SOLVED The gremlins return: LED-based switch pop

[p_wats]

I'm back with the same Arachnid build. Starting a new thread, as I had marked the other switch popping thread solved...because it was!

Everything seemed perfectly fine after replacing C15 (the 1u MLCC in series with the output), but when I got it all back together and boxed up today, I noticed the LED was behaving strangely, and the circuit was popping.

I've once again hit that troubleshooting point of chasing gremlins to the point of making things worse, so I'm here again to let cooler heads prevail.

Where I'm at:

• Everything works fine in terms of sounds, controls, etc.
• There is a very slight pop without the LED, but nothing I would complain about
• The pop gets very loud with the LED installed

I've already replaced C15 again, just in case, but it didn't help (last time the pop was present regardless of the LED). I've also tried different LEDs.

I was sure it was all working fine, as I kept testing while re-boxing it up and looking for the LED, so I'm curious if something shorted as I crammed it all in the box, but hard to tell.

Knowing my previous C15 was leaky, is it worth replacing the others, just in case?

 

[p_wats]

I'm measuring a small amount of DC on the output (5mv or so), but have none on my other Arachnids.

However, the really curious part I can't wrap my head around is my CLR (4.7k) seems to measure 9v on both sides! There is no continuity between the two sides according to my DMM and I've replaced the resistor, just to be sure. Same issue. This is definitely wrong.

There's nothing else nearby that measures 9v, so I'm confused.

 

[phi1]

It measures 9v on both sides of the CLR when the pedal is in bypass right? So the led cathode isn’t connected to ground... this is how it should be, there’s no where for the current to go, so it’s pulled to 9v on both sides or the R. With the pedal active (LED on), the voltage at LED/CLR junction should be something much less than 9v.

Sorry this doesn’t address the pop sound... just thought I’d mention it quickly.

 

[p_wats]

It measures 9v on both sides of the CLR when the pedal is in bypass right? So the led cathode isn’t connected to ground... this is how it should be, there’s no where for the current to go, so it’s pulled to 9v on both sides or the R. With the pedal active (LED on), the voltage at LED/CLR junction should be something much less than 9v.

Sorry this doesn’t address the pop sound... just thought I’d mention it quickly.

Argh! How did I miss that?

I removed the LED to test and got confused. This is what happens when I'm grasping at straws. Ha.

Now back to chasing the DC on the output I guess. Any ideas?

 

[phi1]

R11 is measuring 100k to ground? You could experiment with a lower value resistor there (just put a 10k in parallel) to see if that helps (and if tone is affected). Following an op amp stage (low output impedance), 10k might still sound fine.

Did you use relay bypass for this or normal 3PDT?

also, if you know it’s mostly the LEDs fault, there’s an old AMZ article specifically about LED popping, he recommends an extra R and C to make the LED turn on slower, I’ve never used it personally.

does it matter how long you wait between turning it on and off again? I think a while back I had an issue with pop, but only if i turned it off and on quickly. If I waited at least ten seconds or so, there wasn’t any pop. I don’t remember if I ever tracked that one down all the way or maybe decided to ignore since it was just a for fun build for me, and I didn’t need to turn it on and off quickly anyway.

 

[Nostradoomus]

Once in a blue moon stomping the switch a few times quickly gets rid of it too.

 

[p_wats]

R11 is measuring 100k to ground? You could experiment with a lower value resistor there (just put a 10k in parallel) to see if that helps (and if tone is affected). Following an op amp stage (low output impedance), 10k might still sound fine.

Did you use relay bypass for this or normal 3PDT?

also, if you know it’s mostly the LEDs fault, there’s an old AMZ article specifically about LED popping, he recommends an extra R and C to make the LED turn on slower, I’ve never used it personally.

does it matter how long you wait between turning it on and off again? I think a while back I had an issue with pop, but only if i turned it off and on quickly. If I waited at least ten seconds or so, there wasn’t any pop. I don’t remember if I ever tracked that one down all the way or maybe decided to ignore since it was just a for fun build for me, and I didn’t need to turn it on and off quickly anyway.

I've got the relay board installed, but this same board was used in another build that was not popping at all (and this build wasn't popping until a recent bout of troubleshooting).

R11 measures fine. So far jumpering a 10K in parallel hasn't done anything.

The pop is consistent no matter what I do (wait, stomp quickly, etc.).

I've read the AMZ article, before, but was hoping to find the gremlin in this build, as I swear it wasn't popping at one point. Alas.

Once in a blue moon stomping the switch a few times quickly gets rid of it too.

No such luck here, unfortunately.

 

[Nostradoomus]

I’d change C15 out for a film cap in your situation, this usually is the fix on all my popping pedals...on some such as the DuoPhase I change the 10uf electro out for a 1uf film and all is well. Not sure if that fucks with the output impedance or not but it sounds fine and doesn’t pop anymore so...yeah.

 

[p_wats]

I’d change C15 out for a film cap in your situation, this usually is the fix on all my popping pedals...on some such as the DuoPhase I change the 10uf electro out for a 1uf film and all is well. Not sure if that fucks with the output impedance or not but it sounds fine and doesn’t pop anymore so...yeah.

Ah, right. You had mentioned this, but all I did was sub it out for another MLCC---in your experience film vs MLCC in this position makes a difference?

 

[Nostradoomus]

I don’t know if it will but that’d be my next Hail Mary haha

 

[p_wats]

I don’t know if it will but that’d be my next Hail Mary haha

Worth a shot! Socketed C15, but no luck.

About to try the AMZ tricks and see if that helps.

 

[p_wats]

Wow. Tried the AMZ suggestion (extra resistor in series with the CLR and cap to ground) and still no change. This is a persistent gremlin.

Out of ideas. Might have to put this one away for a while.

 

[TheSin]

Wow. Tried the AMZ suggestion (extra resistor in series with the CLR and cap to ground) and still no change. This is a persistent gremlin.

Out of ideas. Might have to put this one away for a while.

Did you try removing the LED the see if it still pops? Ive usually found that its not the LED, in my cases at least...worth a shot though.

 

[p_wats]

Did you try removing the LED the see if it still pops? Ive usually found that its not the LED, in my cases at least...worth a shot though.

Yep, as per my initial post, without the LED there's no pop (at least it's not as audible).

Not sure if my eyes are playing tricks on me, but it seems like the LED is still dimly light when bypassed. I'll have to dig into that a bit.

 

[p_wats]

Well, I've been all over the place on this one. In the end, I tried absolutely everything, but ended up taking off the relay board (pin headers. Argh) and it worked without popping. I have confidence in this relay circuit (it's my favourite of all I've tried), so I tried a new one I have ready for something else and it worked fine.

Somewhere along the way I must have damaged something on the salvaged board, so I'll trouble shoot that another day or just pull off the useful parts and buy a new one.

Thanks again to everyone who helped with this thread (and the others)!