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Sunn Concert Lead Burnt Resistor

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  • Sunn Concert Lead Burnt Resistor

    Hello!

    After having luck repairing a few peavey VTMs with help from here, I decided to try to go back and fix a busted Sunn Concert Lead (Silverface). I got this knowing it was having issues as a learning project, but haven't had much luck. The issue occurring is R322 keeps burning instantly. I've tested the transformer for shorts between secondaries and the primary, as well as case without finding anything. The output transistors were replaced and all, including Q5, seem to be fine testing with a multimeter. I'm kind of stuck on where to go next or how to test further with limited equipment. If anyone has any advice on how to move forward or some suspicions, I would greatly appreciate it!

    All carbon and wirewound resistors been replaced as well as electrolytic caps .

  • #2
    Do you have a speaker or load connected? If so, remove it and check for DC on the output.
    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    • #3
      ^^^^^^ Yes, that. I'll add- leave the speaker disconnected until you very that there is no DC on the output and until the amp is repaired.
      "Yeah, well, you know, that's just, like, your opinion, man."

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      • #4
        And make yourself a light bulb limiter
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          Q5 is shorted Base-Collector

          Disconnect its base from R322 and retest.

          Thereīs no other path for important current through R322

          And yes, make and use a lamp bulb limiter, running through a minefield blindfolded is not the best way to find them.
          Juan Manuel Fahey

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          • #6
            Sorry for a late response!

            I replaced Q5, as well as Q2 and hooked up to a dimmer to the same results. I'm assuming the short would have to be mechanical then, but I'm not certain where that would be as I believe all the interconnections are correct. The only missing thing is that the mounting brackets only have one screw, but I assume that would suffice temporarily as both screws of the collector are connected.

            Ive cleaned it up since taking the photo attached but connections are the same
            Attached Files

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            • #7
              Originally posted by Jynk_O View Post
              I replaced Q5, as well as Q2 and hooked up to a dimmer to the same results.
              I hope that doesn't mean you used a phase control dimmer instead of the bulb limiter?

              - Own Opinions Only -

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              • #8
                No no sorry, I did use the light bulb limiter, I'm just mixing my words up!

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                • #9
                  Are you maybe missing an insulator on one or more of the output transistors? Check transistor connections to ground and make sure nothing is grounded that shouldn't be.
                  "Yeah, well, you know, that's just, like, your opinion, man."

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                  • #10
                    The only missing thing is that the mounting brackets only have one screw, but I assume that would suffice temporarily as both screws of the collector are connected.
                    BOTH screws are NEEDED, I couldnīt care less about the "electrical" connection, those screws hold transistors tight against the heatsink.

                    And follow/check WHAT is connected to the burning resistor, high current enough to burn it must reach it somehow.

                    As shown on schematic, ONLY path is through a shorted BC junction.

                    If not, then there is some other path, find it.

                    Current flows through metal paths (track/wire/shorted Silicon junction), or low resistance resistor (a few ohms).

                    Please measure Volts across R322 and post results.

                    For sake of completeness,showing the only possible paths for current into R322
                    and that .....

                    Click image for larger version

Name:	R322.jpg
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                    * Red through R301 must pass through 910 ohm in series, not enough to burn R322

                    * Blue through C301: ... itīs a capacitor! ... only possible if shorted.

                    * Green through shorted Q5 Base-Collector junction, which I suggested.

                    none other.

                    UNLESS you disconnected T1 driver transformer Blue-Vio windings and connected them to some crazy place,

                    AGAIN: measure voltage across R322 or report smoke/sparks/red glow.
                    Juan Manuel Fahey

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