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Mackie M1400 poweramp problem

Discussion in 'Electronic Repair' started by powerampfreak, Jun 3, 2008.

  1. Hi all. I had a shorted driver stage (MJE15032/33) in one of the
    channels which resulted a lot of open resistors in the output stage,
    Some components in the clip-detector circuit blew too. After replacing
    all the broken stuff, the channel operates, but with high frequency
    ringing superimposed on the fundamental sinewave. Does anyone have a
    specific tip for this failure? Some of the compensation capacitors
    gone bad? Thanks a lot for any advice!
    Regards
    Steve
     
  2. N_Cook

    N_Cook Guest


    Assuming a constant hf then a matter of divining, by literally floating your
    hand/finger over, until zeroing in.
     


  3. Yes, you haven't found all the broken stuff yet. Don't start guessing,
    trying to link symptoms with components, you'll be there all day and get
    nowhere. Go back and check very carefully all the components from the
    output transistors back as far as it is possible for the damage to have
    gone.

    One technique I use is to use the diode check function on my multimeter,
    giving a reading of forward voltage drop. This gives an indication of
    resistance and semiconductor junction integrity at the same time. As this
    is a stereo amp you can directly compare the working channel with the faulty
    one.

    Check each transistor junction and all the components using BOTH test lead
    polarities, starting from the power devices backwards. (You may be measuring
    a base emitter junction with a 100K resistor between the two. In one
    direction you will read 100k, the other you will read a 0.6V diode).

    Investigate any discrepancies between channels, allowing for changing
    readings due to charged/charging capacitors. Both sides should read and
    behave the same. This may seem at first to be a very labour intensive
    procedure, but with practice can be carried out surprisingly quickly, and
    discrepancies easily located.


    Gareth.
     
  4. Eeyore

    Eeyore Guest

    At all power levels ?

    Graham
     
  5. Actually, I run it without the dummy load for initial testings, in
    order to operate the amp more safely.. until I get a decent output
    signal.
    Then I go for power tests.
    I think I drove the amp up to somewhere 30-40V rms and the high
    frequency oscillation is riding on top of my sinewave test signal.
    The OK channel doesn't behave like this, if anyone may think it's just
    because I've no dummyload connected.
    Besides, there seems to be different version of the M1400, since my
    schematic isn't exactly like the amp...
    Maybe anyone has a few schematics for this unit in PDF?

    Regards
    Steve
     
  6. Eeyore

    Eeyore Guest

    But at lower levels ?

    Ooooh, I did at one time I think. I'll look.

    But anyway, do you know what I mean by the 'output zobel network' ? An RC
    series combination. The R should be of several watts rating. Do make sure that
    the R isn't burnt out (open).

    Graham
     
  7. The Zobel network is all fine.
     
  8. Eeyore

    Eeyore Guest

    Which it needs to be to counter the effect of the output inductor or you'll
    wreck the phase stability of the amplifier.

    Graham
     
  9. Eeyore

    Eeyore Guest

    Every amp has one prior tot he output inductor.

    Graham
     
  10. Eeyore

    Eeyore Guest

    I wouldn't expect it to work for very long.

    Graham
     
  11. Eeyore

    Eeyore Guest

    http://fileshare.eshop.bg/downloadsm/35996/Mackie_M-1400, M-1400i.html

    No schematics though

    Here we go
    http://www.eserviceinfo.com/downloadsm/27014/Mackie_M1400.html

    Graham
     
  12. Eeyore

    Eeyore Guest

    They both are.

    I design power amps btw.

    Graham
     
  13. Eeyore

    Eeyore Guest

    Who is JC and why would his opinion change the facts ?

    Grahama
     
  14. Eeyore

    Eeyore Guest

    One of my designs has been sitting in the local venue for over TEN YEARS. One
    level pot went scratchy in that time and we also gave the heatsinks a
    blow-through.

    $300 doesn't buy you very much in the way of a power transformer or decent output
    devices and heatsinking + fans.

    I've heard of these amps in the UK. Not 'Warrior' brand is it ? They came with
    ten year warranties but the company only lasted about 18 months. Killed by the
    warranty claims.

    Graham
     
  15. Arfa Daily

    Arfa Daily Guest


    Isn't using a vacuum cleaner as the rack cooling blower, a bit noisy ... ?
    :)

    Arfa
     
  16. Tim Phipps

    Tim Phipps Guest

    Seems to me that you have a problem with anything made in China. We all
    know that you get some crap made there but sometimes some good stuff
    comes from there. Most of my amps are British made (Matrix) but I have
    a couple of Chinese made amps, one Audiohead and the other T.amp brand
    from thomann.de which look like the same amp to me, both have been in
    service on the road for about 3 years now without trouble.

    You might also remember some time ago when I posted for help with a dead
    Studiomaster 700D. The fault turned out to be a bad power transformer
    which was not economical to replace even by substitution with off the
    shelf parts. That amp got replaced with another Chinese made T.amp from
    Thomann which has also given me no problems.
     
  17. Tim Phipps

    Tim Phipps Guest

    Seems to me that you have a problem with anything made in China. We all
    know that you get some crap made there but sometimes some good stuff
    comes from there. Most of my amps are British made (Matrix) but I have
    a couple of Chinese made amps, one Audiohead and the other T.amp brand
    from thomann.de which look like the same amp to me, both have been in
    service on the road for about 3 years now without trouble.

    You might also remember some time ago when I posted for help with a dead
    Studiomaster 700D. The fault turned out to be a bad power transformer
    which was not economical to replace even by substitution with off the
    shelf parts. That amp got replaced with another Chinese made T.amp from
    Thomann which has also given me no problems.
     
  18. James Sweet

    James Sweet Guest


    Isn't iRiver Korean? We did some work with their products a while back,
    had a couple of their engineers over here, I'm pretty sure they weren't
    Chinese but I could be wrong.
     
  19. Tim Phipps

    Tim Phipps Guest

    I was beginning to think my posts were not making it out there until I
    saw your reply (Arfa was right about the Virgin Media news server being
    troublesome!)

    I don't remember looking closely but I don't think they were
    particularly domed. It was at least 2 years ago when it died. I do
    remember that everything looked very clean in there. The amp had an
    easy life in a church install but for some reason the power transformer
    got shorted on the primary thus blowing the mains fuse every time.
    Since it was a custom part I enquired with Toroid International about
    purchasing a replacement but it would have cost about £150 as I guess
    they were not making these in volume any more. When I saw that I could
    just replace the whole amp for the same money it was a no-brainer.
     
  20. The Mackie amp was repaired. A tiny tiny pcb trace/via hole for one of
    the predrivers were open circuit.
    This happened probably when the drivers shorted. Now everything's
    OK!

    Regards
     
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