FS4 just burned up HELP

Discussion in 'The Workbench' started by Rmmccoy71, Oct 17, 2021.

  1. Rmmccoy71

    Rmmccoy71 New Member

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    Was playing at church this morning, just bought a Two Notes Captor X, was play in through my 410H dimed out sounding sweet through the captor, halfway through sound check lost everything, came home to check fuses, got a real education on the whole FS3 & FS4 situation, FS4 fuses was smoked and charred the board, does anyone think it fixable, I will upload a pic, it did burn a trace, I figured wherever a trace was burnt I could replace with a wire but I’m a novice I have modded a few pedals so I don’t know, really bummed, I bought tha amp used a few years ago and haven’t really had the chance to use it much, guess it’s my fault maybe I shouldn’t have had it dimed out any suggestions
     
  2. Rmmccoy71

    Rmmccoy71 New Member

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    Can not upload pic
     
  3. South Park

    South Park Well-Known Member

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    You need to rule out the power tubes . Pull the tubes replace the fuze and power it up if the fuze blows it is not the tubes
     
  4. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Try emailing the pics to yourself, that’s an easy way to resize them. Copy them from the email and paste them into the reply window, that usually works for me. They need to be below 1 Meg I believe.

    But that may not be your problem as you may need a few more posts before you are allowed to upload pics.
     
  5. Matthews Guitars

    Matthews Guitars Well-Known Member

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    What is an FS4? I haven't heard of any amp by that name.
     
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  6. Rmmccoy71

    Rmmccoy71 New Member

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    Finally got the pic up loaded thanks Chris in LA 1A43E7BF-5EB4-4B9E-A6A7-2F270E347A01.jpeg
     
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  7. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Follow the damaged trace. What's on both ends of it? Post a picture of the whole board.
     
  8. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Damn. That looks toasty.
     
  9. thetragichero

    thetragichero Well-Known Member

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    10A fuse? i'm guessing those are on the filaments? probably dc heaters? can you check the bridge rectifier with a multimeter set to diode test?
     
  10. Spanngitter

    Spanngitter Active Member

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    These fuses should have been removed as per Marshall Service Bulletin and replaced by a solid wire.
    F4 is the filament fuse and it will develop contact resistivity over time and then burn the PCB.
    BTW: it looks to me like somebody already made a repair at this point not being aware of the requirement to remove the fuses...
     

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  11. XTRXTR

    XTRXTR Well-Known Member

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    The schematic shows what looks like a raised heater CT, the design seems unstable with all that transistor logic, and no Choke to help capture a current spike or keep it from blinking out with every change. Current drain as a tube in saturation heats to the point of max dissipation and blows. A situation where you have a flyback where a fuse is blown? Maybe that was what they saw that drove the change in the bulletin. Page two of the schematic:
    https://el34world.com/charts/Schematics/files/Marshall/Marshall_jvm410_sch.pdf
    Imagine a serious anode current drain and then pop, flyback, what happens to the heater fuses?

    Is the church electric wiring dependable? Were there any lights flickering? Who knows exactly.

    A few components look bad but they may not be as bad as it looks. As previously suggested, pull all tubes, plug into a light bulb current limiter (you can build one cheap), if it lights up bright stop. If it doesn't keep tubes out then test your heater voltage at the PT secondary, then test your HT voltage or your B+ rail. If that checks out you should be able to put humpty dumpty back together again, if this is too much bring it to a respected tech and have him look at it. The light bulb limiter could save you a bill for a tech to tell you.
     
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  12. Pete Farrington

    Pete Farrington Well-Known Member

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    As I see it, HT current can only get into the heater circuit if there’s a short in one of the valves. But there are high value resistors, R82 and 83, that then prevent any more than a few mA taking a return path. And both of those look ok. Unless the elevation decoupling cap C53 has shorted?
     
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  13. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    "If this is too much bring it to a respected tech and have him look at it".

