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Old 9th May 2010, 04:07 AM   #1
semi932 is offline semi932  United States
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Default Forum rookie ? about LM3886 amp prob

Hi to all. My first post at DIY audio forums. Been a member for a little while but just lurking.
Got the suggestion about the forum from folks at AK.

I've got an amp that I've had for a long time that was built by another diy'er 15-20 yrs ago.
Dual transformers 25v output ea. Power supply caps are 13,000uf going to two bridged 3886 modules.
32.5 VDC.

This model PM21 Dual Audio Power Amplifier Module
The boards I have are the same but the connections are a lot more beefy than on the link.

To shorten this the problem is a loud hum in the left channel, not affected by volume control.
I've done a lot of trouble shooting and PS is fine and steady.
I took the good channel board and changed the jumpers to allow for stereo operation and I'm powering it by itself and everything sounds very good. No hum whatsoever.
But I would like the power of dual mono.

Seems like I've got the hum in the bad module. There are not a whole lot of parts so I'm looking for some guidance to troubleshoot the board.
I don't see any fractures or any suspicious joints.
I'm guessing either bad cap, and there are only 5 lytics, or ic chip itself.

Sorry to be so long winded on first post but all help is appreciated.

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Old 9th May 2010, 10:54 AM   #2
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
since it is working but with hum, it is unlikely to be the chip.

What measuring instruments do you have?
Can you compare the stereo good channel to the stereo hum channel?
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 9th May 2010, 03:31 PM   #3
semi932 is offline semi932  United States
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I've got DMM's and analog meters but no scope.
Maybe Ill try hooking the board with the hum up in stereo mode and see if it still has the hum.
All connections are fine. I have spent some time with this and it doesn't seem to be a grounding problem.

Thanks for the response.
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Old 10th May 2010, 12:04 AM   #4
semi932 is offline semi932  United States
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Thanks for moving me in the right direction.
I tried the module that I though was bad in stereo mode by itself and it's fine.
At least I don't need any new parts!!!!!

I'm going to rewire the whole setup when time is available and see what happens.

I'll post results.
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