MarkBass LM800 - No sound, switch-related?

Discussion in 'Amps and Cabs [BG]' started by bdplaid, Nov 16, 2017.

  1. bdplaid

    bdplaid Supporting Member

    Aug 31, 2007
    The other day I say a few light sparks emerge from the top of my LM 800's switch as I turned it on. Now there is no sound coming from it; but, If I play while flipping the switch off and the amp is powering down, a bit of sound comes out . Thi makes me think there is nothing wrong with the output section, but rather that it's just not getting power via the switch, because the switch might have fried.

    Before I open it up, do you think there might be something amiss in the switch?


    PS: also, tell what to NOT touch inside there. I learned that the heard way fooling with a Genz Streamliner!
  2. For starters, stop doing that flip the switch thing.

    A proper diagnosis cannot be made UNTIL you open it up.
    Once open, if you can SAFELY bypass the power switch that will tell you if it is the switch.
    If you don't know how to SAFELY bypass the power switch, then take it to a qualified technician.
    If you have to ask if it is the switch, before you open it up, take it to a qualified technician.
    Unfortunately, you failed the first question on the qualified technician test.
    You don't want to fail question number two. It may be harmful to your health.
    bdplaid likes this.
  3. bdplaid

    bdplaid Supporting Member

    Aug 31, 2007
    Yeah, I can do that. thanks.
  4. BassmanPaul

    BassmanPaul Inactive

    Reading the OP, my suggestion would be leave the amp alone and take it to a tech before you make a $100 repair into a $500 one.
    Old Garage-Bander and bdplaid like this.
  5. bdplaid

    bdplaid Supporting Member

    Aug 31, 2007
    Yeah, but I like to futz around and this looks like a good learning experience. I need a source for the MB switch.

    Plus i haven't been bitten by 120 since I replaced all the outlets in my house a few years back. That 300V from the Genz streamliner was a little hard to take, tho... :)
  6. ddnidd1

    ddnidd1 Supporting Member

    It may be a learning experience, but from your question and your stated approach to the problem, not a good one.
  7. BassmanPaul

    BassmanPaul Inactive

    Getting a shock was the least thing on my mind when I wrote my post. “Futz” is not something you should do with a piece of good quality electronics. I stand by my recommendation: take the damn thing to a tech!
  8. basscooker

    basscooker Commercial User

    Apr 11, 2010
    Northern KY
    Cab fan, hobbyist
    Static electric shocks from rubbing your feet on the carpet can be 3000 volts. Very low current. Voltage is not the threat, current is.

    Amps have high current. Please don't kill yourself "futzing". Take it into a tech.
  9. bdplaid

    bdplaid Supporting Member

    Aug 31, 2007
    OK, enough fooling around, I'm not going to fix it, I'll probably open it up to have a look but nothing more.
    But really my question is simple, and I'm not asking how to fix it:

    Can it be so that the single on/off switch provides electricity to the preamp section and the power amp section separately? I'm not really looking for how to fix it myself, Just want to understand what went wrong, which seems clearly a bad switch.

    In my head, having not opened the amp, the switch would control the flow of electricity to a single item, perhaps a relay, that would then provide power to the pre and power sections. Just want to understand that the single switch might control the electricity flow to two separate circuits directly, since when I turn the amp off, the power section comes on for just a moment.

    Not sure I'm being clear... bear with me.

  10. ddnidd1

    ddnidd1 Supporting Member

    Instead of guessing - take it to a qualified tech and get the right answer and a working amp.
    BassmanPaul likes this.
  11. bdplaid

    bdplaid Supporting Member

    Aug 31, 2007
    I wanted to follow up that I took it to a local shop, well-regarded and with MarkBass experience but not factory authorized. They found two blown resistors and cleaned it all up for about $150. The amp sounds great. Money well spent, and a shout-out to Diversified Audio in Tampa!

    As an aside, they said they don't see many MarkBass amps, they thought speaking to their reliability.
    agedhorse likes this.