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Old 14th Dec 2017, 3:43 pm   #1
ITAM805
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Default LM3886 question

Hi all

I have a job here, a KRK powered monitor with no outputs from the two LM3886 chips, I'm getting a signal to the inputs of both. Before I spend time and money replacing both chips, could somebody kindly confirm a question regarding the mute circuit, as I'm unclear.

When powered on, pin 8 (mute) gets about -1.7V, does this enable or disable the mute, that is, is the chip output enabled or muted?

Many thanks
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File Type: pdf lm3886.pdf (1.15 MB, 65 views)
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Old 14th Dec 2017, 4:07 pm   #2
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Default Re: LM3886 question

The pdf states that drawing less than 0.5ma from pin 8 will mute the sound, page 17 decribes it in detail.
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Old 14th Dec 2017, 4:09 pm   #3
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Default Re: LM3886 question

From the data sheet you posted.

Pin 8 Open or at 0V, Mute: On
Current out of Pin 8 > 0.5 mA
Mute: Off


This seems to indicate that the condition you have gives mute disabled. To be sure unsolder pin 8 and measure the current, if it is greater than 0.5mA the mute should be disabled allowing signal through the IC.

The dreaded crossed post strikes again, sorry Frank.
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Old 14th Dec 2017, 4:24 pm   #4
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Default Re: LM3886 question

Thanks guys,

Colin, I shall try and lift pin 8 but this board is a cramped mess, the resistor from pin 8 is covered, like much of the rest of the wretched thing, in black rubber like compound

edit: the resistor to pin 8 measures 22k and the voltage across it is about 55mV, so it aint drawing 0.5mA?

Last edited by ITAM805; 14th Dec 2017 at 4:36 pm.
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Old 14th Dec 2017, 4:55 pm   #5
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Default Re: LM3886 question

That's right, I make it 25 micro-amps. So it looks like it is indeed muted.
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Old 14th Dec 2017, 5:35 pm   #6
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Default Re: LM3886 question

Ah right, thanks Colin. Now the question is why? There is no schematic for this amp and as it's so cramped it's like a nightmare Crystal Maze episode
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Old 14th Dec 2017, 6:11 pm   #7
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Default Re: LM3886 question

Could the black goo be starting to conduct? This is a known problem with some types. Try removing it if you can. If the currents are so small anyway, a little bit of leakage across the black stuff may be upsetting the circuit.
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Old 14th Dec 2017, 6:27 pm   #8
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Default Re: LM3886 question

Quote:
Originally Posted by ukcol View Post

The dreaded crossed post strikes again, sorry Frank.
Crossed posts are not a problem Colin, better crossed posts rather than no posts.
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Old 14th Dec 2017, 8:05 pm   #9
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Default Re: LM3886 question

Hi Biggles, I tested the black stuff at 40MΩ with the probes 1mm apart and there was no reading.

However, I removed the transistor that feeds the resistor supply chain to pin 8, (sorry I didn't get the number and impossible to read it in circuit) and it tested good? So I refitted it and now there's -3.4V on pin 8 and the amp works!
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Old 14th Dec 2017, 9:46 pm   #10
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Default Re: LM3886 question

Perhaps a fault in the transistor (temporarily?) cleared when it was heated by the iron? It may be prudent to change the transistor anyway.
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Old 14th Dec 2017, 9:53 pm   #11
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Default Re: LM3886 question

Quote:
Originally Posted by Nuvistor View Post
Crossed posts are not a problem Colin, better crossed posts rather than no posts.
True.
Also, it's more common than you realise. Some members just delete the second one if it doesn't add anything.
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Old 14th Dec 2017, 10:03 pm   #12
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Default Re: LM3886 question

Thanks Colin, yes I think I'll do just that. Something I didn't mention, while testing the components in circuit I noted that the said transistor had a resistance of around 350R between base and emitter, which is why I pulled it to test. The junction now reads normal, the low resistance has gone, so as you say the hot iron maybe did something to clear it
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Old 15th Dec 2017, 12:44 am   #13
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Default Re: LM3886 question

That sounds like a contaminant on the pcb.

When you removed and refitted the transistor, it probably cleared the contaminant (possibly leaky flux), I've had that before.

A clean with a bit of isopropyl might be in order.
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Old 15th Dec 2017, 12:53 pm   #14
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Default Re: LM3886 question

Thanks Terry, shall do. This transistor though was covered in the aforementioned black goo so it's a puzzle how contaminants got there? Anyhow, a new device is going in there as well as a good clean up, as I dont ever want to see thing back again!
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Old 16th Dec 2017, 1:56 am   #15
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Default Re: LM3886 question

Not impossible for something to have been on the pcb when the 'black goo' was put on that became leaky over time is one possible theory.

Main thing is - problem solved
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