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Post subject: 5150 III 50W EL34 with Green/Blue Channel Switching Issue
Posted: Tue May 07, 2019 7:50 pm
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I took a job for a friend and am going in circles trying to figure out the fix. It's a 5150 III 50W EL34 amp. Essentially, the Green channel is seemingly bypassed for the Blue channel. The 'outter' controls just control the same channel, and the 'inner' (concentric) controls do nothing. When switched between Blue and Green, the gain and volume levels are the same. Seems like a short, or a switching issue, but I can't find it. The preamp tubes all test with healthy GM and no shorts, and they're not microphonic. Somebody said something about the concentric pots not being grounded properly, but I added solder and still no dice. All the other pots show continuity between the pot back and the shield.

Are there any ideas where I should look/test? I'm pretty thrifty with a soldering iron, but I'm not really an amp tech. I've done mods and simple troubleshooting, but this one has me spinning my wheels.


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Fender Play April 2019
Post subject: Re: 5150 III 50W EL34 with Green/Blue Channel Switching Issu
Posted: Wed May 15, 2019 3:27 am
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Hello !
A measurement, oscilloscope, would help you a lot.
( You can borrow the instrument from a colleague. )

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varga jano


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Post subject: Re: 5150 III 50W EL34 with Green/Blue Channel Switching Issu
Posted: Wed May 15, 2019 11:22 am
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Aspiring Musician
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Since you admit to not really being an amp tech, I would advise your friend to take it the nearest service center. If the amp is less than 5 years old, and your friend bought it new, it is still under warranty.

http://www.evhgear.com/dealers/service-centers/usa/

Sounds like a switching issue to me. The LEDs may switch, but that doesn't mean that the relays are switching.


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Post subject: Re: 5150 III 50W EL34 with Green/Blue Channel Switching Issu
Posted: Wed May 15, 2019 3:02 pm
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Aspiring Musician
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Joined: Thu Aug 07, 2014 3:25 pm
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From your description, it is working properly. Ch1 is both the inner knobs. Ch2 is both the outer knobs. If the outer ones are working, the inner ones shouldn't. Not sure about the volume difference.
It's a digitally controlled footswitch, so it usually works right or not at all. If the lights are changing properly, then it thinks its OK. The relays are usually reliable, so that leaves the relay driver transistors. You can check the drive voltages to most relays by measuring the voltage across the nearby diodes that are across the coils.


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