|
Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
|
Thread Tools |
14th Dec 2017, 3:43 pm | #1 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
|
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 |
14th Dec 2017, 4:07 pm | #2 |
Dekatron
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,427
|
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.
__________________
Frank |
14th Dec 2017, 4:09 pm | #3 |
Rest in Peace
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,944
|
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. |
14th Dec 2017, 4:24 pm | #4 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
|
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. |
14th Dec 2017, 4:55 pm | #5 |
Rest in Peace
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,944
|
Re: LM3886 question
That's right, I make it 25 micro-amps. So it looks like it is indeed muted.
|
14th Dec 2017, 5:35 pm | #6 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
|
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
|
14th Dec 2017, 6:11 pm | #7 |
Rest in Peace
Join Date: Feb 2004
Location: Hexham, Northumberland, UK.
Posts: 2,234
|
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.
Alan. |
14th Dec 2017, 6:27 pm | #8 |
Dekatron
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,427
|
Re: LM3886 question
Crossed posts are not a problem Colin, better crossed posts rather than no posts.
__________________
Frank |
14th Dec 2017, 8:05 pm | #9 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
|
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! |
14th Dec 2017, 9:46 pm | #10 |
Rest in Peace
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,944
|
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.
|
14th Dec 2017, 9:53 pm | #11 |
Dekatron
Join Date: Nov 2003
Location: Heckmondwike, West Yorkshire, UK.
Posts: 9,637
|
Re: LM3886 question
|
14th Dec 2017, 10:03 pm | #12 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
|
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
|
15th Dec 2017, 12:44 am | #13 |
Nonode
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,324
|
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. |
15th Dec 2017, 12:53 pm | #14 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
|
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!
|
16th Dec 2017, 1:56 am | #15 |
Nonode
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,324
|
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 |