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  • Popping Sound JCM900 4102

    Hi Folks.

    I'm getting a pop sound from my amp. The V7 LED lit up, fuse blown. I looked inside the chassis and the 1.5K that goes from pin 4 to 5 of V7 smoked. (what is this resistor called BTW?) I replaced the resistor and still had the pop. I got a new tube, still popping. I replaced V4, still pop.

    I had some volume fluctuations before the pop occurred.

    I'm an engineer/tech but I'm very unaware of tube amp repair.

    Any help? Thanks.

  • #2
    1k5 sounds like the grid-stopper...

    Is this the correct schematic? http://www.techofsound.com/links/Mar...ll/jmp50c2.gif

    Comment


    • #3
      Originally posted by MikeH View Post
      1k5 sounds like the grid-stopper...

      Is this the correct schematic? http://www.techofsound.com/links/Mar...ll/jmp50c2.gif

      Well the schematic is pretty darn close except there are violet wires instead of white going to r 31 and 32.
      The resistor that smoked comes from pin 5 ov V7 and goes to pin 5 of V6.

      the board identifier is jmp50c

      Comment


      • #4
        Pin 5 of an EL34 is the grid.
        You may want to mionitor the grid negative supply voltage after replacing the resistor.
        You want to see a steady dc voltage (-40 to -35 volts) with very little Volts ac ripple.
        The bias cap may be going bad.

        Comment


        • #5
          Originally posted by Jazz P Bass View Post
          Pin 5 of an EL34 is the grid.
          You may want to mionitor the grid negative supply voltage after replacing the resistor.
          You want to see a steady dc voltage (-40 to -35 volts) with very little Volts ac ripple.
          The bias cap may be going bad.
          Yeah - AFAIK, those resistors generally don't see a lot of juice unless a coupling cap from the previous stage is dumping DC, the bias supply has gone rouge, or a tube has some sort of short and is putting plate voltage on the grid. Either way, it wont bias properly if that resistor is damaged.

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          • #6
            Remove the output board (the one with the speaker jacks) and inspect for burning, arcing, check the low / high and impedance select switches..
            This amp tends to burn that output board, and it's an expected trouble spot.
            Once carbon forms on that fiberglass, it has to be cut or drilled out.

            Comment


            • #7
              Originally posted by soundguruman View Post
              Remove the output board (the one with the speaker jacks) and inspect for burning, arcing, check the low / high and impedance select switches..
              This amp tends to burn that output board, and it's an expected trouble spot.
              Once carbon forms on that fiberglass, it has to be cut or drilled out.
              I stuck a newer tube into V6 and it stopped popping. Looking at the old tube I see that it wasn't in too good of shape and was probably arcing. Ordered 2 more JJs to complete the set.

              Comment


              • #8
                Originally posted by norby3 View Post
                I stuck a newer tube into V6 and it stopped popping. Looking at the old tube I see that it wasn't in too good of shape and was probably arcing. Ordered 2 more JJs to complete the set.
                Inspect the board and socket for carbon traces. The carbon traces will make it arc again.
                Arcing is often the result of the wrong speaker impedance, bad speaker cable, a defect speaker.
                Last edited by soundguruman; 03-17-2012, 02:25 PM.

                Comment


                • #9
                  Originally posted by soundguruman View Post
                  Inspect the board and socket for carbon traces. The carbon traces will make it arc again.
                  Arcing is often the result of the wrong speaker impedance, bad speaker cable, a defect speaker.
                  I have 4 new tubes in it and I biased it, the tone is great, and it's now doing a little crackle and small pop sounds, which before led up to the big pop sounds. I inspected the hell out of the board and socket and saw nothing. Beats me, man.

                  Comment


                  • #10
                    Originally posted by norby3 View Post
                    I have 4 new tubes in it and I biased it, the tone is great, and it's now doing a little crackle and small pop sounds, which before led up to the big pop sounds. I inspected the hell out of the board and socket and saw nothing. Beats me, man.
                    Or it could be the dark stuff on the impedance switch. Maybe I shoulda paid more attention to that.
                    http://i569.photobucket.com/albums/s...tput_board.jpg

                    password:100w4102

                    Comment


                    • #11
                      Crackle and pop could be a lot of things; you could have some failing solders. Have you tried chopsticking the board?

                      Comment


                      • #12
                        Originally posted by norby3 View Post
                        Or it could be the dark stuff on the impedance switch. Maybe I shoulda paid more attention to that.
                        http://i569.photobucket.com/albums/s...tput_board.jpg

                        password:100w4102
                        It's true that those switches oxidize internally, and should be replaced now and again
                        the jacks on the board have switches that can also be cleaned

                        Comment


                        • #13
                          Originally posted by MikeH View Post
                          Crackle and pop could be a lot of things; you could have some failing solders. Have you tried chopsticking the board?
                          Believe it or not I've never heard the term 'chopsticking' in my life. Tap lightly with non-conductive long skinny thing?

                          Comment


                          • #14
                            Originally posted by norby3 View Post
                            Believe it or not I've never heard the term 'chopsticking' in my life. Tap lightly with non-conductive long skinny thing?
                            You can tap around on the boards and see if there's a loose solder connection
                            and a non conductive tool, other than your finger, is recommended, such as wood chop sticks...
                            you can try playing/ plugging audio into the effects return, try to narrow it down
                            into preamp or power amp?
                            Take out V1, V2, is the noise still happening?

                            Comment


                            • #15
                              Originally posted by soundguruman View Post
                              You can tap around on the boards and see if there's a loose solder connection
                              and a non conductive tool, other than your finger, is recommended, such as wood chop sticks...
                              you can try playing/ plugging audio into the effects return, try to narrow it down
                              into preamp or power amp?
                              Take out V1, V2, is the noise still happening?
                              I'm gonna observe it here for a couple days, it's so slight...before I open the thing up again. May try the valves.

                              Comment

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