Sound Craft FX16ii, sacrificial electrolysis?

[Sowhat]

I hate working on mixer boards and keyboards, main reason is that I never have enough bench space to do it properly. I need to improvise and in doing so more often than not I break something that's hard to replace because I'm obviously a clumsy idiot… none the less, I have this Sound Craft FX16ii mixing board with some missing channels and crackle (oh god, what am I in for now?) Well it's a ribbon cable nightmare initially but once you've taken it apart and put it back together a dozen times, yeah, you heard me: a dozen times, the mechanical design begins to make some sense. Actually this mixing board has been in the shop before, that time some previous tech replace the electrolytic caps in the power distribution which were surface mount cap cans. Problem was I don't think he was equipped with micro-soldering equipment or didn't know how to use them effectively because he ripped pads and traces up all over the place. As a solution he tacked in some thru hole caps to the trace stubs and declared it a success! I guess that fix lasted about 6 months before it shook apart which is when I stepped into the picture.

I cleaned and resoldered the thru hole caps because we're not going to get into the labor charges for me to meticulously glue in tiny copper strip and rebuild the trace pattern, anyway that type of fix never works real good. I gobbled on some component dope and sent it back out to the customer, suggesting he trade it for some new digital doo-hickey. Well, he didn't and it went right back into the venue and worked flawlessly for about a year when all of a sudden it appeared in my incoming mail again but this time with a different complaint as I mentioned above.

DON'T DRINK OVER THE MIXING BOARD!!!

This is something that should be tattooed on the arm of every "sound guy". Once I gutted the mixer and propped it up on the bench it was clear that somebody didn't heed the aforementioned commandment. I'm not really sure but I think it was rum and coke or something all down the 3rd and 4th channels. After probing and measuring it looked like a chip that is shared by both channels (a dual OP amp, a pair of TL072's) was not talking and was caked up with dried party drink liquid residue. I hot aired the chip off and started cleaning up all the residue with hot flux baths and alcohol. There were two power rail filter caps (little mylars) next to the chip that had this gunk incrusted under them so I removed them and one of them fell apart. After cleaning the board surface I could see that the negative supply (V-) had an open connection where the copped was eroded away right at the lead of the filter cap that fell apart. At this point I figure that the chip was OK but I replace it anyway with a new one, I like things nice and tidy. I did a tiny jumper on the trace, put in new caps and once I fired it up the channels came right back.

Does copper in rum and coke under the influence of V- cause sacrificial electrolysis? Sure seems so.

BTW: The vibration crackle was probably from dirty ribbon cable connections… somewhere… anyway It don't do that anymore after a dozen disassemblies and assemblies.

[Enzo]

Rarely did I put mixers and synths on my bench. Unless the mixer is the size of a rack toy. Once in a while I might stand one on end and lean it on my riser, but mostly I use my cart.

I have something like this:
Easy-Up Four Wheel All Purpose Service Cart | EZ45G | Easy-Up

Easy enough to make your own. I have carpet on top to help prevent scratching things. It sits in the middle of the shop. I do most work on it, leaving the bench for chassis or boards. I sit a combo, like Fender twin reverb, up there, makes taking the chassis out easy at eye level, then swing around and put chassis on bench, leaving cab on cart. But keyboards sit on this quite well, and I can spin the whole thing around to get to back or front easily. It is on wheels. With the carpet, I can lay a synth on its face, won't hurt the keys.

But a big mixer shines there. With 200 knobs, the weight on any one is tiny, so you can lay them on their face for disassembly. I work on them from the front largely, but to check them out, I spin them around and run from the rear. SO I can plug signals into any jack simply, and then reach down and work the sliders and controls. No way a 48 channel mixer fits on my bench.

I got a long scope probe to reach from my bench riser to the cart, but if I need to scope a lot, I can pull my scope off the shelf, sit it on the floor next to me facing up next to my knee, and scope in the mixer.

[Sowhat]

Rarely did I put mixers and synths on my bench. Unless the mixer is the size of a rack toy. Once in a while I might stand one on end and lean it on my riser, but mostly I use my cart.

Yeah, I have some carts and even my old back saver, the hydraulic lift table I have from when I did slot machines and had to heft those monsters around. My problem is vision and that means that I use an inspection microscope for micro-soldering work and that thing is on the bench. I have about 40 feet of bench space but I always seem to have stuff on it in the way. I muddle through best I can but I'm always looking for better. Give me a nice busted Marshall head anytime. :-)

[drewl]

Good on ya' mate.

Anyway, my bench is wide enough but I always rest things like that on a towel or foam rubber with my custom made wooden amp supports underneath, strategically placed as to not break anything.

This also allows you to move any knobs or faders underneath.

This works well for guitars also as you don't want to nick or chip someone's guitar or bass as you work on the active electronics.


Don't get me started on removing molten globs of beverage residue from equipment.

Don't Drink Over The Mixer!

Unless that's the secret to certain masterpieces?