Mackie 820i CH2 buzz issue- Reaper project linked

[Om_Audio]

Had Reaper and 820i (v1) working great for years on laptops, desktops, etc.

Recently CH2 on the 820i has an odd behavior- it buzzes when you plug anything into the 1/4' input. I plugged a 1/4" to RCA adapter in when I first noticed it. If you plug a balanced cable same but if balanced cable is connected to a piece of gear the sound is still there but almost inaudible. I have recreated settings exactly on CH1 and it is silent in all scenarios as it always has been (and CH2 until recently).

I am posting a link to a zipped Reaper project with screenshots of freq display of audio examples. One screenshot here.
dropbox.com/sh/wz9lf2vhq8hq6y0/sBnQMOaXkA (copy and paste link- won't let me post full link as I have not been active on these forums enough yet- less than 5MB zip file w project)

Anyone have any idea what could be wrong? I will open the unit soon to look inside- I am good with a soldering iron so if I burnt an op amp or something I may be able to repair (although if surface mounted components might be tricky)

Thanks-
Youtube:
http://youtu.be/wpGJP8y0tvI

Dropbox link to project-
https://www.dropbox.com/sh/wz9lf2vhq8hq6y0/sBnQMOaXkA
Screenshot links showing frequencies:
https://plus.google.com/u/0/photos/105266388149148864163/albums/5988955244292586913

[rullywowr]

Ground loop hum from the switching power supply is my guess. You can get a 3prong to 2 prong adapter for your computer at the hardware store and try it to see if the noise goes away. Had a similar issue with a hp laptop a while ago. This is of course not the safest (since you are lifting the ground) but worthwhile to see if it helps. Total investment: about $2

[Om_Audio]

No it is not that, it is only on 1 channel and new issue. I took the mixer apart tonight to check solder joints of connector etc. Bunch of SMC in there. :/
I will give your suggestion a try once I have it all back together but I suspect some part in that input got damaged. I never go 3/grounded to 2/un-grounded permanently- only for trouble shooting- just not a good idea.
The manual only has a rudimentary routing schem- so I have no full service schem to offer the more experienced users here.

Thanks-
C

[Om_Audio]

Here is another observation- if I plug in this ground loop isolation transformer the noise is almost inaudible. All I do is plug in one side of the transformer- all other connections remain unused.


Also, if I plug something into the insert the noise is gone. I am wondering if a damaged component could be shorted and causing the issue. The noise id produced before the EQ and Aux secions. The XLR is normal- no noise. I am using simple 1/4" male to RCA female adapters for testing.

[Om_Audio]

Bump

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