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Old 22nd June 2010, 09:25 PM   #1
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Default LM3886 issues

Ok so this amp was working fine both channels. Then a couple months ago one channel stopped working. So I took the cover off and it looks like one of the bridge rectifiers on the PSU got real hot because all the hot glue that was on it melted and spread out all over the bottom of the chassis. The other channel that the glue hasn't melted on works just fine. So I'm wondering, do you guys think the rectifier just went bad? How do I go about testing it? I need to order whatever parts to fix this. There wasn't any burning smell like something burned up so I'm thinking it's just the PSU possibly.

Here are a couple pictures and btw I did not build this my friend bought it from someone.

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Old 22nd June 2010, 09:48 PM   #2
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I'd replace them with some of the metal-cased 25/35A types and solder the wires on them,ditch those crappy connectors..
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Old 22nd June 2010, 09:50 PM   #3
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I didn't know electronic components could be glued. That's a fire waiting to happen.

Maybe the glue went somewhere inside and since electrons can't swim they died.
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Old 22nd June 2010, 09:54 PM   #4
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What are the part numbers of the ones you're speaking of? And do you think that's probably the issue? I don't want to just start replacing parts and find that they aren't fixing the issue. My friend wants it back but I don't want to give it back to him broken lol. Never did anything to it though other then use it on some 8ohm speakers.
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Old 22nd June 2010, 10:04 PM   #5
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Have you got access to an oscilloscope? You could disconnect the amp boards and look at the output of the rectifiers if so. Maybe just the rectifier blew or maybe it blew because the amp took it out. Do those gainclone boards have a zobel network on the output?
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Old 22nd June 2010, 10:14 PM   #6
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They are the chipamp.com boards, other then the PSU everything else is just the standard stuff it seems. So unless they had a zobel stock then no there isn't one.

No I do not own an oscilloscope. I have a DMM and soldering station, that's about it.
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Old 22nd June 2010, 10:45 PM   #7
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Those particular gainclones were notorious for having problems in the early days due to omission of a zobel network, later ones have one. But I think I can see one on those boards - the red cap and big blue resistor at the front of the board. Can you confirm those values?
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Old 22nd June 2010, 11:02 PM   #8
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Red cap is a WIMA .22, and I think the resistor is 4400 ohms? or maybe 44k?
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Old 22nd June 2010, 11:08 PM   #9
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I was thinking of something along the lines of this:
GBPC3502-E4/51 Vishay Semiconductors Bridge Rectifiers

There are several 'flavors' (voltage ratings) but any of them should be fine for the low-ish voltages inside of a SS amp. Digikey or your other favorite supplier should also stock something similar.

You might want to check the resistance between the +/- rails and ground (0V) of the 'dead' amp board (with the bridge rectifier disconnected),just to make sure there are no shorts.The readings should start out low,and gradually climb to something higher (at least few K ohms,or so.),it may take a bit,as the DMM has to charge up the larger filter caps.

If all that checks out,and there are indeed Zobel networks on there,I'd think it's just a dead rectifier.
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Old 22nd June 2010, 11:12 PM   #10
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Quote:
Originally Posted by Dougie085 View Post
Red cap is a WIMA .22, and I think the resistor is 4400 ohms? or maybe 44k?
That value seems awfully high for a Zobel,what are the colors of the stripes on those resistors? 4.4ohms would be a more logical value. (usually 4-10ohms,or so.) You can double-check them with your DMM if you need to.
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