LM3886 troubleshooting help needed for newbie - diyAudio
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Old 2nd January 2012, 01:27 PM   #1
franke is offline franke  United Kingdom
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Default LM3886 troubleshooting help needed for newbie

Hi all,

This is my first attempt to build something like this, I have only put together some small kits before and nothing to do with audio.

I have put everything together now but I need help fixing it. I checked the power supply and it's ok, get the correct voltage out. I connected some speakers to the amp, the selector works, volume pot is good and I get good audio out from the right channel. The left amp is silent though. If I only connect the left speaker I can hear something very weak coming through only, otherwise it's near silent.

I need help troubleshooting that amp pcb to find out where it goes wrong. I'm not sure how to check the components or what else otherwise to check. The solder joints look ok to me but obviously it goes wrong somewhere.

I have taken some pictures of the whole thing, please let me know what other info you would want me to post. All help is appreciated here. It would be fantastic to be able to finish this great project. I have learned a lot and it would be great if I could get it working as I feel I'm very close now.

One thing I can tell you is when I check DC output on the left out while source is connected I get a steady DC out, something like -5 mV, I think. I can check again.

All pics here
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Old 2nd January 2012, 01:52 PM   #2
AndrewT is offline AndrewT  Scotland
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-5mVdc is very acceptable for an output offset.
It probably indicates that there is no serious error that could damage your chip nor your PSU, nor your speaker.
Remove your speakers insert a dummy load. Power up via a bulb tester.
Let it warm up and while doing so repeatedly check the output AC and DC voltages.
Now apply a quiet music signal. Measure the output voltages again. Are they near zero?
Apply a louder music signal. Check the voltages again. If there is still near zero output, Check around the chip amp to feel if any component is overheating. Do not feel temps around your transformer. It is at mains voltage !!!!
Power off.
Check the underside of the PCB very thoroughly.
Check the component names and values and orientations.
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Old 2nd January 2012, 01:55 PM   #3
franke is offline franke  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
-5mVdc is very acceptable for an output offset.
It probably indicates that there is no serious error that could damage your chip nor your PSU, nor your speaker.
Remove your speakers insert a dummy load. Power up via a bulb tester.
Let it warm up and while doing so repeatedly check the output AC and DC voltages.
Now apply a quiet music signal. Measure the output voltages again. Are they near zero?
Apply a louder music signal. Check the voltages again. If there is still near zero output, Check around the chip amp to feel if any component is overheating. Do not feel temps around your transformer. It is at mains voltage !!!!
Power off.
Check the underside of the PCB very thoroughly.
Check the component names and values and orientations.
There is a steady output DC while I have a source and speakers connected, it doesn't change, but the other one plays fine.
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Old 2nd January 2012, 02:04 PM   #4
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Long shot, but make sure none of the RCAs are shorted to the chassis.

Some types of those plastic shoulder washers are very shallow and the barrel might have shifted while tightening the nut.

Unrelated, but I would use the same gauge wire for the outputs as for the DC power.
I would also insulate all the exposed 230V terminals, and re-arrange the RCA/selector wiring to minimize the possibility of noise pickup.
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Old 2nd January 2012, 11:55 PM   #5
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Since you have one good channel and one that's inoperative, this is a good opportunity to do "A to B" comparisions. With a digital voltmeter, take comparative readings from each of your amp PCBs. It would be interesting to see where they differ (worth reporting back to us on the thread), as that might give a good indication of the problem. Even more worthwhile, if you have an oscilloscope you can use for similar "A to B' measurements while providing a common audio source to both inputs. With the simplicity of the BrianGT boards, the problem should become easily evident. (In addition, you can do a good-board-to-bad-board comparision for resistor values, capacitor orientations--and don't forget a GOOD visual inspection of all solder joints (and potential bridges) on the bad board.

The best troubleshooting tool you have in a two-channel amp, when one channel is giving you problems--is the other (good) channel....!!
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Old 3rd January 2012, 12:15 AM   #6
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Looking at your pics, there are a number of PCB pads that look as though they could use more solder. (Might be the source of the problem, if you have a poor/no connection. I'd hit those pads again, and run more solder on them (don't be afraid to fill the pad with solder on each of the PCBs.....but do be careful not to form any solder bridges. (Bridges can be recoverable--but it's easier not to create them in the first place).

As we look at the amp pic that shows one amp module to the left, one to the right, and the PSU and rear apron/RCA connectors in the rear middle, which amp is NOT working--the one on the left, or the right?
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Old 3rd January 2012, 12:44 PM   #7
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Is the transformer center tapped? Looks like extra connection to ac1 input in pic from transformer to the rectifier diodes.
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Old 3rd January 2012, 01:49 PM   #8
franke is offline franke  United Kingdom
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Quote:
Originally Posted by CanAm Man View Post
Looking at your pics, there are a number of PCB pads that look as though they could use more solder. (Might be the source of the problem, if you have a poor/no connection. I'd hit those pads again, and run more solder on them (don't be afraid to fill the pad with solder on each of the PCBs.....but do be careful not to form any solder bridges. (Bridges can be recoverable--but it's easier not to create them in the first place).

As we look at the amp pic that shows one amp module to the left, one to the right, and the PSU and rear apron/RCA connectors in the rear middle, which amp is NOT working--the one on the left, or the right?
The one that's not working is the one on the left. The one that's behind the transformer.

Quote:
Is the transformer center tapped? Looks like extra connection to ac1 input in pic from transformer to the rectifier diodes.
Extra connection?! There's no extra connection, from transformer there's the 4 connections to AC1x2, and AC2x2.

I'll take some pics of the undersides of the amp PCBs, and I'll take note regarding more solder. I'm not sure it's needed but resoldering the faulty amp where it looks sketchy might be good.
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Old 3rd January 2012, 02:21 PM   #9
franke is offline franke  United Kingdom
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Here are some more pics, they are also in the album:

Faulty amp, top
Faulty amp, bottom 1
Faulty amp, bottom 2
Faulty amp, bottom 3

Some pads look quite bad, I will resolder some now, and check again.
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Old 3rd January 2012, 02:38 PM   #10
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Re-sweat those joints with a bit more solder and see what happens.
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