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GK MB115 only works cranked...

Discussion in 'Amps and Cabs [BG]' started by Bluezmax, Dec 19, 2018.

  1. Bluezmax


    Dec 19, 2018
    Hi everybody !

    My friend bought a GK MB115 and he loves the amp but it just has this one issue :

    It will only work if the master volume is cranked all the way. There is no output at all until you reach the very end of the master volume knob, regardless of where the gain knob is.
    So the only way he can use it is by controlling his volume with the gain and have the master cranked all the way but that also triggers the thermal protection afer a while since the power section is always cranked...

    I understand this is a digital amp. It feels like a component isn't turning on or letting the signal through until it reaches a certain threshold on the power amp. The master volume knob isn't dirty or scratchy, it's just dead quiet until you reach about 90% of the range.

    I've already unplugged and re-plugged all the internal connectors and also sprayed and cleaned all the pots and switches.
    Everything works fine (all inputs and controls) except for this master volume issue.
    Not really sure what the next step to troubleshoot would be except maybe changing the master volume pot ???


  2. Not normal.

    If it is new return it for a replacement. May need the board replaced.
    jeff7bass likes this.
  3. mmbongo

    mmbongo Five Time World Champion Supporting Member

    Aug 5, 2009
    Definitely contact GK support, as mentioned that is not normal. And do not spray anything into the amp.

    And it's not digital :)
  4. Bluezmax


    Dec 19, 2018
    Appreciate the quick answers !
    munkeyfish, Amp is not new so returning to GK isn't really an option.
    mmbongo, I already cleaned the pots and switches out with contact/tuner cleaner, without spraying everything of course !
    I fix and mod tube amps all the time so I have experience with that but rarely work on solid state amp. I have a multimeter and most other tools. I assumed it was digital because of how recent it is... My mistake !
    I'll send an email to GK to see what they say.
    Any suggestions other than sending the amp to GK ?
  5. portpc


    May 31, 2011
    Good chance the pot wiper is hanging up internally, really changing the pot is the only way if that's the issue.
    If you are handy with such things you can disconnect the pot from the circuit & test the resistance with a meter.
    Nevada Pete and InhumanResource like this.
  6. FrenchBassQC

    FrenchBassQC Supporting Member

    Jul 13, 2011
    Gatineau QC CA
    Not normal, this comes from GK web site:

    Level and Master Volume settings: Set the
    master volume to 12 o'clock. (MB210, MB115,
    MB410 and MB212 only) While playing, turn
    the input gain up to the desired volume level.
    At this point you can experiment with different
    EQ, contour, horn and boost settings.
    bassbully likes this.
  7. All that needs to be said has been. Take the amp back.
  8. Call GK technical support they are very helpful.
    bassbully, Aqualung60 and pht2356 like this.
  9. pglaser01


    Mar 19, 2013
    St. Louis, MO
    If only I were that good....
    It’s a terribly basic question, but just to get it out of the way for more in depth answers.

    What kind of bass is he playing? Active?
  10. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    If it was me, I'd check the condition of the pot. But this kind of work should be done by a qualified tech. If it's under warranty, then it should be fixed under warranty.

    The pot could have a broken resistive element.
    BassmanPaul and portpc like this.
  11. Bob Clayton

    Bob Clayton valar morghulis Staff Member Supporting Member

    Aug 14, 2001
    Philly Suburbs
    doesn’t matter if the amp is new or not, they’ll still fix it, albiet not for free
    Aqualung60 and pht2356 like this.
  12. pht2356


    Apr 28, 2018
    Los Angeles
    I believe that you will be pleasantly surprised with assistance from gk customer support
  13. Zoolion


    Oct 31, 2014
    +1 on checking the pot if you are savvy with such things.
    portpc likes this.
  14. Artto


    Oct 3, 2018
    While all the above suggestions are your mostly likely course of action, consider that you may have an "intermittent" connection somewhere in the signal path that only works when the gain is turned up.

    Example: I was using a pair of Klipschorns (extremely efficient speaker systems) with a high power Crown power amp and pre amp. At the time Klipsch recommended using a fuse between the power amp and speaker. The system developed an issue where one of the speakers would not play unless I turned the pre amp volume up to a certain point (as in way to too loud). So naturally I assumed the problem was in the pre amp. It actually turned out to be a hairline crack in the speaker cable fuse holder. The crack was just enough to let some tension off the spring loaded fuse so that it would not make a connection - unless - the gain was turned up enough where the current/voltage could "jump the gap". Removed the fuse/fuse holder and everything worked fine. In your case this could be caused by faulty jack, internal jack wire on the amp or speaker cabinet etc.
  15. Ric5

    Ric5 Supporting Member Commercial User

    Jan 29, 2008
    I grow organic carrots and they are not for sale
    maybe you can clean the volume pot … but probably this is a worse problem that will get worse and not better

    This is why people should have 2 bass heads … because sometimes one will die
  16. Bluezmax


    Dec 19, 2018
    Thanks for all the replies.
    I did send an email to GK asking if they believe the pot replacement is the correct fix.
    I've already located the correct pot and am about to order it. Changing a circuit board mounted pot is child's play for me. I've had to redo entire burnt out boards from some tube amps, even redoing some of the burnt traces...
    I will take a closer look to look for any intermittent connections as Artto suggested as well.
    I really believe it's a damaged pot since there are no noises as you crank the master volume pot until you reach the 90% travel point. And when putting sound out, it is clear, undistorted and just sounds great.

    Thanks for all the help. I will give an update once the new pot is in.

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