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Mixers/Consoles Hum in Mackie CFX20MKII

Discussion in 'Sound, Music, and Intercom' started by way2go, Nov 12, 2008.

  1. way2go

    way2go Member

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    Hi there. I'm wondering if anyone here has experience repairing Mackie
    boards. My CFX20MKII has a 60hz hum in all output channels. I haven't
    been inside the unit yet, but am qualified to repair the power supply.
    I have done the basic tests for ground loops or noisy inputs. with
    all inputs off, the LF hum is in main, aux , sub, etc. I am assuming the
    PS capacitors need replacing, or there is a broken ground.

    Anyone have a schematic?


    Thanks.
     
  2. domls

    domls Member

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    I have exactly the same mixer.
    Just some initial thoughts, but have you tried muting the effects? What about the power supply coming from the wall...is it dirty? Did you try on a different circuit?

    domls
     
  3. way2go

    way2go Member

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    Thanks. Yes, we did try another circuit in the adjacent building with
    a long extention cord and had the same level of noise.

    If I remember correctly, I think the power cord plugs directly into the
    rear. I don't remember having a separate transformer on this one,
    but I better go check for sure.

    This thing may even be in warranty, but who knows where the
    receipt is!:confused:
     
  4. avkid

    avkid Not a New User Fight Leukemia

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  5. museav

    museav CBMod CB Mods Departed Member

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    Perhaps you have tried these, but did you actually disconnect the inputs and outputs? How did you monitor the outputs? Does the noise track any adjustments such as the group or master fader? Is there anything inserted on the Main Insert? Basically, have you taken the unit out of the system and tested it in a 'test bench' environment?

    Schematics are sometimes difficult to get unless you are an authorized service center. This came up recently on another forum and although it addressed a totally different product, the manufacturer in that case responded that they consider the schematics to be proprietary intellectual property and will not release them. So their availability is not always a given.
     
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  6. jkowtko

    jkowtko Active Member

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    Yes, I got fooled with a ground loop hum last year (it happened to be on a CFX20, but that's probably immaterial) through my PC's Delta 66 sound interface card, even with the PC turned OFF, because the PC was accidentally plugged into a different circuit under the table. I naturally thought turning off the PC would break the circuit, but it did not. Once I unplugged the PC from it's power supply, the GLM went away.
     
  7. FMEng

    FMEng Well-Known Member Fight Leukemia

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    It could well be the electrolytic capacitors in the power supply section. (See my comments regarding the Ashley EQ.) If it is indeed the power supply, you should be able to hear hum in the headphone output, with nothing else connected to the console.

    If the power supply is the cultprit just replace all the electrolytics at once.
     
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  8. Anonymous067

    Anonymous067 BANNED USER

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    Haha. Once I called up mackie with an inquiry on modifying the pre fade mutes on the oynx (spelling) xx80 series and they gave me a password to some online deal to download the schematics!! They seemed nice about it.
    Posted via Mobile Device
     
  9. dafunkmonster

    dafunkmonster Member

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    My initial thoughts are: check unbalanced inputs, check all inputs for that matter, check main outs, rule out faulty cabling, rule out ground loop noise.

    In my experience, 9 times out of ten, noise in the system at 60Hz is just interference, not something wrong with the mixer.
     
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  10. leistico

    leistico Member

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    Got the same board. Are you using powered PA's or an amp driving passive speaker boxes? Could the hum be coming from whatever's powering your speakers? I turn my board off and all of a sudden I've got 60hz hum coming from my (Mackie) powered PA's in the absence of signal. Turned on with everything potted down, it's perfectly silent. Weird. Let us know what you find.
     
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  11. way2go

    way2go Member

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    First, I want to thank everyone that has responded! I appreaciate all the
    great information. The hum is in the headphone output with all inputs unplugged, including trs; I have monitored while unplugging the outputs also.
    I am pretty convinced that it is not a loop, but taking it to a 'bench test' is
    a great idea. Some of my speakers are passive, a couple of stage
    monitors are powered and of course amplify the hum. The hum is in all
    speakers and didn't really bother us much until we tried the powered stage
    units. Then I knew this had to be attacked and is when I began troubleshooting the problem.. The only wierd thing is, that I recently recorded off the tape
    out and there was no hum on the CD recording. However, an amp connected to the tape out has the hum. I will continue with the bench test and the
    power supply capacitor replacements and report back.
     
  12. museav

    museav CBMod CB Mods Departed Member

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    Some years ago Mackie mixers were pretty infamous for having inputs very susceptible to induced hum and RF pickup, but newer models should not be as bad. However, I do not think that they are Pin 1 (AES48-2005) compliant.
     
  13. way2go

    way2go Member

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    did the bench test and no hum found! Back to the site to check power and
    amp connections.
     
  14. Chris15

    Chris15 CBMod CB Mods Premium Member

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    Not really all that weird. When you turn the power off, the output becomes floating and so picks up whatever. When it's turned on, the electronic balancing is in play and so the op amps pull the outputs to ground. Does that make any sense?

    As to power supply noise, with a conventional supply, there will be mostly noise at 100 Hz or 120Hz depending on your mains frequency because of the rectification process. To me mains noise indicates poor shielding or the like somewhere...
     
  15. jkowtko

    jkowtko Active Member

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    Here's another sanity check --

    Also make sure you don't have any TS mic cables (XLR to TS, not TRS) lying around that accidentally got plugged in. Just had that happen last night while setting up band monitors on stage. Didn't take long to think to look at the 1/4" jack, but when tossed into a pile of XLR-TRS cables, an XLR-TS cable is hard to distinguish. And boy, what hum ...!
     
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  16. way2go

    way2go Member

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    :)
    jkowtko said it best! my problem was in fact unbalanced cables feeding
    balanced inputs on amps. I should've caught that sooner!
     
  17. museav

    museav CBMod CB Mods Departed Member

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    Glad you got it resolved!
     
  18. TimmyP1955

    TimmyP1955 Active Member

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    The same thing happens if our DSP is powered down and the amps are on. I think the output impedance of the output circuit goes unbalanced, causing the pickup of noise in the lines.
     

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