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Thread: Marshall JCM2000 DSL50 Channel LED Illumination Question

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    Member SYSDOA's Avatar
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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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    Don't forget the joker g1's Avatar
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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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