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Thread: 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!

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    Lifetime Member Enzo's Avatar
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    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.

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    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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    Lifetime Member Enzo's Avatar
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    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.

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    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.

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    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.

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    Thanks, I've sub'd in now that all pass diode test out of circuit and am still getting a dim light.

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

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    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.

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    Can you post the emitter and collector voltages for TR1 with channel off and on?

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    Sure,

    Off c-17.59v
    E-9.8mv

    On c-20mv
    E-5mv

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    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.

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    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.

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    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.

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    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

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    With TR1 off the board, there is no path for current through LED2, the red one, other than something outside the circuit. That can only mean things like contaminated board, or some unrelated circuit elements touching each other on the board somewhere.

    With TR1 still out, measure resistance to ground from the anode of LED2, the point where the collector of TR1 used to be. Should read open.

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    Resistance to ground from anode of LED2 measures as open.

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    My apology, I meant to say cathode, the end tied to the transistor. In other words looking for any resistive path to ground the LED current might take.

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    aww bummer, this case is closed, but will refer to this thread for notes next time. Thanks for all the replies.

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    JCM2000ís red light dim

    Quote Originally Posted by SYSDOA View Post
    aww bummer, this case is closed, but will refer to this thread for notes next time. Thanks for all the replies.
    I have a jcm 2000 100 watt doing that,did you figure it out? Thanks

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