Mackie 8 Bus noise - help
Have an older Mackie analog 8 Bus 8x2x2 board that has random computer noise occurring. Little 1/2 to 3 seconds of random frequencies, very low volume, but showing up on 1-2 LED segments across all 8 channel meters and the L/R as well. Definitely something internal to the board or external power supply. Not coming from any specific channel or inputs (have muted/disconnected everything. Any known problems like this? Sent a support email to Mackie through their website but says right up front they don't support this model anymore (wasn't even a choice for the support request) so I don't expect to get an answer - other than take it to a service center. I have the tools and am qualified to do repairs, even at the component level, but need some guidance as to the cause - if already known. Or if not, a contact that could help. Thanks!
You're not setting your phone on the console, are you?
The last time I had one of those Mackies in for repair I had to replace many of the bulk smoothing capacitors on the internal power rails (not the external PSU). The problem was showing up as strange noises on all outputs - not quite as you describe it, but sufficiently similar to make it worth investigating that area.
I would open up the mixer and go round the power rails with a scope (a.c. coupled). It should soon be apparent from the waveforms if any of the caps have gone. If you don't have a scope, you can get some idea by using a DI box as an audio probe feeding into a separate interface or mixer. Put a 100nF capacitor of at least 63V rating in the line from your probe and a pair of reverse-connected silicon signal diodes to ground as limiters at the DI input. Connect the DI box input ground to the mixer ground.
Does the board produce this noise with nothing connected to the inputs? I've not owned this mixer but I seem to recall that noise on the 8 group buses was a major knock on the board.
The other thing which can cause this problem and is a pain to find, is/are a ground loop.