LinearPower PA2, Right channel louder than Left. - diyAudio
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Old 18th January 2012, 09:20 PM   #1
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Angry LinearPower PA2, Right channel louder than Left.

I have this hooked up and when testing I notice that only the "Rear Fader" RCA outputs both are equal, but the "Front Fader" & "Main Ouput" RCA's are not the same, the right channel is okay but the left channel is about 1/3 of what the right channel is. For example if the output signal is 2.5v the right is just that but the left channel has 0.85v . I try adjusting the input gain trimmers but I notice those dont function at all. I can move them all the way counterclockwise then clockwise all the way and NO EFFECT to output signal.


There is two opamps (PMI OP275GP) near the input RCA's and below is the readouts for both.

1)2.495 AC
2)2.495 AC
3)2.495 AC
4)-13.11DC
5)0.098AC
6)0.098AC
7)0.098AC
8)13.13DC

1)0.983AC
2)0.983AC
3)0.983AC
4)-13.11DC
5)0.038AC
6)0.038AC
7)0.038AC
8)13.13DC

Dont know if the one with lower readings is damaged, both Left & Right input signals (RCA) are the same so it cannot be my signal source.

Any help is appreciated. Below is a picture of the actual item in question. The SkyBlue Arrows are input & out signal that are okay.
Red Arrows are the outputs that are very low.
Red "X" are on the trimmer input gains, have no function or effect when rotated.
Red Circle are the opamps, the one the right (close to the edge of the board) is the one that have the low readings.
Click the image to open in full size.

Last edited by evianamp2011; 18th January 2012 at 09:29 PM.
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Old 18th January 2012, 09:22 PM   #2
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I want the trimmer's to function so I will have to work on those for sure. I also will add IC sockets where the opamps are so I can switch them out with others to see if I can hear any differences.
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Old 19th January 2012, 12:38 AM   #3
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Do the jumpers make a difference?

To what are the two blue capacitors near those op-amps connected?
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Old 19th January 2012, 04:10 AM   #4
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Not messed with the jumpers. I've gone through the manual a few times and still get confused sometimes, the PA2 had two source inputs and two remote cables so once one source is on thats the one the PA2 will allow through and filter. Which is also the fader/rear input but the jumpers have to be moved from "S2" to ""PB".

The two caps are 4.7uf 50v that are inputs to the RCA's. I checked each with my Fluke 111 for capacitance and both read 4.8uf (thats as far as I can test with my meter,lol).

I can replace them though because I have some in hand.
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Old 19th January 2012, 04:25 AM   #5
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According to the manual, you have the option jumper set incorrectly. The jumpers set for PB (patchback) means that the jumpers for "Internal Fader" need to be removed.

More likely, you want to just remove the "internal fader" jumpers so you can take measurements that will agree with your diagram.

Linear Power - PA2 Manual
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Last edited by EnvisionAudio; 19th January 2012 at 04:29 AM.
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Old 19th January 2012, 04:42 AM   #6
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I know I drew skyblue arrows on the second set of inputs (fader/second source) but I only had one set of channels input, which where connected directly by the yellow relay box.

The other set was unconnected. I dont know if that makes a difference, maybe I have to mess with it some more.

But I do know that the following traces were cut. Dont know if thats making a difference on the trimmers not functioning. Below is a pic, the red traces have been cut, and the yellow traces are good and remain on the board.
Click the image to open in full size.
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Old 19th January 2012, 06:28 AM   #7
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Just checked both trimmer pots (input gain control) and they are good, but are not controlling the input signal. Like I posted before, I rotate them back and foward and no effect to output signal, which is weird because the traces are good, expect for those two red traces I drew on the last pic.
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Old 19th January 2012, 12:09 PM   #8
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If you measure the AC voltage on the various terminals of the pots, do you see any AC voltage?

Do you read continuity through the two glass devices (no RCAs connected)?
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Old 19th January 2012, 03:03 PM   #9
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Click the image to open in full size.
Both are three pins but they only use two of the three pins. I will power it up and check it out. Since the pins are below the board could I check it through the top on the traces?

"Do you read continuity through the two glass devices (no RCAs connected)? "

-By the light bulbs? By continuity you mean would they be "on" if there is no RCA's then yes they do turn on.
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Old 19th January 2012, 03:07 PM   #10
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Am assuming there is AC voltage because its connected to pin 1 & 2 on the op amps, same on both op amps.
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