Soundcraft MFXi Mixer-Hum in the outputs

[jpglyde]

I have on the bench a Soundcraft MFXi mixer with 60hz hum in the main and aux outputs. There is nothing connected to the mixer except some headphones. I’ve plugged it into different outlets in different buildings and can still hear the hum in the headphones. Using the AFL buttons I’ve determined all the channels are clean. The hum is only present in the outputs.

I disassembled the chassis. The switching power supply is mounted and grounded to the bottom half of the chassis while the rest of the circuits are in the top half. The two halves are only connected by the power supply output ribbon cable. When the two halves of the chassis are separated there is no hum. When they touch and the top half is connected to earth ground, the hum returns. Lifting the earth ground doesn’t help either.

Any ideas?

[nevetslab]

I was curious to see what this was, and fetched a couple images off the internet for visual aids:



When you have this opened up, does anything look like it's missing, or cable routing suspicious? Any hardware missing? Such as shielding that might have been in place to shield the RFI these switching supplies will generate? I've never been inside this model before. Also, somewhat odd that using the AFL monitoring, all the input channels don't exhibit the hum, while the buss outputs do. Now, buss amps can pickup stray fields and have lots of gain. Cable routing on the power supply from the SMPS to the system may be critical. There is lethal voltage on that power supply, so make sure it's not connected to the AC mains when trying to vary the lead dress in searching for the source of hum injection.