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  • #1

    Deluxe VM

    Fender Deluxe VM. First power supply dropping resistor goes open almost immediately upon applying B+. No tubes installed. Should I be looking beyond the first leg of the power supply? I can't find any problems in the output stage. Any help appreciated.
    Schematic can be found here:Amplifier Schematics
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  • #2
    R73?

    Is C39 shorted?
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Yes, Enzo, it is R73 that is cooking. I have subbed a new cap for C39 and lifted R69 and R68 with the same results. It sure points to the output stage don't you think? I can't find a thing wrong though.

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      • #4
        Found the problem! PCB jumper W39 had burned through a trace and was shorting R73 to the filament supply. Do you think this might explain the odd coincidence of two 12ax7 tubes failing simultaneously? I moved the offfending jumper to the top side of the PC board. The amp is working great now. Thanks for your advice, Enzo. I always value your opinion.

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        • #5
          yes, that would be a reasonable conclusion.

          Once you subbed the cap and lifted the R69 and no power tubes in the sockets, your troubleshooting then leaves the schematic.

          I am glad you found it, my next suggestion would have been start looking for arcs in the tube sockets, arcs on the pc board or little solder beads between two points. And those of course could have been many places on the amp.

          As to thinking about the output stage, the resistor serves the screen node on the B+, so it is after any OT issues, so other than screen pins on the sockets, the output would not be involved.

          All of which is now moot.
          Education is what you're left with after you have forgotten what you have learned.

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          • #6
            Hi, just want to thank both JVM and Enzo. Been beating my head against the wall for two days, same amp blows R68 this time though. I rebuilt the tube power supply to no effect. After trying some other dumb things I read this and gently pried up those twelve uninsulated jumper wires. Resistance at R68 returned to normal. I removed the wires and install insulated ones on the other side of the board and all is well.

            Can't thank you guys enough! I doubt I would have found this. I never did find any burn or indication of which one, guess I should have pried one wire at a time, but all is well now because of JVM's struggle.

            Thanks so much guys!!!!!

            David

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            • #7
              Glad that one of my posts actually helped somebody, Stratjeep. Enzo is the best and has helped and educated me on several occasions. Thanks to everyone here for sharing their insights.

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