BACK FROM REPAIR MAN ,HEADED TO THE TRASHCAN JCM 800

Discussion in 'The Workbench' started by Exotic, Nov 22, 2019.

  1. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    guitars are simple, tube amp circuits not so much...
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Are you able to swap out 1 tube at a time?
    Do you know how to plug / unplug the tube?
    Do you have the old tubes?
     
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  3. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    did you save the old ones? (you should have)

    Open the amp up, turn it on & w/ a pencil, tap the tubes. If 1 tube makes a bunch of noise, it's no good (new or not)...

    makes sure the speakers are hooked up properly, etc...
     
  4. SkyMonkey

    SkyMonkey Well-Known Member

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    Ahhh...Hung, Drawn and Quartered.

    Takes me back to the good old days!

    Hung (carefully) by the neck until nearly dead.
    Dragged (drawn) behind a horse over cobbled streets.
    Tied to a scaffold and belly cut open, in 2 slices, to quarter the flesh.
    Organs removed and burned before your eyes if still alive (great care taken to make sure that you are!).

    Too good for 'em!
     
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  5. myersbw

    myersbw Well-Known Member

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    Exotic, LA...now I get the price soaring...especially with brick & mortar buildings there. Almost save you to ship it somewhere else. ???
     
  6. Exotic

    Exotic Active Member

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    ok guys thank you so much for your help ,i'm done messing with it . i tried to tap on the pre amp tubes and it didn't work so i put everything back together and i'm taking it to another tech ,gonna look on the list and if not, it's going to sam ash music on hawthorne blvd in torrance ca .
    i will post another thread when it gets back to let everyone know what the second tech said . appreciate all your time and help very much .
     
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  7. Lance Chambers

    Lance Chambers Well-Known Member

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  8. Matthews Guitars

    Matthews Guitars Well-Known Member

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    If you're in the LA area you should not have a problem finding an amp tech who knows his way around a Marshall!
     
  9. RickyLee

    RickyLee Well-Known Member VIP Member

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    I will try to find some time to look at the schematic later today. If the reverb pot is affecting volume, then something around the reverb/fx loop branch is not right. Should not be that big an issue. Hopefully nothing wrong with the PCB traces.

    Are you experienced in soldering and working inside these things?
     
  10. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    the problem around here though, is everybody w/ a soldering iron & a multi-meter is a "tech"...

    finding the real deal among the fray, is the difficulty, as the OP is unfortunately finding out.
     
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  11. RickyLee

    RickyLee Well-Known Member VIP Member

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    Start out by checking R31 is correct at 56K and not jumpered or shorted.

    The reverb pot affects the volume to where it will make the amp go silent? Has anyone done any mods to this amp?

    What happens if you turn off reverb with footswitch?
     
  12. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    he doesn't have any experience w/ soldering gun/multi-meter inside a tube amp...
     
  13. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Quoting a post from another site...


    THE BEST AMP FOLKS I CAN THINK OF
    Los Angeles Area - Valley - Glendale - Burbank - Hollywood
    Tone Man - Don Butler
    Toneman does repairs, modifications, general maintenance and restorations to tube amps, guitars and effects
    pedals. With over 25 years of hands on experience as well as being a professional musician, Toneman knows
    not only how your equipment should sound, but how and why it works ! Don is some of the "braintrust" behind
    the great 65amps London amplifier.
    Don Butler/Toneman
    23040 Hilse Lane
    Newhall, Ca. 91321-1009
    Don Butler's Website
    Phone number: (661) 259 4544 . Please Call ONLY between 10am-6pm Tuesday thruSaturday-Pacfic Time Zone. Please do not
    call on Sundays or after hours. My shop is in my home so please respect my privacy during my non-business hours. Thank you!
    Your courtesy is greatly appreciated in this aspect.
    Cell Phone: (818) 486-9264. Call ONLY if you do not reach me at the above number during the above hours. Always try calling the
    other number first before calling this one. My cell phone reception at home is pretty poor.
    Thank you!

