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very weird mini3 problem

post #1 of 10
10/21/09 at 4:20am
Thread Starter 
This is my 3rd build and the other 2 went flawlessly but this one has me mind boggled a bit.
I've been soldering for a few years and I have triple checked my points.

All "initial checks" pass full. It's a 2.01 revision of the board.

Here's the problem.

1. There is NO static on any of my other 2 amps, but with volume down all the way on the amp, there is slight hiss/static ONLY on the left side.

2. As I start to increase volume, I can hear it crackle pop in the LEFT side and after turning it about 10% volume would kick in but hiss stays in the left side.

3. at about 40%+ volume, the left goes out COMPLETELY and I only get sound out of the right side.

4. at 40%+ volume, As I begin to insert the output plug, i get sound in both cans(sounds mono), as I insert it more, the sound shifts to left side and when the plug is all the way in, i only get sound in the right side.


Ideas?
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post #2 of 10
10/21/09 at 4:31am
Reflow the solder joints on the volume pot and headphone jack.
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post #3 of 10
10/21/09 at 4:58am
Thread Starter 
Quote:
Originally Posted by johnwmclean View Post
Reflow the solder joints on the volume pot and headphone jack.
done that many times, and removed/resoldered another pair of headphone jacks
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post #4 of 10
10/21/09 at 5:36am
Measure the DC offset of the left and right channels as you slowly rotate the pot to confirm U5 is the problem.
And.. it would be a very good idea to stop plugging the phones into the amp when it is turned on.
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post #5 of 10
10/21/09 at 5:46am
Thread Starter 
Quote:
Originally Posted by MisterX View Post
Measure the DC offset of the left and right channels as you slowly rotate the pot to confirm U5 is the problem.
And.. it would be a very good idea to stop plugging the phones into the amp when it is turned on.
I normally do not plug them in while it's on but I was trying to try different things.
Measuring DC offset

followed those directions.
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post #6 of 10
10/21/09 at 5:50am
Here are the instructions from the Mini3 "Initial check" page:

Quote:
Output DC offset check
Install the battery, and turn on the amplifier, but set the volume control to its minimum position. Set your multimeter to measure DC mV.

1.With the meter's black probe on IG, check the voltages at OG, OL and OR. Use the output jack's sleeve, tip and ring solder pads.
2.The measured voltages should typically be less than 5mV in each of these locations. The Mini³ high performance edition has lower DC offset. The extended runtime edition may exhibit somewhat higher output DC offset.
3.Turn off the power, remove the battery, and discharge the rail capacitors by turning the power on again and wait a few seconds. The power LED may illuminate momentarily and then fade out.
4.If you have high DC offset on any channel, check your soldering for cold joints and/or bridges. If the problem cannot be corrected by reflowing the solder joints, then the opamp associated with the channel with the high DC offset is most likely damaged. See notes below about how to desolder and replace an opamp.
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post #7 of 10
10/21/09 at 6:02am
Thread Starter 
on the output, left channel. Im breifly seeing a reading of 150+ mv and then it changes to solid 1. ON the Right channel, I'm getting a reading of -3mv.

to confirm, it's the u5 that needs to be replaced?
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post #8 of 10
10/21/09 at 6:52am
Yeah, looks like U5 is the problem but it is a good idea to "double check" you have the resistors in their proper locations and all of the other stuff before replacing it (rather sucks when you go through the effort of replacing that op-amp only to figure out it was not the problem).
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post #9 of 10
10/21/09 at 6:56am
Thread Starter 
that was a doozy. Boy is it hard to get to the opamp with all the caps and pot and jacks.

I got it out, got a new one in and this time go 70mv dc offset on the right chan wonderful .

Wicked up some excess solder from the pins and down to 8mv on the right and about 4mv on the left.

I think that should be ok no?
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post #10 of 10
10/21/09 at 7:51am
Quote:
Originally Posted by xsoundx View Post
Wicked up some excess solder from the pins and down to 8mv on the right and about 4mv on the left.

I think that should be ok no?
Yes, those are acceptable readings.
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