    Been watching this thread and at last someone had said the sensible thing, as if this was posted by a Tech, well then it would not be posted at all.

    Its clear to me that the guy, and with total, respect, who posted it would not have a clue in what to do, so buddy do the right thing.
     
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  14. ibmorjamn

    ibmorjamn Well-Known Member

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    Someone put a big overrated fuse. The fuse should have burnt before the board.
     
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  15. Rmmccoy71

    Rmmccoy71 New Member

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    Thanks for comments guys, and your right I am not stupid I do a little electronics experience, but I do know enough to know when I’m in over my head, I have made contact with an amp guy and I’m taking to him this afternoon to see if it can be salvaged, I totally feel that it can be, and even made better, I have been wanting to learn about working on alps and possibly even building one of my own but this ain’t the one to start on, and if anything this experience and you guys have shown me this can be a lot complicated than modding a few crybabies, lol and maybe a career in amp repair is not for me, obviously I don’t even know where to start, maybe ITT TECH
     
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  16. Rmmccoy71

    Rmmccoy71 New Member

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    And I do do know enough to think that I agree the fuse should’ve gone first, otherwise why is it even there, but someone did mention problems with the fuse holder and I could see that not allowing all the current to get to the fuse to pop it
     
  17. thetragichero

    thetragichero Well-Known Member

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    unfortunately most of the stuff in guitar amps (even solid state) is EXTREMELY old tech and only very briefly touched on (as a foundation) for electrical/electronics engineers. there's pdfs floating around for a NEETS correspondence electronics course that'll start you off at the bare bones stuff one may think is dumb ('what is an atom?') and build up to passive components and then valve and transistor technology. that's a good foundation for the technology to add guitar tube amp-specific books by many folks who came before (merlin, kevin o'connor, etc). soldering experience with pedals is definitely helpful. i always suggest practicing on old home electronics (bet you've got an old vcr or crt tv gathering dust) because pcb soldering/desoldering is not nearly as forgiving as turret/eyelet/point-to-point construction you'll find on most diy builds. (as a plus old electronics are a great source of obscure components to build pedals with)
    but yes, recognizing that you may be beyond your depth before you plug in your iron and potentially fix your complicated amp more broken is an incredible skill. i learned the hard way way back when that jcm2000-series amps were NOT a good first amp to work on!
     
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  18. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    Good luck, and just remember one thing, start at the bottom and dont try to run before you can walk.

    What you are going to do will not be quick but well worth it in the long run, to get to the first run of the ladder will be if done properly about 5 years.

    Next week i will be in my 75th year and started when i was just 12, seems like just yesterday.
     
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  19. Rmmccoy71

    Rmmccoy71 New Member

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    We’ll guys you’ll be glad to know that I dropped my amp off at a tech yesterday and we spent some time together looking at it, both fuses were blown to hell, I left it with him he checked a few things out didn’t have any 10 amp fuses so he ordered some, I called him this morning and told him about the repair advisory for taking the fuses out, he checked all the tubes out and some other stuff(I don’t know what all) he tracked drown the repair advisory bulletin and performed that operation and I went to pick it up today and put the chassis back in the cab and she is good to go, he said it seemed like the scorched board was from before and all seems fine, he charged me 60 bucks, I told him I was so happy I wouldn’t come pick it up for less that a 100 bucks, he agreed, ( I guess I showed him), and she is nice and snuggled in back in the music room. He was so nice he didn’t even call me a dumbass for dime-it out, I think I’ll stay around 7-ish now just to be safe
    Thanks so much for the well informed advice and the good wishes you guys are awesome
     
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  20. XTRXTR

    XTRXTR Well-Known Member

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    Its always good news when your amp is fixed and no disaster. That's a good tech for sure. I would keep his contact information for future needs. Its good you are going to treat it better. If it is biased cooler you probably won't have any further problems. You should ask him what he biased the amp at so you can have that in your records. 60%-65%. What was the plate voltage, and current? The tubes were fine I guess?
     

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