Post mortem of an ESI nEar 05 studio monitor with pictures - diyAudio
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Old 15th September 2011, 03:31 AM   #1
stube40 is offline stube40  Australia
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Default Post mortem of an ESI nEar 05 studio monitor with pictures

One of my ESI nEar 05 studio monitors has recently stopped working. I noticed it wasn't working a few days after a 5min REALLY loud playing session and I'm thinking that I may have blown it, although it might just be coincidence.

The symptoms are that it switches on OK, but the only output is a fairly quiet mid-freq buzz that you can't hear until your ear is about a foot away from the speaker. Adjusting the master volume pot makes no difference to the level of the buzz. The other monitor still works fine.

I'm now armed with an oscilloscope, multimeter and some limited electronics knowledge and I'm hoping to debug the problem. I can obviously use the good speaker as a reference if need be.

I've noticed that the internal electronics are fairly simple - there are only 3x ICs on the board - 2x LM3886TF power amps and 1x unknown 8-pin IC marked 4558D JRC 45E0311.

I was thinking that I'd maybe just replace the 2x LM3886TFs and hope for the best. If I knew what the 8-pin IC was too I'd replace that. Other than this, I could try debugging it with the scope, the good monitor and an audio source, but I wouldn't know where to start since I don't have a schematic.

For those that are interested, here are some photos:

http://i800.photobucket.com/albums/y..._0232Large.jpg
http://i800.photobucket.com/albums/y..._0231Large.jpg
http://i800.photobucket.com/albums/y..._0230Large.jpg
http://i800.photobucket.com/albums/y..._0229Large.jpg
http://i800.photobucket.com/albums/y..._0233Large.jpg
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Old 15th September 2011, 03:39 AM   #2
kevinkr is offline kevinkr  United States
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The 4558D is a very common inexpensive audio op-amp made by new japan radio corp amongst others.. Data sheet here: http://semicon.njr.co.jp/njr/hp/file...?_mediaId=7903
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Old 15th September 2011, 04:13 AM   #3
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OK, so you have a deep-fried self powered 2 or 3 way mini-monitor? First check that there is no DC on the speaker outputs to burn out the drivers.

Then check that you do have DC fom the power supply by measuring its output from the power electrolytics and does this appear again at the power Chip-amps (LM3886)? Take real care not to short pins here - one short and its bucks + an awful job refitting if you aren't a tech. or experienced repairer.
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Old 15th September 2011, 04:19 AM   #4
stube40 is offline stube40  Australia
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Quote:
Originally Posted by Ian Finch View Post
OK, so you have a deep-fried self powered 2 or 3 way mini-monitor? First check that there is no DC on the speaker outputs to burn out the drivers.

Then check that you do have DC fom the power supply by measuring its output from the power electrolytics and does this appear again at the power Chip-amps (LM3886)? Take real care not to short pins here - one short and its bucks + an awful job refitting if you aren't a tech. or experienced repairer.
Hi Ian,

Thanks for the advice.

I've ordered new LM3886 and 4558D chips from Digikey ($7 and $0.50 each) Will be here in a few days. I've ordered a few spares too in case I blow some up.

I see what you mean about fitting them, but I think I'll manage OK. Would you recommend I just go ahead and fit them or would it be worth me trying to debug as per your instructions first of all?

Stuart.
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Old 15th September 2011, 05:37 PM   #5
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Hi Stuart
Without some feedback on the basic condition there, it won't help to guess what next. With no schematic either, we fly blind and hope to collide with the problem. However, I would be surprised if the power chips failed - they are almost bulletproof for the very reason you are in this predicament - you know....turn it up until....etc.

Make sure the power supply is working first and check the output for DC. If this is not easily determined, you might reconsider the sense of just replacing the semiconductors in the hope that this will solve the problems.

Consider this is a set of amplifiers at different power levels driving separate speakers and somehow all 3 amps have failed? You could replace them and immediately farewell your new parts the moment you power up too, if the power supply is badly at fault.

