Alesis M1 Active, repair

Discussion in 'DIY' started by Fredrik 1977, Jun 25, 2018.

  1. Fredrik 1977

    Fredrik 1977 Super Member

    Messages:
    1,190
    Location:
    Sweden!
    ok will do!
    all caps desoldered from the pcb. all test good with cheapo component tester. but i know that probably dont mean a thing in this case :)
    didnt have all values at home so placed a order, have to wait....again:)

    also pulled the fets, tested just fine. but however originals were irf840, i used irf841 as replacement the first round in here. could that have anything to do with it blowing up again?
    anyhow i ordered irf 840 just for the sake of it.
     
    Last edited: Jul 5, 2018

     

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  2. dr*audio

    dr*audio Fish fingers and custard!

    Messages:
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    Location:
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    In Sweden the line voltage is 230V. The 840 is rated for 500V and the 841 is rated for 450V. Probably ok but I would use the 840.
     
  3. elnaldo

    elnaldo Addicted Member

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    Location:
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    When I face a shorted SMPS, I usually replace the Fet, oscillator IC and opto coupler in a single operation, along with small diodes in the oscillator circuit.

    Check overheated components, even transformer coils, check it's not shorted. The same with the filtering chokes and X2 caps at the AC side, check the AC rails are not shorted. Anyway if the diodes are burned the short is probably ahead of those filters.

    I also try this PS with a step down transformer at 110V.
     
  4. Fredrik 1977

    Fredrik 1977 Super Member

    Messages:
    1,190
    Location:
    Sweden!
    new caps and rectifiers. and poof!....rectifier (D2, D3) and fuse toast.
    there was no short on the ac rails when i had the diodes out.
     
    Last edited: Jul 9, 2018
  5. dr*audio

    dr*audio Fish fingers and custard!

    Messages:
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    Location:
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    Do you have an oscilloscope and DC bench supply?
     
  6. Fredrik 1977

    Fredrik 1977 Super Member

    Messages:
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    Location:
    Sweden!
    hi!
    yes i do.
     
    Last edited: Jul 9, 2018

     

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  7. Fredrik 1977

    Fredrik 1977 Super Member

    Messages:
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    D14 was shorted. must have missed that:whip:
    I replaced all toasted components and installed psu with 2A fuse (5A stock) now its powered up and working...lets see for how long:)

    if this thing still works tomorrow i´ll recap the other speaker.

    oh! and i didnt have that "ultra fast" MUR160 diode so i replaced both D14 and D16 with BYV26C they looked closest lol. is this gonna work? or would regular 1N4007 do better ?
    or do i have to get ultra fast ones?
     
    Last edited: Jul 9, 2018
  8. Fredrik 1977

    Fredrik 1977 Super Member

    Messages:
    1,190
    Location:
    Sweden!
    and thanks so far dr.audio and elnaldo. i "think" i have learned something about these psus. maybe not much but atleast something:)

    exactly what does the "opto coupler" do? gives a "good to go" signal from output (low voltage) section to the main (high voltage) section?
     
    Last edited: Jul 9, 2018
  9. dr*audio

    dr*audio Fish fingers and custard!

    Messages:
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    Location:
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    Usually the opto connects the regulating voltage to the controller IC so it can properly sense the output voltage and regulate it.
    Get the proper diodes. If they can't turn on and off fast enough they may overheat.
    When working on a switching supply its a good idea to plug it into an isolation transformer and then look at the drain of the switch with your scope. The duty cycle should be 50% or less and you don't want any high voltage spikes on the square wave, those can exceed the D-S voltage rating of the fet and kill it. That's why you replace ALL the caps and use high frequency 105C caps.
     
  10. Fredrik 1977

    Fredrik 1977 Super Member

    Messages:
    1,190
    Location:
    Sweden!
    thanks!
    well the speaker has now been on for 24h so i consider it working.
    as u said from the beginning.... replace the caps:D
    old ones were probably tired. these psus eat caps for lunch?

    i will get new high speed diodes when ordering stuff for some other project. i still have the other speaker to recap before i can use them anyway.
     

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