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old mixer cleaning/ trouble shooting

Discussion in 'Pro Audio Equipment' started by regularmike, Aug 17, 2008.

  1. regularmike

    regularmike Guest

    hi,

    i brought an old (1980) allen and heath 16-4-2 mkII mixer on ebay for barely anything and im trying to get it working.

    i dont know much about electronics, but i succeeded in building the power supply based on the diagram in the user manual, so i am reasonable competent.

    the problem now is that the output volume is fluctuating when nothing is being moved, and at its best is still too low, ie, am recoridng snare drum and no matter how high the gain and fader go i cannot get the channel to peak, with any mic, and the variations are across all 16 channels simulataneously. when i say variation, i mean that all channels will simultaneously drop in volume, and will not achieve anything higher than -20db on the VU meter.

    when i give the board a bit of a bang near the master faders that can effect the problem, but it is unpredictable.

    does anyone have any suggestions on what it might be that could effect the level of all inputs/ outputs together?

    cheers

    mike
     
  2. RemyRAD

    RemyRAD Well-Known Member

    I know you said you built a new power supply. And, I don't believe this unit contains any VCA's? So perhaps, your new supply is slightly underrated? You may also have swapped out some capacitors in the wrong polarity?

    You need posted on this project
    Ms. Remy Ann David
     
  3. Kapt.Krunch

    Kapt.Krunch Well-Known Member

    Recheck your soldered joints that you did. Check ALL soldered joints and
    connections.
    Clean all pots and faders, if possible.
    Clean all socketed joints, and ensure that they mate securely.
    Clean all input/output jacks. Check all switches, and clean, if possible.

    There also may be certain places you can inject a signal to see if it happens before or after that stage, while eliminating earlier or later stages. Like, INTO a submix or master bus, or direct OUT OF channels, etc. If you've bypassed earlier stages, and it still happens, you know the problem is in or after the injection stage. If it DOESN'T still happen, it's before that stage.

    Could be one or more bad/dirty pot, fader, switch, connector, or switching jack. Could be bad solder joint or loose connection. Might as well clean and tidy up as much as possible while it's open, anyway. Could be a wrong value or incorrectly installed component. Could be a bad component on the circuitboard.

    If none of that works, you'll have to run a signal through, and follow the schematic to see where it bugs out at.

    Check all of above.

    Kapt.Krunch
     
  4. regularmike

    regularmike Guest

    thanks for the replies,

    when i first plugged in my power supply i did have one of the electro caps backwards, but i realized pretty soon when the room filled with stinky smoke. but i have replaced that and it seems fine now.

    i am pretty sure that the power supply is supplying the correct voltage, because the +- 15v on the circuit diagram corresponds with writing on some of the circuits inside. i think i will have to do as kapt crunch suggested and go through and clean and check everything and all the soldering joints, and maybe get a new mixer. i think this one has been through its fair share of modifications. it was once a 24-4-2, but at some stage it was cut in half and made into a 12-4-2. so there are several foreign circuits inside and odd patch leads that obviously weren't part of the original mixer.

    thanks again for the advice

    mike
     
  5. RemyRAD

    RemyRAD Well-Known Member

    If it was a 16 x 4 x 2 and it was cut in half, your math is bad. That's eight, not 12. There's your first problem. And no, Enzyte will not make your penis grow larger unless you look closer at it. Maybe a circus mirror will help? So if somebody hacksawed a mixer in half, I hope you didn't offer them more than $25? You purchased something already pre-screwed, pre-broken, pre-stupid, pre-fooled. What were you pre-thinking about? Now that you're post thinking?

    So really, with the thing powered up, you have to start injecting a known signal source such as a CD player, into and out of every stage, throughout the device. It would actually be preferable to have a signal oscillator & volt meter for steady-state measurements. Active, dynamic music as a signal source already has changes in volume and so is hard to use as a reference. But chances are, you don't have a tone generator nor volt meter?

    Sorry you can't six stupid.
    Ms. Remy Ann David
     
  6. GeckoMusic

    GeckoMusic Guest

    Sounds like a power supply issue to me. Have you checked all the voltages under load?
     

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