Mesa LSS popping when switching channels

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by gregit, Feb 1, 2008.

  1. gregit

    gregit Member

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    I don't remember it being this pronounced. Any ideas on what to look for? I've done the obligatory preamp tube swapping with no success.

    Thanks!
     
  2. trdlasvegas

    trdlasvegas Member

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    Mesa's use a mute circuit to quiet the switching noise when changing channels. Basically they have FET's in 2 or 3 spots throughout the circuit which short to ground momentarily when they receive a mute pulse from the switch circuit. It's kinda of complex if you don't know what your looking for. I'd take it to Mesa Service Center as they know the inner workings of these circuits.

    -Tony
     
  3. donnyjaguar

    donnyjaguar Member

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    Gimme a schematic and I'll tell you where to start your search.
     
  4. gregit

    gregit Member

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    pm sent w/ schematic link
     
  5. trdlasvegas

    trdlasvegas Member

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    Gregit,
    I wasn't trying to dis you just explaining the circuit is a bit difficult but if your up for it here's where to look:

    Anytime a channel is switched a positive voltage will go down the "Mute Pulse Buss" which will feed the base of 6426M transistor causing the collector which is normally high to momemtarly switch to ground.

    This is the first place I would look to see that you are getting the "Mute Pulse" and that 6426M is switching to ground.

    If you get that, then the collector feeds a couple of FET's, J175M and J175RM which will short Source to Drain when they see the low on their Gates. If either of them are open then it won't mute the switching noise on the audio.

    There are no layout diagrams so you will have to search for those components but they will be marked on the PCB's

    Let me know if this helps.

    -Tony
     
  6. gregit

    gregit Member

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    I didn't get a dis vibe from your post Tony, it's all good.

    Thanks for the info and I'll let you know.

    Greg
     
  7. Red Planet

    Red Planet Member

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    A mute pulse? What the heck. I understand the concept but man what has this here amp world come to.
     
  8. Metalmike117

    Metalmike117 Member

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    I had the same issue with my Road King and other boogies. When you turn them off they store some sort of charge and when you turn it on and switch channels it pops.

    Now before you play you should cycle through the channels back and forth with the amp off standby before you play your set or record or whatever your doing, cycle through and it will go away. Now if it happens all the time, then you might have a bigger issue.
     
  9. trdlasvegas

    trdlasvegas Member

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    The first time I saw something like this was on a Peavey 5150. On The Peavey schematic it's called "Clamp", I guess because it clamps the audio signal to ground for a second while you switch channels. There may be more manufacturers doing this but you don't usually find out until your troubleshooting a problem related to this. My introduction to the Peavey clamp was because the FX return FET was shorted all the time and the amp wouldn't pass any signal.

    Certainly a long way Leo's first Fender designs...

    -Tony
     

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