My Mackie VLZ mixer's got a bad socket: when the mixer is switched on, the socket goes on and off intermittently. If it's just a loose connection inside, I think maybe I could fix it, although I'm only an enthusiast when it comes to electronic wiring. Do you have any ideas?
Via SOS web site
SOS Reviews Editor Matt Houghton replies: I've owned quite a few VLZ Pro series mixers over the years, as I've found that they're ridiculously good value for a rack of preamps of that quality, particularly when I've bought second‑hand. Of course, buying second‑hand means that I've encountered quite a few crackly and intermittent channels, too! In my experience, usually, these symptoms are down to the insert jack sockets becoming a bit dusty or grimy, which results in a bad contact when a jack is put in there, and can fool the mixer into thinking you've inserted something when you haven't! 
Cleaning the sockets on your mixer (or any other piece of equipment, for that matter) should be relatively simple. Using a slightly 'roughed‑up' jack with some Deoxit and plugging and unplugging the jack repeatedly should get rid of any oxidation or debris that's interfering with connections.
If this is the case with your mixer, all you need do is give the insert socket a good clean. In fact, you might as well clean them all anyway, to prevent this issue occurring on other channels. A spray of Deoxit (which is slightly different from, and better than most, 'contact cleaners') and a very lightly 'roughed up' jack plug are all you need. By roughed up, I mean that it just needs to be lightly scratched with a wire brush or coarse sandpaper (don't overdo it!), so that it can lightly scratch the socket when placed inside. Squirt some Deoxit into the offending socket, then plug the jack plug in and out of the socket 10 or 20 times, or so. That should get rid of any muck and oxidised surfaces, ensuring a good clean contact when in use, and that there are no problems when it's left empty.
If that isn't the issue, it's hard to say precisely what is without examining the mixer in question. It could be the state of the input sockets, or of any of the pots and switches. It could be trickier to solve such problems, since the Mackie not a modular desk, and that means that it will take time to open it up, trace the problem, fix it and put everything back together again. In fact, I'd venture to suggest that it might be cheaper to buy a replacement second‑hand version of the same mixer than it would be to get it serviced!