Marshall DSL 50 LED question

[AdmiralB]

I have a Marshall DSL 50 (that works fine) with an unusual issue - the red channel LED stays partially illuminated when the green channel is selected. It dims appreciably but doesn't totally turn off.

This is true whether the channels are selected via the footswith or the front-panel button. I've traced voltages as far as possible without pulling the front-panel PCB, and everything looks OK except that there is always a little voltage on the base of the MPSA13 associated with that LED - about 18V when it's on, about 0.2V when 'mostly' off.

Do I need to replace the transistor? I had a DSL 100 last week that I'm 99% sure did NOT do this.

[Ptron]

I've traced voltages as far as possible without pulling the front-panel PCB, and everything looks OK except that there is always a little voltage on the base of the MPSA13 associated with that LED - about 18V when it's on, about 0.2V when 'mostly' off.

That sounds funny to me. You're talking about the base voltage of TR1 in this schematic, right? http://www.drtube.com/schematics/mar...jcm2-61-02.pdf The MPSA13 is a darlington transistor. In that configuration it should really only have 1.5v or so on the base when full on.

[AdmiralB]

Yeah, it's TR1 and you're correct, I meant the collector, not the base.

[Ptron]

I think you reversed the votages for the on and "off" states, but anyway, what are the base voltages? Is the base going all the way to ground when switched off? If the base is going all the way to ground and there isn't some sort of short from the cathode of the LED to something then it pretty much has to be that the transistor is leaky.

[AdmiralB]

Thanks. I did jumper the base to ground and it didn't help. I'll replace the transistor.

[AdmiralB]

Fixed...problem was, the PCB mask doesn't match the schem. I've seen this on other Marshalls, and this one is admittedly a first- or second-year DSL.

The 100K resistor (which was actually 47K on this amp) shown between the top of the red LED and the base of TR2 is actually connected below the LED, between it and TR1's collector. A little board surgery fixed things.