Repair Question - Why am I blowing fuses? | The Canadian Guitar Forum

Repair Question Why am I blowing fuses?

Discussion in 'Amp Building/Technical/Repair' started by Sneaky, Dec 16, 2019.

  1. Sneaky

    Sneaky

    Joined:
    Feb 14, 2006
    Location:
    Calgary
    A 500mA fuse in my JTM45 (Friedman modded Metro Amp) keeps blowing when I power it up. It’s the one between the turret board and the transformer like in this pic.

    [​IMG]

    Any ideas?
     
  2. KapnKrunch

    KapnKrunch Gold Member

    Joined:
    Jul 13, 2016
    Location:
    Yes
    Are you using slo-blo?
     
    Frenchy99 and jb welder like this.
  3. Sneaky

    Sneaky

    Joined:
    Feb 14, 2006
    Location:
    Calgary
    Not this one. The main fuse is slow blow and it is ok.
     
  4. jb welder

    jb welder

    Joined:
    Sep 13, 2010
    Location:
    Melonville, Canada
    Should be slo-blo.
    And try without power tubes.
     
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  5. Lincoln

    Lincoln Gold Member

    Joined:
    Jun 2, 2008
    Location:
    Fort Saskatchewan, Alberta
    Try powering it up with the tubes removed. See if it still blows
     
  6. Sneaky

    Sneaky

    Joined:
    Feb 14, 2006
    Location:
    Calgary
    Will try that. I’m on my last fuse so I hope it doesn’t blow.:(
    The original wasn’t slo blo so I just replaced what was in there. Amp worked fine then sat unused for 3-4 years and now this.
     
  7. Frenchy99

    Frenchy99

    Joined:
    Oct 15, 2016
    Location:
    North Pole
    Love owning a Variac just for these situations...

    +4 on the slow blo fuse without power tubes in...
     
  8. Sneaky

    Sneaky

    Joined:
    Feb 14, 2006
    Location:
    Calgary
  9. jb welder

    jb welder

    Joined:
    Sep 13, 2010
    Location:
    Melonville, Canada
    That should be right, though I can't 100% verify the size from the pic. 5x20mm is the smaller size commonly called 'mini', but yours looks like 6x32mm(1&1/4 inch) size.

    The 35A break means the current that is blowing the fuse. Your wall breaker is probably only 15 or 20A so no worries. :)
    If it blows and there is more than 35A blowing it, the cartridge may not be able to handle it.
     
    Sneaky likes this.
  10. DaddyDog

    DaddyDog Gold Member

    Joined:
    Apr 21, 2017
    Location:
    Mississauga, ON
    I'm no amp tech, but when that happened to me, it was a bad power transformer. The builder replaced it, and good ever since.
     
  11. dtsaudio

    dtsaudio

    Joined:
    Apr 15, 2009
    Location:
    Hamilton, ON
    Schematic shows it to be a slo-blo.
    Does it blow right away, or after the rectifier tube warms up?
     
    sulphur and jb welder like this.
  12. nonreverb

    nonreverb Gold Member

    Joined:
    Sep 19, 2006
    Location:
    Ottawa, ON
    I'm with jb welder and Lincoln....pull the power tubes and fire it up. They're usually the problem.
     
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  13. Sneaky

    Sneaky

    Joined:
    Feb 14, 2006
    Location:
    Calgary
    Immediately on switching it on. I’ll get a slo blo before I try it again. Probably won’t get around to it until after Christmas now, but I’ll come back to this thread with the results.
     
    greco likes this.
  14. dtsaudio

    dtsaudio

    Joined:
    Apr 15, 2009
    Location:
    Hamilton, ON
    Hate to be the bearer of bad news, but the likelihood of it being the power transformer just went up dramatically.
    Usually if it was a tube, it wouldn't show up until they warmed up. There is always a chance it is tubes, but transformer now seems more likely.
     
  15. Doug Gifford

    Doug Gifford

    Joined:
    Jun 8, 2019
    Location:
    Gananoque
    Check at your local Home Hardware for the fuse. They have fuses, though maybe not your slo-blo job.
     

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