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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!

Comments

Boswell Thu, 10/01/2015 - 15:51

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.

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