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Electronics Help Sought - Lack of Sound Through Homemade Fuzz Pedal

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I made myself a clone of a Green Russian Big Muff from this layout.  It sounds great when it works, but it has a weird problem with not sending any audio out sometimes.

In true bypass mode it works fine 100% of the time.

If I plug it in and press the foot switch, half the time it works perfectly and half the time there's just no sound.

When it's not working sometimes tapping it gently on the floor can get the audio going.  Once it's working, you can play it forever and it won't cut out.  However next time you click it off and back on, it may or may not give any sound.

Because of this I thought it might be a problem with the switch, but in either case the LED lights up so I'm guessing that means the switch is working? 

 

Can anyone suggest where I can look for the cause of this intermittent lack of sound?  I'm thinking it might be something mechanical, because of the tapping "solution", but nothing appears to be loose and I can't see any shorts anywhere.

Thanks in advance!

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Armchair diagnosis practically impossible, other than dry joints I dunno! Have you got a illuminated magnifier or microscope?

Good luck

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1 minute ago, KiOgon said:

Armchair diagnosis practically impossible, other than dry joints I dunno! Have you got a illuminated magnifier or microscope?

Good luck

All I have is a spy glass and a cheapo multimeter I'm afraid.

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as Kiogon says its almost impossible to diagnose.

It sounds like it could be a loose connection of some sort if you say tapping it can make it work. i would be looking for loose wires or bad solder joints first off. maybe look at whether it shorts on the enclosure. go back to basics and omit the footswitch if you think that is the issue. a couple of pictures might help people diagnose an obvious fault but to be honest it is very hard without looking at it. 

 

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That’s a verified layout, and if it works some of the time then it’s working all of the time. I’d hazard a guess that the problem is the off board components and/or wiring.

when you soldered the potentiometers were they set to minimum (fully counter clockwise), as soldering with them while they are anywhere but this setting makes it easier to damage the track.

Just because the led lights up doesn’t necessarily mean that the switch is fully working - just that the two lugs for the LED are. 
 

If bypass is ok then I’d have a look at the in and out of the board first, then check all the other off board wiring.

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37 minutes ago, paul_5 said:

when you soldered the potentiometers were they set to minimum (fully counter clockwise), as soldering with them while they are anywhere but this setting makes it easier to damage the track.

I didn't know to do that, thanks!  I've checked with the multimeter and I can measure resistance across all of the lugs on them.

50 minutes ago, paul_5 said:

Just because the led lights up doesn’t necessarily mean that the switch is fully working - just that the two lugs for the LED are. 
 

If bypass is ok then I’d have a look at the in and out of the board first, then check all the other off board wiring.

I got it into a non-working state, then unplugged it and checked continuity from the solder joint of the board input to the jack socket and it worked.  Likewise with the output jack socket to lug 2 of the Volume pot.  So I think that would prove the switch was ok?

 

Like you say, if it works sometimes then it seems I've at least got the layout as it ought to be.  Perhaps I should just reflow all the solder joints?

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When it decides to work, grab a pen or some other small non conductive, hard stick and gently tap the components and or solder joints, one at a time. With a bit of luck, you'll find your intermittent problem. You may also try a "freeze mist" spray, if you have one. 

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Sounds like the switch is dodgy. Many of the cheap chinese switches are prone to fail.

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