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Alesis M1 Active, repair

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

  1. Fredrik 1977

    Fredrik 1977 Super Member

    Messages:
    1,193
    Location:
    Sweden!
    hello!
    while having some time to spare i decided to have a look the powered monitors i bought last year at a yard sale.
    one is working the other was dead.
    opened it and replaced the fuse, it blew again...sigh!

    pulled the psu, hit the power on the solder iron,and pulled out the fluke...

    lets start by saying i dont know much about swich mode psus, except that they bite! if you pat them the wrong way:) im into 70´s SS amplifiers.

    from what i have gathered the Q1,Q2 shorted (IRF840 fets) i replaced these with IRF841 because thats what i had at home:)
    Q3,Q4 was shorted
    R15 0,22 ohm was open
    U1 (UC3844) DIL8 shorted
    3 of 4 rectifier diodes D1-D4 was short!
    and C8 was fried.

    i have replaced all but the IC since i didnt have that at hand. is there any thing i should check before putting it in when it arrives? i mean so it dont blow up again?

    The Alesis 9-40-B601-E psu unit
    001.JPG
     
    Last edited: Jun 25, 2018

     

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  2. Fredrik 1977

    Fredrik 1977 Super Member

    Messages:
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    hmm on a second thought...maybe should have put this under DIY section.
     
  3. bowtie427ss

    bowtie427ss arigato gozaimashita Subscriber

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    Since you're working on a solid state amplifier, what would be more appropriate than the solid state forum?

    The mods here are some of the best folks on the planet. Although they don't need an increased workload, i'd wager that if you started a conversation with the mod for this forum and asked them, they would happily move this thread to another forum for you.

    You might also be able to use the "report" link if it gives an option for a short explanation.
     
  4. Fredrik 1977

    Fredrik 1977 Super Member

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    your right...
    i reported myself:)
     
  5. Celt

    Celt Peanut Head Staff Member Super Mod

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    I moved it to DIY...if this doesn't work, I can move it to SS. Lemme know.
     
    Fredrik 1977 likes this.
  6. dr*audio

    dr*audio Fish fingers and custard! Subscriber

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    Any time you work on a switching supply, replace all the electrolytic caps with high frequency 105C caps. they go bad and usually caused the failure. Use Panasonic or Nichicon.
     
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  7. Fredrik 1977

    Fredrik 1977 Super Member

    Messages:
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    hi
    well i was quite surprised to see that all caps was nichicon PL and the 2 big ones is panasonic UP
    the fried (C8) cap sat next to(as in one mm from) some chernobyl_hot resistor (R4) so the plastic wrapping had melted:)

    usualy this type of stuff has all kinds of stange brand lytics...
     
  8. elnaldo

    elnaldo Addicted Member

    Messages:
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    Location:
    Buenos Aires GMT-3
    Hello. I've fixed this same PS last month, but no ic were shorted, just two bad capacitors, the one you already replaced and that small one (1uf I guess) in your pict at front, beside that ferrite coil.

    Another problem it had was the very first ferrite coil after the AC input, the wires were so tiny that acted as a fuse and one wire was cut . I removed the coil, unwond one turn of wire and installed it again, glued with epoxy to avoid vibrations.

    I retouched every soldering too. Too many overheated points.

    Other people had reported bad main filter caps. Mine were "Jun-Fu" brand... made me remember Kung-fu, but they are made in Taiwan, not china.

    Make sure the power switch is connected at the right connector. There are two similar connectors very close to each other.
     
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  9. dr*audio

    dr*audio Fish fingers and custard! Subscriber

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    Panasonic UP is rated at 85C and should be replaced with 105C.
     
  10. Fredrik 1977

    Fredrik 1977 Super Member

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    ok i´ll take a closer look at those. something must have caused this catastophic failure.
    or maybe its just poorly designed?
    if transformers were cheaper i´d build conventional (proper):D psus for the speakers.
     
  11. mhardy6647

    mhardy6647 AK Subscriber Subscriber

    Messages:
    52,767
    Maybe just poorly implemented -- I have a pair of those I picked up cheap; one of the pair is immolated (I mean, really immolated inside).
    The other one seems to work just fine.
     

     

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  12. Fredrik 1977

    Fredrik 1977 Super Member

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    IC arrived, shoved it in the ic socket and the speaker powered right up!
    now lets see if they ROCK to:)
     
    Last edited: Jun 27, 2018
    elnaldo likes this.
  13. Fredrik 1977

    Fredrik 1977 Super Member

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    i desoldered those pana UP and meashured them. they were pretty good, and i didnt have any 200V snapins at home so i put them back on the board.
     
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  14. Fredrik 1977

    Fredrik 1977 Super Member

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    well...they played for about 45minutes before the same speaker blew the fuse again :)
     
  15. dr*audio

    dr*audio Fish fingers and custard! Subscriber

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    Not saying this is what's wrong in this case, but my experience has been that if you don't replace ALL the caps with 105C HF caps, there can be spikes on the supply that exceed the voltage rating of the switching transistors and they blow.
     
  16. Fredrik 1977

    Fredrik 1977 Super Member

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    hi!
    ok i´ll try that out
     

     

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  17. dr*audio

    dr*audio Fish fingers and custard! Subscriber

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    Check the fets, too.
     
  18. Fredrik 1977

    Fredrik 1977 Super Member

    Messages:
    1,193
    Location:
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    so i have the psu on the bench...again...
    All 4 rectifier diodes are shorted...again...

    fets "looks" ok from beeping them from the foilside.
    same with the ic. atleast they arent shorted:)

    what the hell is going on here? i´ll replace the caps and try it again? i really dont like these types of psu´s

    maybe its time to recap the working psu in the other speaker as well....before that one also blows up.?
     
  19. dr*audio

    dr*audio Fish fingers and custard! Subscriber

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    Heres what to do:
    1. recap.
    2. Plug it into an isolation transformer and probe the fets with a scope to make sure they are switching. Needs to be loaded.
    3. Check the output voltages.
     
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  20. stoN_Cold

    stoN_Cold Super Member

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    Check all again.

    [​IMG]
     
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