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Mixer/console repair/cleaning
Mixer/console repair/cleaning
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Old 18th January 2018, 10:07 PM   #1
curtis73 is offline curtis73  United States
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Default Mixer/console repair/cleaning

I am TD for a theater and I inherited a few million problems and its high time I solved this one. The theater owns an Allen and Heath GL2200 24 channel. We have a Mackie 8-channel that I use as a backup or sometimes for a pre-mix in an orchestra.

The A&H isn't healthy. When I originally started here, it was living in the shop uncovered and had some dust on it. I blew it off externally (didn't disassemble) and it worked well for a year. Now all of the inputs (I think I checked 14 channels before assuming that they would all behave the same) are heavily distorted; 1/4" and XLR both. Almost like you have the gain cranked the whole way, but it isn't. Small inputs from the fader will peg the LEDs even at very small volumes and the sound (even when quiet) is clipping badly. Problem seems to be the same for all channels. It's not a source problem because I plugged in a laptop, CD player, and a few other sources into both consoles and the problem only exists in the A&H box. No cracking or fuzz from the Pots or faders.

The actual SPLs seem to be logical. That is to say the volume seems right with the gain and fader positions, but the LEDs peg on clip as soon as you bring up the fader and the program is distorted.

There is zero budget for a pro fix but we have a show coming up in March where I'll need more than 8 channels from the Mackie. What can I test/try/clean? I'm no stranger to repairing things, just haven't ever done a console before and don't want to mess it up. Tips? Pointers?
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Old 18th January 2018, 10:45 PM   #2
Enzo is offline Enzo  United States
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My first couple reaction are you are losing a low voltage supply. I assume teh signal circuits run on +/-15v or +/-12v. Some run on 17 or 18v. WHatever your uses, if one of those supplies is collapsing or losing filtration, distortion results. That is where I start.

If you ever see a mixer sitting there with no input and faders at zero yet all the peak lights are pegged, I will bet my lunch money for the week that you have lost a 15v supply.
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Old 18th January 2018, 10:50 PM   #3
curtis73 is offline curtis73  United States
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Ok. I might be able to check that if it looks logical when I pull it apart.

The LEDs aren't actually pegging until you start to bring up the channel, but that still could be the issue. Pretty much after you get the slider up about 1/8" and you start to hear audio, the LEDs peg and the sound is distorted even though its very quiet.
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Old 18th January 2018, 11:14 PM   #4
lakeside is offline lakeside  United States
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I agree check the power supplies. Link below to the service manual.

http://www.allen-heath.com/media/GL2...ICE-MANUAL.pdf
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Old 19th January 2018, 12:21 AM   #5
curtis73 is offline curtis73  United States
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Sweet. Printed. Thank you
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Old 19th January 2018, 08:49 PM   #6
curtis73 is offline curtis73  United States
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Hey, while I'm in there... I know it has been at least three years since it has been cleaned, and I found it in the shop without a cover on it.

Quick tips and primers on cleaning while I'm in there? Dry compressed air, deoxit, and a screwdriver? Anywhere that shouldn't have deoxit? I don't want to make it worse, but I might as well clean while I'm in there.
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Old 19th January 2018, 09:09 PM   #7
turk 182 is offline turk 182  Canada
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keep the pressure down on the air and use soft bristle brushes to access/disturb accumulations further in on the channel boards and while your in there don't neglect a close visual inspection with a magnifying glass of solder joints on buss connector points.
from the symptom you describe i'd be looking for a loss of the mix buss ground that is assuming all the supply lines are ok, have you checked those?
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Old 19th January 2018, 10:57 PM   #8
curtis73 is offline curtis73  United States
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Well, I did a side-by-side test. Plugged into my 8-channel Mackie, its all clean. Pull the XLRs and 1/4" out of the Mackie and plug into any of the channels on the A&H and its distorted. All channels the same, tried multiple cables and multiple sources. I have narrowed it down to "something in the console."

I haven't opened the console yet.
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Old 29th January 2018, 06:24 PM   #9
chris marcellus is offline chris marcellus
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Can you insert your audio feed directly into the sub groups or master left and right group? If you can, try that as well. All mixers have summing amplifiers in the sub group sections and in the master output sections. If the summing amps are going crazy, then the problem will be evident no matter which input is used.
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