hoping some experts can weigh in

Discussion in 'Let's Talk Vintage' started by bud light, Apr 6, 2020.

  1. bud light

    bud light New Member Premium Member

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    looking at a late 70's JMP 2203. Wanted to get some experts eyes on em for opinion. Looking for non-original items

    78_2.jpg 78board.jpg 78xfrmr.jpg 78xfrmr2.jpg
     
  2. South Park

    South Park Well-Known Member

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    The power tubes sockets look burnt . Pin arking
     
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  3. neikeel

    neikeel Well-Known Member

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    Looks totally stock and unmolested to me. I agree that there looks to have been arcing between anode (pin3) and filaments (pin2)
    What is the history? Looks like it has been ‘resting’ for a while?
    Highly likely that with two new McMurdo Octals, maybe (and I mean maybe ) new filtercaps tubes and a general check over pot lube it might be as good as new
     
  4. bud light

    bud light New Member Premium Member

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    Really appreciate you guys input. This one probably on the high end of used prices so want to do my diligence.
     
  5. South Park

    South Park Well-Known Member

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    It looks real good . You can pull tubes and power it up on a Varyac to re set the caps if you can.
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    A. You cannot operate the amp until the tube sockets have been changed. Burned sockets are a major problem.
    B. Speaker impedance is wrong, and will cause sockets to arc and burn again.
    C. Input jacks look close to dying. Better replace those.
    The amp is pretty clean other than the maintenance issues.
     
  7. bud light

    bud light New Member Premium Member

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    Thanks for the information @ampmadscientist! You're referring to impedence set to 4 ohms?
     
  8. Seanxk

    Seanxk Well-Known Member

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    There should be two of those selector plugs, looks like one is missing.
    You will only get correct speaker imp by using correct selector plug, which currently is in the mains selector. It's a stupid design as the correct imp is set at 90 degrees to where it's printed.

    If you put the mains selector plug in the speaker selector what does it say in the window of the plug.

    A picture would help.

    These plugs once lost and replaced with incorrect versions have destroyed more Marshall Transformers than any other amp I'm sure, but once you know which is the right point then all should be ok.
     
  9. bud light

    bud light New Member Premium Member

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    Apologies. i don't have the amp in front of me. Also a bit confused as to what you're describing. Will post a slightly magnified pic. Can you explain which knobs/inputs you're referring to? Much thanks. "window of the plug"??

    [​IMG]
     
  10. Seanxk

    Seanxk Well-Known Member

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    Ok all good, you have both plugs and they're correct.
     
  11. Matthews Guitars

    Matthews Guitars Well-Known Member

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    The midrange control is larger than the others. I don't know if that's original or not but I'm pointing it out, one of these pots is not like the others.

    That burn mark on a phenolic octal socket is bad news. That socket is not to be trusted. I recommend replacing all four with ceramics.

    I have a large stock of correct type ceramic sockets (about 70 of them) but the eye mounting plates would have to be sourced separately. However, they
    do have their snap rings with them.

    How long has it been since this amp was last run? I'm asking because the filter caps don't show any signs of overheating or failure, but they're still in
    their mid 40s. They're OLD. If it has been a few years since the amp was last run, then it needs to be brought up on a variac and follow the filter cap reforming procedure. If that is not done, then the caps might fail quickly upon power-up. (Letting old caps sit for years unpowered is the worst thing you can do to them.)

    I would completely dump the selector plugs. Bypass the power selector and have the transformer taps direct wired to specific speaker jacks and label each onf them. Or, replace the impedance selector with a high quality 3 position rotary switch that can handle the full output current load.


    I'd plan to recap it before long in any event.

    It's otherwise an all original, unmolested unit. Depending on the cosmetic condition of the cabinet and front and back panels, I might say it's worth up to around 2000 dollars but I'd try to buy it for less, of course.
     
  12. Matthews Guitars

    Matthews Guitars Well-Known Member

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    See the features on the back panel marked "selector" and "mains"? The one marked "mains" is your input voltage selector. It has a window in it showing the line voltage that is selected. It needs to match your actual line voltage. This type of selector becomes unreliable and the plug can get lost. It's best to have a qualified technician bypass it entirely, permanently set the amp for your local line voltage, and keep the selector in place for strictly cosmetic reasons.

    The "output" selector is your speaker impedance selector. All the same information applies. I recommend replacing it with a rotary switch. A GOOD one, rated to handle the full output power and current of the amp.
     
  13. bud light

    bud light New Member Premium Member

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    [​IMG] [​IMG]
    couple more pics
     
  14. bud light

    bud light New Member Premium Member

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    I should note that this amp was bought in Europe (if the mains selector is set wrong)? Thank you guys for all the detailed info!

    The new pic I posted shows 16 in the impedance selector window. Mains is set to 220/230 for Europe.
     
  15. bud light

    bud light New Member Premium Member

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    MG, et al -
    Thank you guys so much. Great advice. Anyone know a good Marshall tech in Austin, TX?
     
  16. ampmadscientist

    ampmadscientist Well-Known Member

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    It depends on what cabinet you have.
    If the socket burns, there must be something wrong with the impedance.
     
  17. bud light

    bud light New Member Premium Member

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    That's a good copy! Will have that checked.
     
  18. Matthews Guitars

    Matthews Guitars Well-Known Member

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    If the TUBE socket has burn damage, that's due to high voltage arcing. Cause: Almost always a tube that failed and did so in a big way.

    If the IMPEDANCE selector socket has burn damage, that's unusual and would indicate that it was connected to a cabinet that was basically shorted.

    Just being clear about that. Two totally diferent sockets are being talked about.

    I would recommend octal tube socket replacement (all four of them) with ceramics. By my way of thinking, phenolic (plastic) octal sockets have no business in any guitar amp.

    As for those impedance selectors....when the spring contacts lose their tension, they become unreliable. And that can lead to a dead output transformer,
    so I recommend replacing it with a rotary switch or bypassing and hooking the speaker taps directly to individual output jacks. One for the 16 ohm tap, two for the 8 ohm taps (as running two 16 ohm cabinets at the same type is the most common way to run a Marshall with two cabinets) and one for the 4 ohm tap.

    Then, just glue the selector plug into the socket. Use it as a cosmetic feature. Looks right, but not able to cause problems.
     
  19. bud light

    bud light New Member Premium Member

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    I bit on this. A little worried. Will have the tube sockets replaced. Maybe hard wire the mains switch. A rotary switch for the impedance selector. Not sure about the mid pot. If it works, will leave it be. And, will warn the tech about power up with variac.

    Can't thank you guts enough. Invaluable!
     
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  20. Seanxk

    Seanxk Well-Known Member

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    Make sure your tech knows to read what is viewed in the imp plug window corresponds to the pins that connect to the socket, he will wire up the new selector correctly then.
    If he doesn't get the plug then all will be wrong as whats written on the socket is not what corresponds.
    Or better still ask him to tighten up the connections on the back of the socket and use the old plug, you will have a spare plug anyway should you loose one.
     

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