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REPAIR: Marshall 50w Plexi 1967 - DOA

Discussion in 'Amp Input - Normal or Bright' started by vid1900, Nov 17, 2019.

  1. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    Nothing amazing here, but I figure if some of you have never seen an almost untouched Marshall Plexi, you would like it.

    Even the Pots appear to be original. (I'll clean them with tech spray before reassembling).

    IMG_20191116_194557190.jpg
     
    Last edited: Nov 18, 2019
  2. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    Yes, I'll blow all the dust out before reassembly...needs a few 4-40 nuts too.

    IMG_20191116_194601758.jpg
     
  3. clydethecat

    clydethecat Strat-Talker

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    The trem board is new territory for me, thanks for the clear pics.
     
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  4. clydethecat

    clydethecat Strat-Talker

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    The diodes bug me. Okay they've been there for 52 years but at an instinctual level they still look like an accident waiting to happen.
     
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  5. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    So after looking over the amp for any glaring wiring errors, it all looked solid.

    The only thing in the circuit that would interrupt our voltage after the Fuse is the Power Switch.

    Putting the meter on Continuity, I test the switch - and it's good!

    It can't be the transformer, because the Pilot would still light - and we got no pilot.....

    I put one lead of the meter (still on Continuity) on the switch, and the other I put on the HOT lead of the IEC Jack (remember the switch and the fuse are on the HOT lead) - NOTHING.

    Switch probe to the input of the Fuse Holder - NOTHING

    Switch probe to the ass end of the Fuse Holder - BEEP!

    Pull the fuse again, it's still good.

    Fuse holder spring in the cap is strong.

    Tug and bend the Fuse Holder in every direction, it's solid, but somehow broken internally?

    I flip the amp and using my monster desk lamp magnifier, I look down the fuse holder.

    On the bottom is some black something:


    10.jpg

    (sorry about the bad pic angle, it was taken through the magnifier.)

    I try vacuuming it out - nothing.

    I try getting it tangled in the fibers of a Q-tip - not budging.

    I stick a hex wrench in and can feel it crunch - I crunch it more.
     
    Last edited: Nov 18, 2019
  6. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    I pour out the now broken crunchies and they are black glass!

    IMG_20191116_195935162.jpg

    Shiny fine glass pieces.

    I vac out the hole.

    Examine for any other fragments with the magnifier.

    Reinstall the fuse.

    Check for continuity all the way to the switch - GOOD!

    I spray out the controls, vac out all the dust, zip tie the loose cap, install the 2 power resistors, and put a lock washer on the chassis ground.
     
  7. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    Blues guy is laying on the couch playing some sort of Neo-classical noodling as I come around the corner, his amp in tow.

    Guy: You couldn't fix it?
    V: It's fixed.
    G: OMG, my guy said it was dead, like dead, dead.
    V: Did you blow a fuse recently and the fuse broke?
    G: Nope, never had nothing with that amp for 2 years.
    V: But when was the last time you replaced a fuse?
    G: Never. I mean my guy was dicklng with it today, hope he didn't break no fuses!

    We plug the amp into a Randall 4x12 and the amps just roars. It has so much gain, it plays itself.

    G: Lordy! Did you hotrod this???
    V: No, it's exactly the same.
    G: Uh-hummm, you did something, cuz this thing is fncking red hot!
    V: I swear, I made no changes to any part of the circuit, well except the fuse holder.
    G: C'mon, I know this amp like the back of my hand. It my baby.
    V: The controls are cleaned out, fuse holder cleaned out, second speaker jack added, nothing that would change the gain.
    G: Well, if you ain't gonna tell me, you ain't gonna tell.
    V: I'd tell you, why would I keep it a secret? Maybe you got the Ohms set wrong for your speakers? I don't know.

    G: What do I owe you?
    V: $216 bucks
    G: So you mean $516 ?
    V: No, your total for today is $216
    G: I don want no special treatment.
    V: If you were Crapton, I'd charge you $2016
    G: Charge that MF $20,016 you hear me?
    V: Noted.
    G: You do that, ha haggggh! [pointing at me]

    So that was the oddball emergency Marshall repair.

    Not sure how it happened in the first place, but you guys can get an idea of the workflow when doing that kind of troubleshooting
     
  8. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    They do look like trouble, but they can't bend or vibrate into contact with anything else.

    If I was building that amp from scratch, I'd mount them to the side; but here I've got to respect the originality of the amp (as long as it's not an actual safety hazard like the Deathcap) .
     
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  9. nederemer

    nederemer Senior Stratmaster

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    But who wassss ittt!!!?!?!?
     
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  10. Triple Jim

    Triple Jim Senior Stratmaster

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    Thank you for taking the time and putting out all the effort to make this thread. It was informative and I enjoyed reading it. When I'm doing something like this, documenting it in a thread is the farthest thing from my mind. :)
     
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  11. Robins

    Robins Dr. von Loudster Strat-Talk Supporter

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    Great thread but I knew I never charge enough for repairing that old stuff.
    I am just enjoying too much getting those old things to work and being happy not having to overhaul that silicon **** they build these days.
    But for sure I had some really frustrating moments making those dinosaurs bark again.
    Many of those really fall apart - but used many decades on stage and in the studio, badly treated and kicked.

    All the best,
    Robin
     
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  12. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    I took a selfie with him and the amp, but when I asked if I could put it on the net, he said "That's how people get into trouble."

    So I took that as a soft no.

    He was playing some of the guitars I had refretted recently and was gushing over how smooth Stainless Frets bend (I honestly don't think he knew what stainless frets were before yesterday).

    I told him to let me re-fret one of his main guitars, and I'm certain he'll never go back to soft nickel.

    He said he was going to do just that, and asked how shipping would work.

    He said he was going to call me with some show passes, and that never happened, so I'm not holding my breath; lol.
     
  13. Ferpie

    Ferpie Senior Stratmaster

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    A moment for the ages...

    Should have gotten some audio recordings of the amp roaring for our enjoyment.
     
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  14. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    I take pics just as proof if someone says I scratched their guitar, or burned up their amp.

    The Plexi seemed like it would be interesting to ST because it was largely original. (and it's the ultimate Plexi: 50W, Tremolo, Master Volume)
     
  15. Stormy Monday

    Stormy Monday Dr. Stratster Strat-Talk Supporter

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    neat!
     
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  16. Triple Jim

    Triple Jim Senior Stratmaster

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    After doing two of my guitars with stainless, I really don't understand why anyone buys anything else.
     
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  17. Triple Jim

    Triple Jim Senior Stratmaster

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    That sounds like a very good reason. It also explains why I don't... they're usually my amps. :)
     
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  18. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    Had I not had a gun to my head, timewise, that would have been cool.

    Now, if I could have got a clip of him, all animated and surprised like he never heard his amp destroy before - that would be internet gold.

    That mofo can play!
     
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  19. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    It might change their **tone**
     
  20. sssstratsss

    sssstratsss Strat-Talker

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    I enjoyed this thread very much.
     
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