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FDP Forum / Amp Mods, Repairs, and Projects / Fender Blues Deluxe Channel Switching Issue

Hammond101
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So. Cal. USA

Oct 26th, 2017 09:49 AM   Edit   Profile   Print Topic   Search Topic

I replaced the switch (S1) and relay (R1) however the amp has a mind of its own and will cycle between clean and drive at random. Looking at the schem, this is an original Blues Deluxe, it could be an IC however I'm unsure as to which one.

Anybody have any experience with this? BTW plugging in a foot switch seems to stop the problem.

Peegoo
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Oct 26th, 2017 12:02 PM   Edit   Profile   Print Topic   Search Topic

Many amps use a 4049 hex inverter IC for channel switching. It's a fifty-cent jellybean part.

Look for one (or similar) in your schematic.

If it's in a socket, pull it and re-insert it to see if it's simply dirty contacts that are the problem.

Peegoo
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Oct 26th, 2017 12:06 PM   Edit   Profile   Print Topic   Search Topic

Also: if plugging in a footswitch stops the problem, that indicates the poor connection causing the intermittent issue could be a switch inside the footswitch jack.

Sticking a plug into the jack clears the problem because it opens the 'normally closed' switch that allows channel switching to be operated from the front panel when no footswitch is plugged in.

willie
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Too Near Atlanta GA

Amp Tech Emeritus
Oct 26th, 2017 12:31 PM   Edit   Profile   Print Topic   Search Topic

I recall that problem being commonly caused by a defective JFET...I think it was Q1 in the earlier model which is a J111.

w

Hammond101
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So. Cal. USA

Oct 26th, 2017 01:21 PM   Edit   Profile   Print Topic   Search Topic

Thanks guy! I will apply this new knowledge and see where it leads me.

Peegoo
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Oct 26th, 2017 01:57 PM   Edit   Profile   Print Topic   Search Topic

There ya go.

Peegoo: guessing.

willie: definitive answer!

Hammond101
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So. Cal. USA

Oct 26th, 2017 01:59 PM   Edit   Profile   Print Topic   Search Topic

I figured willie would chime in and sure enough he did! Awesome.

willie
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Too Near Atlanta GA

Amp Tech Emeritus
Oct 26th, 2017 02:49 PM   Edit   Profile   Print Topic   Search Topic

Hope that is the fix,...I found it to be quite common so I kept a large supply of J111, J112, J176 etc JFETs in stock. They are sometimes hard to check so I would usually just change them out and more often than not it would be the fix.

w

FDP Forum / Amp Mods, Repairs, and Projects / Fender Blues Deluxe Channel Switching Issue




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