This Mackie mixer came to me with a terrible and pronounced distortion on the outputs at all times, and also with several dead channels. On disassembly, I was able to find blown surface mount op amps, bulging coupling capacitors, and several dead transistors on the two dead channels. Once these were repaired, the channels came back and the unit sounded fine...including the dead channels...
Until I put it back together completely (ie: placed the cover on the bottom and screwed it in)... as soon as I did that, the noise was right back!
Thinking maybe I had a loose connection or solder joint, I pulled the cover off and turned it on again. Now, the sound was clean as a whistle. I poked around on the circuit boards, trying to bend things, trying to find a failed connection, but everything was rock solid again.
But as soon as the cover was replaced and screwed in, the thing sounded terrible, and the same two channels failed.
I realized, after some thought, that the ribbon cable, shown below, which is encased in a black rubber sleeve, and comes from the power supply, presses down on some header pins on the FX circuit board (also shown below, header pins are in the blue square). The pressure is pretty intense when the cover is installed on the back (not a lot of extra romm in there). After time, those sharp pins had worked their way through the rubber sleeve and into the cable, and were making contact with the voltages coming from the power supply. They sent this voltage into various points in the signal path for the channels. This had made the noise, and eventually blown out those components on those channels.
This is clearly a design issue.
I cut those header pins down (obviously they are made for a connection which will never be used in this mixer), and then placed some hot glue over them, so that there is nothing sharp left to press into the ribbon cable.
This instantly fixed all issues, and fortunately none of the new components were destroyed in the brief time that I had the unit powered on in this condition.
The bottom line is, if you have a PRO FX8 which is starting to get noise all around, or in a few channels, this is very likely the culprit!
