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Bought a broken Mesa S.O.B. What’s this noise?

Discussion in 'Shock Brother's DIY Amps' started by Outlaws, Dec 1, 2018.

  1. Outlaws

    Outlaws Tele-Meister

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    I bought this as a non working amp and as one of two things...a future host for a new amp, or more importantly as something to learn more troubleshooting on. Its a Mesa Son of the Original Mesa/Boogie.

    It’s making a horrendous noise that cuts in and out. I don’t think I am getting a guitar signal either.

    There are not too many of these out there from what I understand, yet still not uncommon to find for sale. Mesa has no documentation for them. But I found a schematic I cannot verify the source of.

    It’s 2x 6L6.

    This doesn’t seem to have a presence knob though. Maybe it was an option.

    Is this “motor boating” or just some form of self oscillation?

     

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  2. Danjabellza

    Danjabellza Friend of Leo's

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    My egnater tweaker made a similar sound. It was a blown resistor and bad tubes.
     
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  3. jhundt

    jhundt Doctor of Teleocity

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    listen to 120 Hz; play it along with your recording.

    a 120 Hz hum is coming from the rectified power supply. (60 Hz x 2)

    filter caps

    A good friend of mine played one of those in a band with me - back in 1982! those filter caps are around 36 years old.
     
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  4. dmagalhaes

    dmagalhaes TDPRI Member

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    Killer amp when you get it working right. I had one with a EVM-12L and often wonder why I sold it.
     
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  5. Outlaws

    Outlaws Tele-Meister

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    Ya I’d say it’s a 120hz. Seems more viscious like a triangle almost. I imagine the tubes are toast though also as there is no sound from the input making it out. I’ll rip it open and assess the innards tomorrow maybe.
     
  6. Outlaws

    Outlaws Tele-Meister

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    This one has a 150w Black Shadow MS12. I am not well versed in anything outside Celestion stuff and the current C90 I think they have a Black Shadow sticker on.
     

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    Last edited: Dec 1, 2018
  7. dmagalhaes

    dmagalhaes TDPRI Member

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    The MS-12 is made by Eminence and I wouldn’t rush to swap it out. It’s about as close to an EV as you can get.
     
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  8. Dacious

    Dacious Poster Extraordinaire

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    The good news is your transformers are likely OK. Don't run it like that - you risk taking out a transformer leg.

    Caps, possibky tubes too. Definitely worth resurrection. Bloody loud amps.
     
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  9. Outlaws

    Outlaws Tele-Meister

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    I think am going to document the attempted resurrection process.
     

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  10. Outlaws

    Outlaws Tele-Meister

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    Any of this look not factory, or at least not of Mesas own work over time?

    I don’t see any tell tale blown resistors. The caps I wouldn’t even say are not bubbled or explodes either (realize that doesn’t = still good).
     

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  11. Outlaws

    Outlaws Tele-Meister

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    December of 1983. I guess this was ordered by Dick Lurie? Googling seems to show he had some kind of music school/shop in Cleveland.
     

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  12. CoyotesGator

    CoyotesGator Tele-Holic

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    Change the Ecaps.

    Use known good tubes.

    Someone loved it long time, you will too when its sorted.
     
  13. Outlaws

    Outlaws Tele-Meister

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    Last edited: Dec 5, 2018
  14. Outlaws

    Outlaws Tele-Meister

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    I was checking out this MS12 speaker with my Mark V head and I am really concerned. I have A/B’d a few 12” speakers in my day next to my 412 Mesa with V30s, and this speaker really seems like it’s missing something. I was under the impression it’s loud. Like EV12 loud. ie 101dB efficiency range. Aside from the cab being a 412, and this being a 112, it’s still lacking volume and bass. My 12” Weber Alnico is probably as loud as this and probably has as much low end or more.

    It measures out at 7ohm and doesn’t “sound” bad, aside from feeling like it seriously lacks low end and overal volume, but do speakers fade away or die in a manner that isn’t distorted? Any tests I can do? There is no way V30s and even my Greenback cab should be louder.
     
  15. Outlaws

    Outlaws Tele-Meister

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    Bias caps. Anything other than age that might trigger one of these to go? Bad tube?

    Replaced it with a 100v. 63v might be in spec for a 50v bias tap but these fit.

    All new power supply caps.
     

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    Last edited: Dec 10, 2018
  16. Platefire

    Platefire Friend of Leo's

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    I've done my share of trouble shooting. I wouldn't necessarily be going through the amp replacing everything.

    You know the speaker works. It may not be up to par, but works good enough to test the amp when you get a signal amplified through it. When you get it up and running then put a good speaker in it.

    First thing I would done would be to pull the chassis and the tubes to get a load off the circuit.
    Then turn it on and check all voltages at key points of the circuit. The voltage would be higher than on a schematic with the load off but would give you an idea if your getting voltages to all key points.

    Don't know if your familiar with a light bulb limiter but you could fire it up with one of those to protect you from any shorts. I would install some known good tubes in it and bring it up with the limiter and see if you got any sound. Good tube are key before you rework the whole amp.

    If you don't have any sound through speaker, then is the time to start trouble shooting. You can test the preamp separately with another amplifier. If you can tag on the preamp out safely(de-coupled from high DC Voltage) in the circuit with alligator clips to a guitar cord and get sound to another amp, you know the preamp is working. If you not getting guitar sound from the preamp, most likely the problem is there. If the preamp is working then you know it's in the power amp section. As the say, divide and conquer. Platefire
     
  17. stratclub

    stratclub Tele-Meister

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    The sound of money. Any PCB amp is miserable and expensive to fix.
     
  18. Platefire

    Platefire Friend of Leo's

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    Well Mesa is known to build a more sturdy pcb. IOW less likely to mess the PCB up by desoldering and soldering .
     
  19. Outlaws

    Outlaws Tele-Meister

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    Here are my Voltages with 12ax7 preamp tubes installed, no power tubes. I haven’t swapped the cathode bypass caps on the preamp board yet, research shows Mesa likes to use Tantalum so they might still be good.


    Wall 120vac

    Filter caps
    1st 220) 479v
    2nd 220) 238v
    1st 30) 475v
    2nd 30) 404v
    3rd 30) 369v

    V1
    1) 231v
    3) 2v
    6) 232v
    8) 2v

    V2
    1) 232v
    3) 2v
    6) 229v
    8) 2v

    V3
    1) 333v
    3/8) 3.5v limit on 0, 7v limit on 10
    6) 327v

    V4 & V5
    3) 476v
    4) 475v
    5) -48v

    Heaters
    6.8vac

    Other than V3 plates seeming to be at max spec for a 12ax7 I think it’s good enough to fire up with power tubes.
     
    Last edited: Dec 16, 2018
  20. jhundt

    jhundt Doctor of Teleocity

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    do you have a light-bulb limiter?
     
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