Problem With Mesa F30

Discussion in 'The Workbench' started by John BNY, Mar 31, 2017.

  1. John BNY

    John BNY Well-Known Member

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    I have a Mesa F30, and I hope someone can help me diagnose a problem I'm having. I took the amp to a tech I use for my amps, and he thought the issue was a bad power tube. That obviously wasn't the issue, as I'm still having the same problem.

    Every once in a while, I'll play through the amp, and suddenly, the volume will slowly fizzle out, and I'll have nothing coming out of the amp. If I shut the amp off, wait a few minutes and turn it back on, it'll be fine. I don't think it's heat related, as I kept the amp on for hours, trying to recreate the problem, but I couldn't recreate it. It'll just randomly fizzle out. It's not a big deal, as I won't sell the amp, and I won't gig with it, but it's still annoying.

    I would appreciate your thoughts on what may be causing this. Thanks.
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    1.
    Resistors laying on top of circuit tracks (no space between resistor and circuit track):
    The bottom of the resistor can burn thru to the circuit track below / insulation failure.
    Look underneath resistors...you might find it.

    2. Burning between 2 circuit tracks: tracks are spaced too close together.
    Look for black spots on the board, between 2 tracks.
    Especially between plate track and grid or plate and cathode tracks.

    LOOK for: plate resistor laying on top of grid track. (in preamp usually)
    Cathode resistor laying on top of plate track.
    Grid resistor laying on top of plate track...etc...

    I have seen these issues in Boogie amps:
    especially in areas of high humidity, near the ocean, where it rains a lot.
    Honolulu, New Orleans, etc...

    Suspect (strongly) this is the cause of the problem.

    After lifting resistors off of tracks, cutting burned spot (if any) out of board (bypass): amp works normal again.
     
    Last edited: Apr 5, 2017
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  3. John BNY

    John BNY Well-Known Member

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    Thank you ampmadscientist. Will check that. Appreciate it.
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    It usually happens in the preamp.
    Use a lighted magnifier to carefully inspect the bottom of the resistor, where it touches the board.
    Look for small signs of soot, or white ash deposit, rising up from the bottom of the resistor. It will leave an ash deposit on the outside casing of the resistor...
    Lift up the resistor and you will see the burned spot on the bottom, where it's arcing to the circuit track underneath.

    Once that the fiberglass has turned brown / black - this is carbon on the board.
    This carbon is conductive, and cannot be "cleaned" off the fiberglass. High voltage will continue to burn across the carbon trace.

    (the carbon forms a resistor across the fiberglass...)

    The carbon must be CUT (scrape, grind, drill) off the fiberglass, and removed until the fiberglass is completely clean again.
    Then, the spot can be bypassed with a wire jumper (if you needed to remove any of the circuit tracks which are in the carbon trace area).

    After the carbon is removed, and the circuit tracks are bypassed w/ wire, the amp will work again normally.

    Well- don't feel too bad about it. Marshall JCM 2000 has the very same problem.
     
  5. Marshall71

    Marshall71 Member

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    Or replace V4 as a quick test.
    Also, plug in a cable in the effect send/return jacks of the effects loop.
     
  6. John BNY

    John BNY Well-Known Member

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    Thanks guys. I've been traveling, so haven't had a chance to open up the amp. I'm hoping to get to it this week.
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    Yes that is certainly true.
    The effects loop jacks (Internal Switches) can fail with age.
    When this internal switch fails, the audio path is broken.
    Plug a guitar cable between effects send jack and effects return jack to bypass these switches.
     
  8. John BNY

    John BNY Well-Known Member

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    Update on the problems with my Mesa F30. When my amp suddenly fizzles out, I check the power tubes, and one of them was redplating. When I shut the amp off for a while and turn it back on, I can't get it to redplate. I played it for hours trying to get it to redplate so that I can try switching the tube position to see if the redplating follows the tube, or whether it's the socket. But I can't seem to recreate the problem.

    The last time it redplated, I had a cable connected to the send and return inputs, so I don't think it's the effects loop jacks. Ampmadscientist, in the possible diagnosis you gave, would resistors lying on circuit tracks cause intermittent redplating like what I'm experiencing? Thanks for your help
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    Swap the power tubes into different sockets.
    if the problem follows the tube, it may be a bad tube.

    BUT usually the problem stays at the same socket, if it does...

    OK then replace the 2 coupling capacitors between the phase inverter plates and the power tube grids.
    The original caps are 400Volt, use 600V caps to replace them.

    It "could" be a bad connection causing bias to fail...but I usually see the coupling caps fail instead...

    Then retest...
    However the circuit board is difficult to work on, and you may need to have a professional do the repairs.
    The circuit board can be damaged by soldering...

    The capacitor has high voltage on one side, and bias voltage on the other side.
    When the cap starts to fail, high voltage will bleed into the bias voltage. This shuts off the bias and the tube red plates.

    The original caps 400V are under rated, and I have seen these fail before.
    Should have been built w/ 600V caps at the factory.
     
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  10. John BNY

    John BNY Well-Known Member

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    Thank you. I appreciate you taking the time to explain all this. The task now is trying to get the tubes to redplate again. Once it does, I'll see if the redplating follows the tube. If not, I'm going to take it to a tech with a printout of what you wrote. Thanks again.
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    It's a question of
    is it a bad tube, or is the coupling cap failing?

    And that is true, intermittent problem is the hardest to solve.
    The problem needs to occur...before you can pinpoint the actual cause.
     

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