Right Channel Problem on LM3875

freeman · 27th January 2011, 10:45 PM

Hi All,
Been listening to my LM3875 premium kit for a week, and it was sounding great. But today the right channel faded out suddenly. There is still faint volume there but not much. I've eliminated speakers and source as problem. Voltages on rails and dc offsets are normal. and all continuity tests are checking out. I will need to remove chip from the board to test...correct? Star grounds are set up exactly as Peter suggests. Speakers are 8 ohms and there has been little detectable heat coming from the chips all week. I'm using a PEC 25K pot. Any thoughts are much appreciated. Thanks.

freeman · 28th January 2011, 12:34 AM

Forgot to mention that I'm using Avel 330/25+25 transformer. Rail voltages measure about 37V. For heatsink, I'm using 2.5"X1.5"X1/8" alum u-channel in almost same configuration as Peter's build instructions. Any advice is welcome. Thanks.

Peter Daniel · 28th January 2011, 04:08 AM

If the chip shows normal offset (20-70 mV) it's usually not damaged. Check if the offset changes when adjusting volume.

I would suggest swapping complete boards between both channels first and see what happens; removing chips might be tricky.

freeman · 28th January 2011, 04:42 AM

Will do the swap. With full volume I'm getting 24mv on right channel 5.5mv on left. With volume off, 3.7mv on right, 33.1 on left. I'm pretty sure these are similar numbers when I brought it up initially. sound ok?

Peter Daniel · 28th January 2011, 01:28 PM

yes, those look normal.

freeman · 31st January 2011, 11:03 PM

Peter. I swapped the boards and the right channel still lacked proper volume. I swapped wiring on pins of the pot and low volume shifted to left channel, which means the problem seems to be in front channel of the PEC. Please let me know if you think I've missed something. Is it possible to repair the PEC? Thanks.

Peter Daniel · 1st February 2011, 01:38 AM

If it's the PEC, I assume you changed the wires after the pot?

Here's the link showing how PEC pot is built:"Silver-Carbon" Pot ex Tawan - Photo - & ignore my ugly Pinky!

One way to damage it is by overheating the pins which may disrupt internal connections.

freeman · 8th February 2011, 12:12 AM

Hey Peter. I swapped the pot out and have it up and sounding great again. Decided not to open up the PEC for now. Now it's on to building a proper chassis if I can pull myself away from listening. Thanks again.

27th January 2011, 10:45 PM	#1
freeman diyAudio Member Join Date: Dec 2010	Right Channel Problem on LM3875 Hi All, Been listening to my LM3875 premium kit for a week, and it was sounding great. But today the right channel faded out suddenly. There is still faint volume there but not much. I've eliminated speakers and source as problem. Voltages on rails and dc offsets are normal. and all continuity tests are checking out. I will need to remove chip from the board to test...correct? Star grounds are set up exactly as Peter suggests. Speakers are 8 ohms and there has been little detectable heat coming from the chips all week. I'm using a PEC 25K pot. Any thoughts are much appreciated. Thanks.

28th January 2011, 12:34 AM	#2
freeman diyAudio Member Join Date: Dec 2010	Additional Info Forgot to mention that I'm using Avel 330/25+25 transformer. Rail voltages measure about 37V. For heatsink, I'm using 2.5"X1.5"X1/8" alum u-channel in almost same configuration as Peter's build instructions. Any advice is welcome. Thanks.

28th January 2011, 04:08 AM	#3
Peter Daniel diyAudio Member Join Date: Jul 2002 Location: Toronto, Canada	If the chip shows normal offset (20-70 mV) it's usually not damaged. Check if the offset changes when adjusting volume. I would suggest swapping complete boards between both channels first and see what happens; removing chips might be tricky. __________________ www.audiosector.com “Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC

28th January 2011, 04:42 AM	#4
freeman diyAudio Member Join Date: Dec 2010	Thanks, Peter Will do the swap. With full volume I'm getting 24mv on right channel 5.5mv on left. With volume off, 3.7mv on right, 33.1 on left. I'm pretty sure these are similar numbers when I brought it up initially. sound ok?

28th January 2011, 01:28 PM	#5
Peter Daniel diyAudio Member Join Date: Jul 2002 Location: Toronto, Canada	yes, those look normal. __________________ www.audiosector.com “Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC

31st January 2011, 11:03 PM	#6
freeman diyAudio Member Join Date: Dec 2010	Peter. I swapped the boards and the right channel still lacked proper volume. I swapped wiring on pins of the pot and low volume shifted to left channel, which means the problem seems to be in front channel of the PEC. Please let me know if you think I've missed something. Is it possible to repair the PEC? Thanks.

1st February 2011, 01:38 AM	#7
Peter Daniel diyAudio Member Join Date: Jul 2002 Location: Toronto, Canada	If it's the PEC, I assume you changed the wires after the pot? Here's the link showing how PEC pot is built:"Silver-Carbon" Pot ex Tawan - Photo - & ignore my ugly Pinky! One way to damage it is by overheating the pins which may disrupt internal connections. __________________ www.audiosector.com “Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC

8th February 2011, 12:12 AM	#8
freeman diyAudio Member Join Date: Dec 2010	Hey Peter. I swapped the pot out and have it up and sounding great again. Decided not to open up the PEC for now. Now it's on to building a proper chassis if I can pull myself away from listening. Thanks again.

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