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Marshall JCM2000 DSL50 Channel LED Illumination Question

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  • Marshall JCM2000 DSL50 Channel LED Illumination Question

    Hello!

    I have this amplifier and it functions as it should but LED for the second channel is faintly on all the time. When it's engaged, it is fully bright, and the first channel 'clean' is completely turned off. I measured on a DMM that the second channel LED isn't getting as much DC when the first channel is engaged. Is this normal? Or, ideally, is there a way to get a few more VDC to the LED so it will fully shut off?

    Thanks in advance!

  • #2
    How would more voltage make the LED go off?

    One LED (Green LED1) shares current with two relays, so the voltage is naturally dropped before the LED. I don't know from channels, can we refer to the LEDs as LED1 and LED2 as on the schematic? Or at least RED and GREEN, which it has printed right next to them.

    If the red LED2 stays dimly lit, then either the transistor that drives it is leaky, or it is not being turned off fully. That is TR1. There is the panel button for channel switch, if it gets dirty you could have this. Also the Footswitch jack has cutout contacts, and if they get dirty, same thing. SO before we get too deep, clean and check teh FS jack and panel switch. MEASURE resistance across their contacts when closed. It SHOULD measure less than an ohm.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      I cleaned out the switches and tested them with dmm as good. Swapped out all of the mpsa13's. I swapped two of the relays with each other since I don't have any on the shelf, red LED still lit dim. I noted continuity all across the channel switch when in circuit and then as expected when out of circuit. Footswitch jack measured as expected in closed position.

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      • #4
        Did you just test for continuity, or did you test for resistance?

        I am not a fan of replacing lots of parts. The LED is lit by current through it, the current is controlled by the transistor. The transistor is controlled by its base. So the thing to do is check the voltages at the bases and see if it makes sense in the circuit.
        Education is what you're left with after you have forgotten what you have learned.

        Comment


        • #5
          Measuring resistance over the contacts for the footswitch jack and the channel switch in the closed position produces less than 1 ohm. Measuring the base on TR1 for DC produces 10 millivolts closed and 1.4vdc open.

          Comment


          • #6
            So the red Led (LED2) is the one that won't turn all the way off? If so, with the TR1 voltages you measured, TR1 must be leaky.
            "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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            • #7
              Thanks, I've sub'd in now that all pass diode test out of circuit and am still getting a dim light.

              Comment


              • #8
                By sub'd do you mean you replaced TR1?
                A diode test may not find leakage.
                "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

                Comment


                • #9
                  Correct, New MPSA13's have been replaced each with the same result. It seems like there would be variance in the degree out leakage, these are performing the same. I measure the same voltage in each instance.

                  Comment


                  • #10
                    Can you post the emitter and collector voltages for TR1 with channel off and on?
                    "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

                    Comment


                    • #11
                      Sure,

                      Off c-17.59v
                      E-9.8mv

                      On c-20mv
                      E-5mv

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                      • #12
                        I hate to say it, but I'd try it with TR1 removed. If the Led still won't go off, you may have some board leakage.
                        But if you see any residue around, clean it up with isopropyl and try again before removing TR1.
                        "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

                        Comment


                        • #13
                          I hate to say it but your idea is correct. The LED glows dim with TR1 removed. bummer. thanks for the help in getting to the route of it.

                          Comment


                          • #14
                            I mean no insult, but any chance the LED is simply passing light through from behind? I have been fooled like that before.

                            otherwise, I agree with a Q-tip and some isopropyl..COuldn't hurt.
                            Education is what you're left with after you have forgotten what you have learned.

                            Comment


                            • #15
                              No this is with the board pulled and a well lit area. My eyes are not fooling me. The board was cleaned prior to removal of TR1. Thanks again

                              Comment

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