Official Luthiers Forum!

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.

_________________
http://www.dickeyguitars.com

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  This part is obsolete, but you may be able to get one from a place that specializes in such parts.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

 

_________________
Tom Barton

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.

_________________
http://www.dickeyguitars.com

Hey Bruce, try http://www.aeri.com/search/LM391N?   ... looks like they've got a a few N-100s in stock.

_________________
Jim Womack

"Be thankful we're not getting all the government we're paying for. "
Will Rogers

Sorry, try this: www.aeri.com/search/LM391N?

_________________
Jim Womack

"Be thankful we're not getting all the government we're paying for. "
Will Rogers

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.

_________________
http://www.dickeyguitars.com

Bruce, can you find a model name for the amp? Schematicheaven.com
might have some information.

Neptune 111
Power Amplifier

Mod. # 111
Ser. # 17244
120V 200W input
2 amp fuse

_________________
http://www.dickeyguitars.com

     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!

_________________
Billy Dean Thomas
Covina, CA

"Multi famam, conscientiam, pauci verentur."
(Many fear their reputation, few their conscience)

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......

_________________
http://www.dickeyguitars.com

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.

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.   

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.

_________________
http://www.dickeyguitars.com