    Rock N' Roll Doctor - Steve Dikun
    Maybe no website but who cares, this is a strong recommendation. If you want any blueprinting done on an
    amp by me it better be in decent shape first. I don't have enough time to clean up mods that were not done
    right or broken things unless you are already a blueprinting client. This is the first guy I recommend to
    everybody in the So Cal area. If you have a amp problem or blew something up call Steve first.
    Burbank / Glendale CA. More at www.rockandrolldoctor.com
    818-956-5413
    Billy Yates - Contact Billy through TrueTone Music at 1-310-393-8232. Amazing guy and he even dors solid
    state! Address and other TrueTone info at www.truetonemusic.com
    NOTE - IF YOU HAVE AN AMP THAT IS GOING TO BE BLUEPRINTED BY ME IT HAS TO BE IN PROPER
    WORKING ORDER FIRST IF NOT ARRANGED PRIOR TO BLUEPRINTING APPOINTMENT. THE THREE
    GUYS ABOVE ARE THE ONES I RECOMMEND AND WILL WORK WITH MOST CLOSELY.
    Mark Cameron - Many of you already know Mark Cameron if you own a Bogner amplifier (super tech there in
    the past), or are a Marshall player in the So Cal Area. Mark is a killer player, and is known for his Marshall amp
    repair, restoration, and killer modifications. He is the "keeper of the Jose mod documents" and tricks, and is
    also a master of repair on Fender and other amps also. Please do not call or write him asking for
    schematics or prints on his mods or those of others. It has taken him many hours of work to compile
    his expertise. This expertise should be limited to his clients (Myles comment). Many times players are
    looking to try to scrape together the cash for one of the new great amps available today. In many cases, it may
    be faster and less expensive to find a Marshall on Ebay or at Guitar Center etc., and send it Mark's ways for a
    trip back to the top of it's tonal history. Mark can be reached at 1-818-500-0286. He is busy, and works very
    late hours, so if you don't get an answer, leave a message on the computerized voicemail. Mark can be
    reached via email at
    cameron_amps@yahoo.com
    Jim Wigle - for Jimmy's phone number, please email him first.
    amptechs@juno.com
    Jimmy is perhaps one of the best anywhere. Jimmy is the designer of the GT Solo-Single, the S-30 reverb, has
    many of his own designs to his credit, stomp boxes and gizzmos. He is the tech and tone to a LOT of well known
    artists. Jimmy is the head designer at GT, but has his own business as noted here. He is also quite a player,
    some of you may have seen and heard him with Ratt recently. Many of his clients can be seen on the GT
    website F&R list at http://www.groovetubes.com/f-n-r.cfm
     
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  14. anitoli

    anitoli Well-Known Member

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    Looking at the schematic the signal goes through FX loop return jack when nothing in inserted, these can get dirty and cause all kinds of cutting out issues. Place a footpedal jumper into the fx loop and try it.
     
  15. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    Think if i was over in the US then i would be very busy indeed. wish i could be there, if nothing to help you guys
     
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  16. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    OK
    Then you can come work in the garage.
     
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  17. MarshallDog

    MarshallDog Well-Known Member Premium Member

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    I had this exact same squeal happen last year on one of my amps. It was fine at low levels. Turn op the gain or volume and look out. This happened with or without a geetar plugged in. Changed out the preamp tubes and it was fine. Put the same faulty preamp tubes in another amp and it was fine. Sometimes its just the amp and the tubes...they dont get along.

    Turn the amp off if you are nervous about sticking your hands in there...its simple.
     
  18. Matthews Guitars

    Matthews Guitars Well-Known Member

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    If it's not a preamp tube squealing then I'd start looking at all the caps on the preamp board. Give each of them a tap with the eraser end of a pencil while the amp is running and squealing. If it's a capacitor then the squealing might change when that cap gets tapped.
     
  19. Exotic

    Exotic Active Member

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  20. anitoli

    anitoli Well-Known Member

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