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Old 27th July 2010, 04:59 AM   #1
alsoran is offline alsoran  Australia
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Red face Help noob fix sub amp

Hi Guys,

Just seeking some direction, find the world of pcbs daunting but learning and reading as much as possible. Have a multimeter thats it.
The amp in question is a Jamo SW1008. The amp failed very shortly after I put very high voltage through the inputs via kx project - as I raced to turn it off it stopped of its own accord. Took it apart, driver fine tested elsewhere, checked what i think are the power transistors and mosfets acording to various methods (bcae the reference), current coming out of transformer 24v per side. No light from lamp led.
Replaced chip amp LM3886TF with LM3886T - led now comes on! No sound though. Have bought replacment power transistors, will try putting these in next although as I say existing seemed to work ok. Am wondering would there likely be damaged caused to a protection circuit - or does all that happen inside the chip? Have i damaged something on the input side rather than the output? Sorry this is all so basic, but am out of my depth. Thanks for any guidence, will add information/pics if someone can tell me what is relevant
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Old 27th July 2010, 07:02 AM   #2
Mooly is online now Mooly  United Kingdom
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It's a circuit we need to see... piccys might help.

What did you connect to the inputs ? A DC voltage, an AC voltage, or just a very high level audio signal.

Fault finding at a basic level involves first checking all supplies are present and correct. Power transistors if they have failed usually go short circuit collector to emmiter.

Look at the data sheet for the LM3886, identify the supply pins and see if the voltage is present. Measure the voltage on the output pin next.

Data sheets available here for most semiconductors,
Datasheet catalog for integrated circuits, diodes, triacs, and other semiconductors, view
just type the device code in the blank box.
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Old 27th July 2010, 02:46 PM   #3
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The LM3886TF is in a plastic case, the LM3886T has an exposed tab which (if I remember correctly) has the negative supply on it. So if the heatsink is grounded, you will cause a short unless you use an isolation like a mica insulator or a piece of silpad and plastic bolt washer.
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Old 27th July 2010, 10:47 PM   #4
alsoran is offline alsoran  Australia
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Hi Mooly & Redshift thanks for you advice,

Will try to put a pick up tomorrow, it was a high level audio signal that caused the problem. Have checked the power transitors and seem ok, as mentioned have switched the LM3886T for the TF and yes redshift you are right, there is an exposed tab, which is grounded, the heatsink attaches to the external metal plate for additional cooling with multiple metal screws, would i have damaged the new chip - could short be the reason it doesn't work (dumb qn probably), could not get the TF and from reading the data sheet thought they were same
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Old 28th July 2010, 01:45 PM   #5
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I don't know if you would have damaged the chips, it probably depends on whether the fuse blew or you turned it off fast enough. You would have to test them to find out.
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Old 28th July 2010, 04:46 PM   #6
Mooly is online now Mooly  United Kingdom
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When you next power up it's worth using a filament bulb in series with the mains supply to the amp of 60 watts or so. If anything draw excess current the bulb lights and it can save burn ups etc.

You need to be logical fault finding. I assume it runs of split supplies -/+ rather than single ended AC coupled... but it may not... without seeing circuit only you know. If it's DC coupled (split supply) the output must be at zero volts DC... is there a protection circuit/relay stopping the speaker connecting because of DC offset (fault) ?
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Old 29th July 2010, 03:18 AM   #7
alsoran is offline alsoran  Australia
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Thanks for your persistance guys, have attached a couple of photos that I hope will clarify. I tried to power up with the chip isolated from ground, the amp led came on briefly but then went out. Tried grounding again and light comes on and stays on. I also tried to get a schematic form Jamo, they said they didn't have one for this model?? Am hoping with the pictures you can give me a lead on what next. I also found a place that can supply a TF - but am wondering if this is the problem
Attached Images
File Type: jpg S5001341.JPG (308.7 KB, 256 views)
File Type: jpg S5001342.JPG (298.0 KB, 249 views)
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Old 29th July 2010, 05:20 AM   #8
alsoran is offline alsoran  Australia
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Also have taken some voltage measurements on the dc side of the regulator - shows 0.5V negative on the positive 35V (in the picture the lower circuit). Is this how it should be? 25V AC out of both sides of transformer.
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Old 29th July 2010, 06:54 AM   #9
Mooly is online now Mooly  United Kingdom
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Can you measure the DC voltage at the four points shown and post the results. Make sure the chip doesn't get hot. Do not connect any speaker either.

The two devices ringed look like transistors used as "switches" to control the main power rails to the IC.

You may well find it's one of these and the IC that's the problem.
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File Type: jpg Amp.JPG (91.1 KB, 233 views)
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Old 30th July 2010, 05:01 AM   #10
alsoran is offline alsoran  Australia
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Hi Mooly,

Results are (am guessing I am measuring from each point to ground)
1. 35.2V
2. 0.5V
3. 35.2V
4. 0V (meter shows negative sign in front of this, reverse lead same result)
I did what I thought were the correct tests on these mosfets before(in circuit), I'm guessing a zero reading is not a good sign?
Also do you think the new IC that is not isolated will be a source of problems, ie because it is the T and not the TF?
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