Blues Jr smoking/burning resistor

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by moonweasel, May 18, 2019.

  1. moonweasel

    moonweasel Member

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    May 18, 2019
    Hey all,

    Blew something in my little amp and hoping someone might be able to help me figure out what’s wrong and how to go about fixing it.

    My amp:
    Fender Blues Jr
    Cream board
    Basic BillM mods done a couple years back (no issues until now)

    One day while playing, there was a loud pop and then smoke and a burning smell.

    I shut it off immediately, and when I took the back off, the only issue I could see was the larger of the two fuses burned out.

    So I replaced the fuse, but then when i turned it on again, the rR33 resistor over the second tube from the left started smoking (and I *think* possibly that tube was glowing brightly too?).

    The burning appears to be where the lower arm of the resistor marked R33 joins to the ribbon cable to that second tube.

    I have a multimeter and a soldering iron and can order parts — anyone have any idea where to start diagnosing or fixing what’s wrong?

    [​IMG]
    [​IMG]

     
  2. jthomas666

    jthomas666 Member

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    Location:
    Downeast Maine
    So... I am just a hobbyist and here is a guess- if you look around the web for a schematic, R33 is part of the plate voltage supply for the preamp tubes. Something over there probably started pulling too much current and thus the sizzle. I have one of these amps. The way that they are designed and constructed (i.e. they are fragile) there could be any number of things that could have caused a mechanical short. The tube sockets are mounted to a PC board for heaven's sake. Preamp tubes may have shorted internally... all kinds of possibilities.

    If it was my amp, I would replace the fuse and R33 again, pull the tubes out and start checking voltages.

    EDIT- Oh... and you need to get all the carbon off the PC board because that can conduct.
     
    pdf64 likes this.
  3. slider313

    slider313 Silver Supporting Member

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    You need to take that to a tech. The trace is burnt and replacing the resistor won't help.

    Did you use the correct value fuse when you replaced it?
     
    HotBluePlates and Darkness like this.
  4. PushedGlass

    PushedGlass Member

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    The nature of the burn strongly suggests that R33 is fine...but what the heck happened here?? R33 is the grid stopper for one of the EL84s; it's not a loaded-up resistor. But the joint next to it, for D11, looks almost as bad as the R33 joint. Either that was a bad solder joint that was suddenly asked to carry a lot of current or something bridged the tube-facing ends of R33 and D11. Something that may have done that is a shorted-out V4 (one of the EL84s) - is it okay??

    Confusion: You said "the larger of the two fuses" but I'm looking at the cream board schematic *and* Fender's service manual and I'm not seeing a second fuse. There's no "F2." There's no HT fuse in the power supply; there's no bias supply fuse.
     
  5. PushedGlass

    PushedGlass Member

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    You must have been looking at the "green board" schematic. The "cream board" is very different w.r.t. part numbering. I almost made the same mistake.
     
    Tone_Terrific and jthomas666 like this.
  6. pdf64

    pdf64 Member

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    This schematic maybe? https://el34world.com/charts/Schematics/files/Fender/Fender_blues_junior_rev_a.pdf
    If so, wow, that's an 'unfortunate' pcb layout (power tube control grid stopper squished between plate protection diode and screen grid resistor). When the amp gets overdriven there will be kV across a tiny area of crappy paxolin.
    Any discoloured board substrate will need grinding out and the componenets remounting somewhere else.
     
  7. Pete Cage

    Pete Cage Member

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    Exactly. They wrap the HV trace around it, just asking for an arc. Which it does. If you don't grind every bit of that carbon out of there, it'll just arc again.

    FWIW, I've never been a fan of those diodes, despite the apparent 'benefits.'
     
    VaughnC likes this.
  8. jthomas666

    jthomas666 Member

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    Oops. My bad. Sorry.
     
  9. VaughnC

    VaughnC Supporting Member

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    Yup, that was PCB accident waiting to happen...voltage is what voltage does!
     
  10. PushedGlass

    PushedGlass Member

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    "kV???"
     
  11. PushedGlass

    PushedGlass Member

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    [Eyes his BJr with suspicion forever after...]
    Seems like even just a little almost-invisible bit of spider silk could trigger an arc situation.
     

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