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Different amp  same trouble. Lm3886 can't get working
Different amp  same trouble. Lm3886 can't get working
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Old 4th November 2018, 08:12 PM   #41
Mooly is offline Mooly  United Kingdom
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Different amp  same trouble. Lm3886 can't get working
Failure modes of chips can be many and varied. Anything is possible.
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Old 4th November 2018, 11:17 PM   #42
Hextejas is offline Hextejas
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I don't have the original zener and capacitor C6 but the values are the same. For the zener, the voltage limit is the same, 16V. I don't know about the other numbers associated with it. I will get the actual part numbers tomorrow.
Would that make a difference ?
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Old 5th November 2018, 07:49 AM   #43
Mooly is offline Mooly  United Kingdom
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Different amp  same trouble. Lm3886 can't get working
Any small 16 volt Zener will be fine. It wont make any difference.
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Old 5th November 2018, 11:12 AM   #44
avtech23 is offline avtech23  Australia
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Different amp  same trouble. Lm3886 can't get working
Quote:
Originally Posted by Hextejas View Post
In the process of removing the LM3886 I saw that 1 pin was folded under rather than thru the hole. Well I straightened it out and pushed it back in. So I am still using the prior albeit it's beginning to look a little abused. On to the reading.
R5 -2.814 -31, voltage drop of 28.9.
P3 -1.9

And the other readings that have changed.
C6 -1.6
P3 -1.9
P4 -1.9
P8 -2.7
P9 -1,8
P10 -1.8

Please ignore the missing C6 as it is mounted on the backside.
I am tempted to remove LM3886 and clean it up.
The reading of -1,9 at the output is encouraging. We're getting closer.
Thanks
With a voltage drop of ~29v across R5, you have about 2.9mA current flow which is above the 0.5mA required to unmute the amp (assuming the 10k was used as current limiting resistor).

You are reading -1.9 on P4 (V-) which should be -31. Check your U4 connector is made properly and has -31v there.

Make sure that you have continuity between each component leg and the next component on the schematic, to ensure good soldering.

When I solder PTH (Plated Through Hole) boards, I will flow enough solder from the back to make sure that the upper surface pad has a good joint too. I've had cheap boards in the past that have not had continuity from one side to the other, so it pays to check the upper surface joints and retouch those as required.
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Old 5th November 2018, 12:51 PM   #45
Hextejas is offline Hextejas
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Quote:
Originally Posted by avtech23 View Post
You are reading -1.9 on P4 (V-) which should be -31. Check your U4 connector is made properly and has -31v there.

Make sure that you have continuity between each component leg and the next component on the schematic, to ensure good soldering.

When I solder PTH (Plated Through Hole) boards, I will flow enough solder from the back to make sure that the upper surface pad has a good joint too. I've had cheap boards in the past that have not had continuity from one side to the other, so it pays to check the upper surface joints and retouch those as required.
Thank you avtech, those are valuable points and I will get to them as soon as my wife turns me loose.
Interesting about soldering PTH, I had wondered about that.
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Old 9th November 2018, 01:53 PM   #46
Hextejas is offline Hextejas
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Default Ok back at it.

I desoldered all the components and restuffed a new board.
It's running now but the readings are odd when I compare it to the other board.
The output varies like this. -2.0, -3.4, -4.n, -5.n, then back to -2 again.
l seem to remember that any DC at the output is bad, so how can I get rid of it ?

The other board has about .02 V at the output.

Thanks
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Old 9th November 2018, 03:21 PM   #47
Hextejas is offline Hextejas
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Default It works !

And thanks to all for the help. After swapping out all the components into a new board,
I think it was a cold or faulty solder joint.
How I found it was by comparing continuity readings to the working board, and reading them from the backside. Between pin 9 and R3, the continuity varied between nil and 45,0,.45,.7. Etc. All over the range. I guess that would the account for the variable readings from the output.
Hallelujah!

So now onward to fit these into the case.
Of note is how darn hot the LM3886 gets. Burned my thumb a few times. While I was testing, I kept a sponge soaked in water and would press it against the chip. It would sizzle and I read a temp of 140c. Wow ! And it got hot in a hurry,

Ok, thanks again for the marvelous learning experience.
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Old 9th November 2018, 06:22 PM   #48
Juergen Knoop is offline Juergen Knoop  Germany
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Quote:
Originally Posted by Hextejas View Post
Of note is how darn hot the LM3886 gets. Burned my thumb a few times.
no heatsink?
Never ever power up an amp without all the heatsinks applied!
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Old 9th November 2018, 07:06 PM   #49
Mooly is offline Mooly  United Kingdom
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Different amp  same trouble. Lm3886 can't get working
Pleased to hear you have it working but DEFINITELY get it fixed to a suitable heatsink or you will be back to fault-finding again
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Old 9th November 2018, 09:04 PM   #50
avtech23 is offline avtech23  Australia
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Different amp  same trouble. Lm3886 can't get working
Glad to hear you got it working.

Definitely test the chip with a heatsink, even if it is a token lump of metal just to slow the heat rise for very short tests.

Max junction temperature of these chips is 150, so getting close there..

Anyway, enjoy the music!
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