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Blues Jr. burned trace

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  • Blues Jr. burned trace

    Click image for larger version

Name:	IMG_20140902_162823_753.jpg
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ID:	868841 I've got an early Blues Jr. in for early breakup and low power. I found that it burned the trace for the plate on V5. So far I've tested the tubes for shorts, checked the diode CR10, checked voltage at CP1 and CP3, checked R20 and R23, checked R24 and 25 and I haven't found anything off. It also had both of the sockets for the EL84's replaced with ceramic sockets in the last year or so. Had to be a bunch of current to take out that trace, but I can't see what caused it. What am I missing?
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  • #2
    Bad tube that shorts at high temperature? Did you replace the tubes sockets earlier?

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    • #3
      You are missing that it almost certainly was caused by an arc. And that was likely not caused in turn by a bad component. Looks to me like pin7, the plate arced over to the ground trace that serves pin 3, which goes right along side the pin 7.

      Only the grossest of shorts in a tube are detectable with a meter. Many won't show up until there are some hundreds of volts present.

      Probably doesn't matter, but what year is printed on the board? Must be 1998 or older.


      What was the proximal cause? Who knows? Maybe a little contamination on the board surface, maybe even a tiny bug stepped on the two surfaces at the same time. The bug evaporates, the arc is struck and the resulting plasma is a dead short to current.
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        Bill, the tube sockets were replaced a little over a year ago.

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        • #5
          Enzo, the main board is dated 1998. This amp gets used hard. The gentleman that owns it runs about 3-4 jams a week and this is one of the jam amps. Like you pointed out, those traces (for pin 7 and 3) run really close to begin with, and knowing how this is used, an arc seems totally probable. And now to try to resurrect this mess, the board is just about burned thru.

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          • #6
            Here's a shot of the bottom of that tube socket btwClick image for larger version

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            • #7
              Wow!

              Is it just me, or does it look to others like the arcing originated on pin7 on the underside of the socket and jumping between pins 6 and 8?? (arcing on the component side as opposed to the foil side). What's odd to me is usually when I've seen arcing on the foil side, the solder's usually blasted to hell, and those joints look fine.

              I'm guessing the bad EL84 tube went up in Murphy style (worst possible way).

              Like Enzo said though..'probably' doesn't really matter at this point. Just make sure to cut out any conductive sections of burnt pcb! And Good Luck! That's an ugly one.
              Start simple...then go deep!

              "EL84's are the bitches of guitar amp design." Chuck H

              "How could they know back in 1980-whatever that there'd come a time when it was easier to find the wreck of the Titanic than find another SAD1024?" -Mark Hammer

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              • #8
                Oh! I meant to ask, is that a part of a component/lead welded to the inside of pin7 in the socket pic? (not talking about the trace pad that came up with the socket...)

                Or perhaps just the angle of the pic...
                Start simple...then go deep!

                "EL84's are the bitches of guitar amp design." Chuck H

                "How could they know back in 1980-whatever that there'd come a time when it was easier to find the wreck of the Titanic than find another SAD1024?" -Mark Hammer

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                • #9
                  I think it's the angle of the pic, I didn't see anything up close and personal. I do see lots of brown marks at the 6-7-8 area. I believe you are all correct with the bad tube issue. As I started to dremel out the carbonized areas, I'm realizing that I may have to cut that whole section out and graft pieces in. Yes it's a mess and the Fender board is really thin.

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