740 and a U1620 both in packaging and how it works internally:

U1620 http://pdf1.alldatasheet.com/datasheet-pdf/view/698789/THINKISEMI/U1620G.html This is a 200V device compared with the others that are 400V devices
IRF740 http://pdf1.alldatasheet.com/datasheet-pdf/view/22400/STMICROELECTRONICS/IRF740.html
NTE2397 http://pdf1.alldatasheet.com/datasheet-pdf/view/10009/NTE/NTE2397.html


Last edited by Alicatt; 12/31/17 10:35 AM.

Chlanna nan con thigibh a so's gheibh sibh feoil
Sons of the hound come here and get flesh
Clan Cameron
#4397618 - 12/31/17 02:32 PM Re: Anyone know anything about electronics repair? [Re: Mr_Blastman]  
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Never been changed or touched before. The old part reads:

top row: 01 34
Middle: U1620
Bottom: AKA

#4397630 - 12/31/17 03:32 PM Re: Anyone know anything about electronics repair? [Re: Mr_Blastman]  
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I've done quite a bit of repair on old boards, and some speaker repair on old AR-3 speaker. There is only 1 guy in the entire world that can repair the spiders on an AR-3 sub-woofer, he's located in NY, and I sent my woofers there a while ago. Very impressed. It's a dying world, and few people have real knowledge on this stuff any more. But I digress..

Instead of wacking out here, there is only ONE place on the internet where the guys are extremely cool and will walk you through any repair on anything vintage stereo/speaker stuff. They will even send you the items you may need at your shipping cost:

http://www.classicspeakerpages.net/IP.Board/

Go there and sign up. Post your issue and I guarantee more than 1 person will be 100% knowledgeable and offer their help.


"College graduates should not have to live out their 20s in their childhood bedrooms, staring up at fading Obama posters and wondering when they can move out and get going with life" - Paul Ryan
#4398170 - 01/03/18 11:27 PM Re: Anyone know anything about electronics repair? [Re: Mr_Blastman]  
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Good catch on the parts, the U1620 and IRF740 are very different parts.

As for the grease, they're both isolated and so long as it's bolted on you can use any pad or grease. You just need something to fill any gaps and facilitate heat transfer from the chip. So yes, regular CPU compound would work.


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