Help Troubleshooting Issue

Discussion in 'Amps and Cabs [BG]' started by Quinten, Jul 29, 2020.

  1. Quinten


    Jul 29, 2020
    I run a MB Blackline 250 through a MB 4x10 cab for three years now and have been running into an issue lately that I haven’t experienced before. My amp turns on fine and all my cables seem to be in decent condition but I’m not getting any sound until right when the amp powers down. It seems to start working fine after being on for 5-10 minutes but I’ve never had any delay like that before. my drummer, who works with electronics professionally, can’t find any noticeable issues on the circuit board. Has anyone experienced this issue before and could offer some tips to get my rig working properly?
  2. BassmanPaul

    BassmanPaul Inactive

    Effects loop? If yes link Send and Return with a signal cable to exclude that from the troubleshooting.

    Pre-amp out/Power amp in? Same thing
  3. Wasnex


    Dec 25, 2011
    I haven't seen schematics for the Backline 250. I do have schematics for my 700RB. It has a circuit that senses the voltage in the output section and keeps the amp muted until the circuit stabilizes. Also, when the power button is turned off, the mute is instantly applied to keep the output from making a loud pop.

    Since your amp was made by GK it may have a similar protection circuit. The problem could be with the protection circuit itself, or the circuit may not be getting an indication that all is safe, so the mute is staying engaged. In other words something else may be wrong with the amp. The problem could also be after the protection circuit, as semiconductors control the mute function in the power amp.

    Here is what the protection circuit looks like in the 700RB

    On the right the tag labeled "Fault" senses the the status of the power amp. On the left you see a Tag labeled "Mute1". This sends a signal to mute the woofer power amp. The signal also loops through the woofer amp to the tweeter amp. At the Bottom is a line labeled "Red Protect". When the mute is engaged, a Red LED is lit.

    The chip (U1) top center of the image, is a LM555CN timer. This is the heart of the protection circuit.

    Here's a video that explains what's inside a 555 timer chip and how it can be used.
    Last edited: Aug 5, 2020
  4. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    In essence, If you do not understand electronics and how the amp is supposed to work, If you do not have the proper tools and knowledge to service the amp, you will be better off finding a QUALIFIED service tech to sort this out rather than creating bigger and more expensive issues.
    BassmanPaul, Wasnex and Reedt2000 like this.
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