I had the same problem. I recently got a MR816X from a friend. Its manufacture date appears to be 2009. The problem was as described in the first post. This includes whether it was connected to a computer or not, and no matter which wall outlet I plugged it into (and no matter what sample rate it was set to, or if any of the input gain controls were turned up etc.) There was seemingly nothing I could do about it.
…or was there…
Since this is clearly out of warranty by now, and I work on electronics devices all the time, I got out my selection of screwdrivers and decided to open it up.
-After not seeing any obviously bad electrolytic capacitors, my first thought was to move the headphone jack assembly out of the way of the power switch in case any noise were being picked up from that (since it’s right beside the jacks). The problem persisted. I then put the headphone jack assembly back where it was.
-I tried moving the wires to the headphone jack assembly around, moving them from their default placement (they were all held in place so they couldn’t move around until I unhooked them from some clever little bendy things). That didn’t change anything. I put those wires back the way they were originally laid out and tested again. No change.
-I checked that the output of the power supply adapter was 18VAC x2. It was.
-I blew out the dust and debris that was inside (almost nothing, it was very clean inside). I tested a few things around the power supply and noticed nothing unusual.
-I then put it all back together (lots of screws in a few different sizes by the way), and plugged it back in.
And the noise was gone!
Rather, the noise was greatly reduced. It’s still there but it’s so faint that it doesn’t bother me. I would only hear it if audio had finished fading out (including any reverb), going down to the noise floor. Even so it’s not distracting like it was before.
I’m left guessing that perhaps there was a bit of dust/debris inside on something, or perhaps one of the chassis screws needed to be undone and redone to make a more effective chassis ground (if one of the screws got a bit of oxide on it preventing the chassis pieces from making a good continuous ground). I had also moved a couple of the electrolytic caps slightly, bending them a bit out of their normal placement (just a smidgen) while I was moving the leads to the headphone jack assembly…although it would be a stretch to think they were causing a ground loop by touching.
That’s my contribution. I doubt it’ll help anyone much since it was a “I somehow used The Force to apparently fix this for the time being” sort of thing, but there it is. If anyone wants to open theirs (which I’m not really recommending, but I’m not stopping anyone either):
get a pen and paper to write down which screws came from where.
use proper sized screwdrivers for each screw head size. Some of these are very tight (probably “tap threaded” into the chassis) and they’re just begging to have their heads ruined if you use the wrong size screwdriver tip.
notice how it comes apart before removing the wrong screws. Some of the screws you see on the outside need to remain in place as you remove the main cover by sliding it off the back. You’ll need to remove all screws on the back (except one that holds a small cover plate in place near the power entry), 3 screws on top (along the front, but not the 2 that are closest to the front), 2 screws on each side (but not the 3rd larger screw on each side), and 6 screws on bottom (3 along each side).
danger Will Robinson–shock hazard! If you haven’t worked on electronics, don’t open this. If you have worked on electronics, you should know the deal by now. You can injure yourself or ruin the device or both if you make a mistake. Don’t mess around.