JTM45 RI no sound... Freaking out

Discussion in 'The Workbench' started by Terry Cooper, Nov 24, 2021.

  1. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Those two blown resistors would explain why the amp isn’t working. What could cause a surge big enough to take them out, a blown rectifier tube?
     
  2. Tatzmann

    Tatzmann Well-Known Member

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    Starting to get a bad appetite about 1987x's
    and JTM45 Reissues.

    The tech should yank out the idiotic humbalance
    pot to prevent the next big brown spot on that
    circuitboard.
     
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  3. neikeel

    neikeel Well-Known Member

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    If you think where those two resistors are they are in the B+ power supply line, downstream of the output tubes/choke etc. Something downstream from both too. Most likely heavy current draw/short will be a failed preamp filter capacitor (the big blue one up top near choke). It could be a shorted preamp valve but unlikely.
    I doubt the rectifier or output tubes are at fault. You need to test/replace the can and both of those resistors. I would use 2w metal oxide (both are 10k).
     
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  4. XTRXTR

    XTRXTR Well-Known Member

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    Well you are taking it to a tech so this is for future reference. The fuses need to be Slo-Blo or "T" for Time Delayed, HT then would be T500mA, and the Mains is T3A, Though the schematic does not call for "T" on Mains which is odd for me to see.

    V2b cathode follower maybe arced, it was cold? It is worth having the diode from grid to cathode, maybe have the tech add that while he is at it.

    It is interesting to see the cathode follower reversed from what I see on 2204 (pin 6 gets the B+ and the 100kR goes to 1 which feeds grid pin 7, and 8 feeds the tone stack) I don't know about JTM45 much other than pictures and schematics. :) Not saying anything wrong, just not used to that layout.
     
    Last edited: Nov 26, 2021
  5. Spanngitter

    Spanngitter Active Member

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    R27/R14 blown => PI had a Plate/Grid short, either by a failing tube or somebody forced a valve incorrectly oriented into the socket.
    It could be also a failing 50uF Cap on the B+ node but then normally it would have popped.
    Replace R27/R12 with 10k/2W Metal Film, replace R14, check R18. Also I would review R31/32 as these are looking to me as well as being replaced in the past and not mounted properly. It is good to have them not sitting 100% on the PCB (same for the dropping resistors) but a small piece of glassfiber tubing (5mm) on the leads would help to support them better. At all normally a job for a tech who's knowing what he is doing and also can measure/check the ESR of your caps (unless you decide to throw them out anyway).
     
  6. Terry Cooper

    Terry Cooper Member Silver Supporting Member

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    Thanks and I will definitely give that a read...I don't think this Tim is the pick up guy,he does build some very fine Amps, http://www.shawaudio.com/
    He did amp repair around Nashville TN for years before that and has a sterling reputation..... And honestly that's as close as I am to knowing anyone that I could take it to, and I'm not crazy about just taking it to a stranger. If anyone knows a good tech in the Knoxville, Cookeville areas of East TN that would be super
     
  7. Terry Cooper

    Terry Cooper Member Silver Supporting Member

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    And after doing some checking like I say according to the serial number the amp is a 2018 will I spoke with the gentleman that I bought it from he said he on the amp for a little over a year well I've had it for a year and a half so I mean that would almost mean that he bought it new but he said he knew nothing about any mods or anything to it and then that was it I've not heard anything else from him which I'm not saying that he does I just the time frame doesn't really seems a little suspicious to me for some reason so not really sure what's going on and again my serial number ends with a number and not a letter my understanding is from like mid 90s on they were letter 10 numbers letter so I'm not sure but if anyone knows anything about that that would be awesome too if any direction that someone could give me on that. And everybody y'all have been amazing I can't think all of you enough for the help and advice and I really can't it's a great community we have here
     
  8. neikeel

    neikeel Well-Known Member

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    If both droppers have gone then the problem HAS to be downstream. But it had to be a short to ground- those resistors took some heat before failing or popping the fuse (was it T500?). If it had been just the one between screens and pi - yes problem with pi/v3. I would be surprised if the cathode follower tube (v2) failing short would do that as the plate resistor over the socket would have died??
    Please let us know what you find.
     
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  9. Eric'45

    Eric'45 Well-Known Member

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    That mess is terribly hard to read...
    If it helps you, my 2245 is also from 2018, and the serial number ends with '1'.
    since the last digit should stand for what voltage the amp is made for, i assume they swiched to numbers instead of letters sometime.
     
  10. Terry Cooper

    Terry Cooper Member Silver Supporting Member

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    I apologize about that,I was driving and talking into my phone when I sent that post. Your absolutely right it's hard to make heads or tails of.. the good thing is the local Marshall dealer has a tech that they have used for several years to work on their stuff. I did some asking around and there's a couple of other music stores that use the same guy so I gave him a call and we spoke at length on the phone and he's going to take care of this thing for me.That makes me feel a lot better.. I'm going to take it to him tomorrow and as soon as I find something out I will definitely post it to let y'all know again thanks to all of y'all for everything.
     
  11. Terry Cooper

    Terry Cooper Member Silver Supporting Member

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    While we're on that subject,once I get the amp sorted out and back right is there anything y'all can recommend having done to it while he has it? Things that help improve tone,lifespan anything like that,within reason I mean..I know that opens up a very large door
     
  12. Tatzmann

    Tatzmann Well-Known Member

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    The humbalance pots in the reissues go
    sometimes up in smoke. Can be replaced
    by fixed resistors of little higher wattage, problem
    is that the underlying source of these pot
    blowouts are not really understood.

    Some say bad powertubes.

    All i know is that older original Marshall
    didnt have these humbalancers and nothing
    went up flaming in those regularly.

    If this was my amp i would replace the
    circuitboard for a handsoldered turretboard.
     
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  13. Terry Cooper

    Terry Cooper Member Silver Supporting Member

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    I've considered that,I actually looked into one that metro amps offered that was supposed to be a somewhat direct replacement, obviously there were some things that had to be done to accommodate it..I heard some not so great,hell not even good,reviews about it and pushed off the idea. I would love to do something like that. Well I would be interested in having it done, I don't think it's something I want to take on myself. Again any advice you could give me on where to begin that process, would be appreciated
     
  14. neikeel

    neikeel Well-Known Member

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    One of my first ‘Marshall’ amp projects was to put a metro turret board in one of these.
    I don’t know how anyone can give the idea a bad review!
    How it turns out would be entirely due to the execution/implementation and the users choice of components and component values.
    If you like the tone as it is I suggest repairing it properly with good quality parts (as above I’d use flame proof not mf resistors in those spots).
    Spend money on good tubes.
    Then build a ptp clone with top notch parts and see what you like best.
     
  15. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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  16. Pete Farrington

    Pete Farrington Well-Known Member

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    Yay, well done to all in getting that website sorted out, thanks for the link :thumb:
     
  17. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    That's all down to Barbara Craven, Dudley`s wife, who is a web designer, living in British Columbia Canada
     
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  18. Gene Ballzz

    Gene Ballzz Well-Known Member

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    Thanks Ken for sharing that link again! We can never have too many references/links to that brilliant snapshot of amplifier history!
    Thanks Again!
    Gene
     

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