Mackie 820i CH2 buzz issue- Reaper project linked

[cnovey]

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-

[cnovey]

Made a Youtube:
https://www.youtube.com/watch?v=wpGJP8y ... 1LMWu-3Ldg

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.