The 4558 dual opamp is a very low power device, a preamp or more likely crossover filter
gain block to split the signal to the amps. Check its power rails which may be +/- 15V at pins 4,8 with OV ( Power ground) as common. Check whether the power supply is a dual rail or perhaps single with isolating caps. at the outputs. (this is often done for low cost)
In that case you may only have a single power rail to th 3886s of somewhere around 40V.

NB: need to know is this a 2 way or 3 way as 2 way config. may be bridged amps. Nasty!
Being actually a Chipamp matter, it might get more suitable attention in that forum.
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Last edited by Ian Finch; 15th September 2011 at 05:41 PM.
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Old 15th September 2011, 10:18 PM   #6
stube40 is offline stube40  Australia
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Thanks Ian - I think I'll take your advice and take a look at the power rails whilst waiting for the components to arrive.

I don't quite understand your question about it being a 2-way, or a 3-way as a 2-way etc. I'm not very well-versed in audio jargon and I guess you are talking about the speaker config? If so, it has a single tweeter and a single woofer - both tweeter and woofer have 2 wires each connecting to the main circuit board but I haven't had a look to see what's going on there in terms of crossover configuration. Does this help, or could I investigate further to get you some more info?
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Old 16th September 2011, 02:16 PM   #7
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Thanks Stuart, for the confirmation. 2-way means 2 speakers here. These are active speakers, so each driver has its own amplifier and the crossover is done at signal level actively (in the preamp circuit) by an opamp. So treble then goes to the tweeter amp and bass to the Mid/bass amp and then direct to their respective speaker drivers. Conventional passive speaker crossover coils and caps. are avoided in this arrangement.

Since you have located the speaker leads, test with a Digital Multimeter for DC across them. I assume you have basic tools to do this or you would not be trying to DIY electronic repairs. There should not be much more than 40 mV DC at the outputs.

Since there are 3 power amplifier chips and only 2 speakers, it's likely the bass amplifier is configured as a bridged pair of 3886s and things now get complicated. So, is there still output of scratchy sound from the tweeter when powered on with programme playing or is that dead too? Verify that the speakers are functional by testing the resistance across their terminals - disconnected from the amplifier! It should read a bit less than the marked impedance and a little scratchy sound should be evident when you test. If you cannot tell if sound is coming from the tweeter, disconnect the bass speaker so you can.

It is important to logically check that everything else works before wasting time and money on guesswork. Might I suggest you read the article pages on Rod Elliott's ESP site to get an idea of how active speakers, chip amps and bridge configurations are done and operate. Elliott Sound Products - The Audio Pages (Main Index)

It's no cakewalk fixing broken gear without some basic knowledge of how it works and how to check that, at least. Good luck - it may be simpler and easier after all!
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Old 16th September 2011, 09:11 PM   #8
stube40 is offline stube40  Australia
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Wow, I certainly came to the right place!! When I first posted my cry for help, my utopian scenario was that some helpful individual would post information like this. However, I didn't actually believe that would happen - but thanks to you it has!!

I have both a scope and multimeter available to me (I work in a company that has an electronics lab that I have access to and occasionaly use).

With your information and the new components, I'm pretty sure I'll get to the bottom of this. I'll obviously write up the findings so that other people can benefit from it.
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Old 19th September 2011, 11:24 PM   #9
stube40 is offline stube40  Australia
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OK, so changed the 2x LM amps and the little JRC amp but the problem has not gone away. Still get a fixed mid-range freq buzz coming from the speaker when I turn it on. The buzz is still fairly quiet, although seems a fraction louder now than before. The buss is still unaffected by the position of the master volume pot.

I guess I will have to debug it now based on Ian's previous instructions. Will report back soon.
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Old 20th September 2011, 03:17 AM   #10
stube40 is offline stube40  Australia
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I think it might be the bridge rectifier. It's a RS406 and I notice that it has a clean 24V coming out of the +ve terminal, but the -ve terminal has a nasty 100Hz distorted sine wave on it with 20V amplitude (actually, it's like a hybrid between a sine wave and a triangle wave).

I guess this means that a diode in the bridge rectifier is gone?

Next thing is to try and find somewhere that sells the RS406.

Or, could I make my own RS406 up from 4 diodes?
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