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Gallien Kruger MB115 Combo distortion

Discussion in 'Amp Tech Center' started by Whale, Jan 19, 2021.

  1. Whale

    Whale TDPRI Member

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    Portland, Oregon
    I am having an issue with my GK MB115 combo and was wondering if any of the tech gurus might be able to offer assistance.

    I'm getting distortion out of the speaker and the headphones no matter what the volume, tone, or boost controls are set at. Also, the distortion happens if I plug in my i-Pad through the AUX port or use any of my basses through either the active or passive inputs. The distortion is worse for the low notes than for the higher notes.

    My fan is working and the issue happens as soon as I play the first note, so I don't think it's a heat related problem.

    I've attached an image of the schematic.

    Any help would be appreciated.
     

    Attached Files:

  2. Jon Snell

    Jon Snell Tele-Holic

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    Jurassic Coast. UK.
    Is it output distortion from the main amplifier or pre amp distortion.
    Take an output from the send socket to a known good amplifier and see if the distortion is gone or not.
    If it has gone, the power amp/speaker is at fault, if not the pre amp is at fault.
    Is it the Mk1 or Mk2 version?
     
    Last edited: Jan 19, 2021
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  3. Digital Larry

    Digital Larry Friend of Leo's

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    I had some weirdness with an infrequently used 200MB. Turned out the foam speaker surround had completely disintegrated in the 30 years I've owned it. Other debugging advice seems sound.
     
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  4. Whale

    Whale TDPRI Member

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    Apr 10, 2012
    Location:
    Portland, Oregon
    Is it output distortion from the main amplifier or pre amp distortion?

    I cannot tell whether the distortion is coming from the main amp or the preamp. The distortion is more pronounced on the lower notes and is very "flubby" in a way that would normally make me think the speaker is bad. That being said, I tested the speaker by connecting another amp and it is fine. Also, looking at the schematic, the output signal is running in parallel with speaker output and the headphones. Both outputs (headphone and speaker) are distorted so the source of the issue is coming from either the preamp or power board.

    Take an output from the send socket to a known good amplifier and see if the distortion is gone or not. If it has gone, the power amp/speaker is at fault, if not the pre amp is at fault.

    Sadly, there are no Send or Return sockets on this amp. There are three inputs (active and passive jacks, plus an Aux In) and three outputs (headphones, Direct Out [XLR], and, of course, the speaker). The issue occurs when I plug in an instrument and/or when I go through the Aux In jack, so I'm thinking the problem is downstream from the Aux jack. The only two items in the remaining circuit flow are the Master volume pot and the power board. The master volume works fine so I'm guessing I either have a bad solder joint somewhere in that area or the power board is wonky (or both).

    Is it the Mk1 or Mk2 version?

    I have the Mk1 version. The warranty period has long since lapsed and parts are nearly impossible to find. Hence my attempt at finding someone who might have encountered this issue before or is able to share insights with regard to the schematic.
     
    Last edited: Jan 19, 2021
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