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| Help on Amplifier, somethings out? http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10102&t=15132 |
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| Author: | Bruce Dickey [ Tue Dec 25, 2007 12:57 pm ] |
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My son brought home an old amp from a garage sale, it only hums. He also brought home a killer 12 channel mixer. He was told the mixer worked the amp didn't. Proved correct. Here is the only thing I found wrong inside: 
I had some other pictures, but they won't load. so, this part appears to have one leg burned to the point of disconnecting. It appears to be some sort of computer memory? Can I get a new one and plug it in for a try? Also, what is it called. Thanks. |
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| Author: | Tom Barton [ Tue Dec 25, 2007 2:04 pm ] |
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Hi Bruce- This is an analog driver chip designed to drive a pair of power transistors that then drive a speaker. This part is obsolete, but you may be able to get one from a place that specializes in such parts. Try googling "lm391" and you'll get spec sheets and a bunch more. I tried my usual sources (Digikey and Mouser) but no luck. I'd bet that there is more damage to the amp than just this part, as it looks like a fair amount of current was flowing through those two pins, which are the power connections to the chip. Hope this helps at least a little! Tom
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| Author: | Bruce Dickey [ Tue Dec 25, 2007 2:30 pm ] |
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Thanks Tom, yes, this is quite old. But looking all over carefully, it's the only thing odd inside, no loose wires, or debris, or burned looking wiring, etc. And this picture is way larger than the part which is 1/4 inch by 3/4 inches. I nearly missed it. But once found, gave me a bit of hope for this old amp. I'll investigate a bit more before tossing it and buying a new amp. Many thanks. Anyone know where to get a part like this? TIA. |
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| Author: | JimWomack [ Wed Dec 26, 2007 12:12 am ] |
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Hey Bruce, try http://www.aeri.com/search/LM391N? ... looks like they've got a a few N-100s in stock. |
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| Author: | JimWomack [ Wed Dec 26, 2007 12:23 am ] |
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Sorry, try this: www.aeri.com/search/LM391N? |
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| Author: | Bruce Dickey [ Wed Dec 26, 2007 2:16 am ] |
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Thanks Jim, I'll check that out. I made one mistake posting on an electronics website of my need for this part. My email box was full of offers from China. Fromm $1.35 to 98 dollars for the part but they'd like me to buy 100pcs. lot. Aaargh! I just wish I hadn't made the post......  everyone in China wants to be my friend now.
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| Author: | James Orr [ Wed Dec 26, 2007 4:57 am ] |
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Bruce, can you find a model name for the amp? Schematicheaven.com might have some information. |
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| Author: | Bruce Dickey [ Wed Dec 26, 2007 5:54 am ] |
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Neptune 111 Power Amplifier Mod. # 111 Ser. # 17244 120V 200W input 2 amp fuse |
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| Author: | Billy T [ Tue Jan 08, 2008 5:59 am ] |
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Bruce you might want to go throught he schematic for this amp and check all the components, if you can get one. I'd like to reiterate what Tom said, there's probably more damage to the amp that caused that burned up IC, in the first place! |
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| Author: | Bruce Dickey [ Tue Jan 08, 2008 9:23 am ] |
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Everyone in China seems to have a pickup load of these chips. Not one for sale in America. I'd fry one just for fun if I could find it. Soon this amp is going in the dumpster, it's a Neptune! One post to China and I've had literally close to one hundred emails from Electronics firms over there...... |
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| Author: | Rick Turner [ Tue Jan 08, 2008 3:20 pm ] |
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What folks are saying is that there was likely a problem upstream of that chip that caused excess current to flow and fry it. Or it could just be that some piece of conductive junk got in there and shorted those pins out enough to have it arc over. What is up-stream of those chip traces? Is it the power supply? There could be a short in a transformer or across other elements in the power supply. |
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| Author: | Rick Turner [ Tue Jan 08, 2008 3:29 pm ] |
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Here's a .pdf of the specs on that chip: http://www.national.com/ds/LM/LM391.pdf You might get in touch with National Semiconductor to see about a substitute part. |
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| Author: | Bruce Dickey [ Wed Jan 09, 2008 2:23 am ] |
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Thanks Rick, I can't imagine it being too expensive. Like a fuse, they blow, you replace it. You could fit my electronic knowledge on the head of a pin and have room to spare. 
I know where the replacements are, in China. They have hundreds of these chips over there. |
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