Seamonk glitches depending upon guitar signal level

flemming

Well-known member
I'm doing my best to avoid all the meme worthy troubleshooting behaviors with this one :) In my SeaMonk Build Report thread I noted an minor issue that I wasn't quite sure about. I decide to revisit it to see if I could resolve it or at least understand why it's happening. Here's the scenario:

Drive: All the way up *** See Update Below
Bottom: All the way down *** See Update Below
Comp: All the way up *** See Update Below

The humbuckers on my guitar are pretty hot. Using the bridge pickup if I hit the strings hard the pedal will start glitching, This happens less often with the neck humbucker or if I split the coils, but you can still trigger it if you try hard enough. Rolling back the volume on the guitar reliably resolves it. Here's a sound sample of hitting a chord with normal force and then pounding on it, and eventually rolling back one of the knobs to make it go away:


When this happens if I roll off the Drive or Comp knobs or turn up the Bottom knob a bit the problem resolves. I started digging into this with an audio probe and here's what I found.

<Deleted, See the updated picture below>

Pots in Green have an impact on whether or not it starts glitching. When actively glitching the spots in red appear to have the issue, while the spots in blue do not. A couple interesting things I discovered on accident. If I short pins 6/7 of IC2 the problem goes away, and if I short pins 7/8 of IC2 the problem appears. Pin 8 is not shown in the schematic but it reads 16v . So to me (a relative noob) this looks like everywhere voltage is introduced (either via pots or shorting) is a control point for the problem. Buuuut, then I guess I don't understand how that Bottom pot comes into play. So here I am, I'm not quite sure where to go at this point. I random guess at this point is either I have a problematic cap or I could stand to use a different size one somewhere to compensated for the signal level that my guitar produces. I thought I'd see if anyone had any thoughts before I contemplate replacing random components. I swapped out both TL072s just because that was easy and no change. The fact that it can be controlled via subtle knob changes in various places doesn't indicate a connection issue (to me), but you can see some pictures of the board in this thread if you're curious.
 
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So revisiting my own question. A couple updates. I've attached a new result of using an audio probe. Not sure what changed or if I was just doing something stupid before, but it seems like I've found the spot where the last bit of clean signal is (once I trigger the problem).

SeaMonk.png


My prior comment about which pots in which positions made it happen and corrected it was wrong (because I'm a dumbass and labeled my enclosure to match the VFE merman and not the actual pots on the PCB). So the correct settings to trigger it are:

Drive: All the way up
Comp: All the way down
Bottom: All the way up

Adjusting any of those in the opposite direction make the problem go away. The same bits from above about which pins when shorted make it appear/go away still apply. And again this ONLY happens with the above settings, my guitar volume cranked up and hitting the strings extra hard. Clearly the answer is to just not do that, but I'm curious as to what could cause something like this in the first place. The pedal works fine with a slight rollback on the guitar volume, splitting the coils, etc.
 
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The fact that it can be controlled via subtle knob changes in various places doesn't indicate a connection issue (to me)

I took a look at your wiring and my first thought is that you might want to reflow some of your potentiometer joints. On the bottom of the board some of the joints look thirsty for more solder, and you lose the non-glitch signal at either a potentiometer (warm) or before a net connected to a potentiometer (10n->drive)

On first read of your thread yesterday I looked at your board and it looked more than fine to me, but after reading through this a couple more times, the behavior reminds me of times my pot connections were "ok" but not perfect. This produced results I had a hard time understanding as well.

If this isn't it, I'm up a creek - you've done a ton of good work trying to figure it out!
 
On first read of your thread yesterday I looked at your board and it looked more than fine to me, but after reading through this a couple more times, the behavior reminds me of times my pot connections were "ok" but not perfect. This produced results I had a hard time understanding as well.

If this isn't it, I'm up a creek - you've done a ton of good work trying to figure it out!

Thanks for giving it a look. I hit all the pots again and even gave them a little extra from the bottom side. I'm still of the impression that it has something to do with how DC is introduced into the signal path, but I don't know enough to know how the level from my guitar could play into that, or what exactly could be freaking out because of it. If I had a strat I may have never noticed the problem at all :)
 
Is R13 the correct value? Just glancing at the schematic my impression is that when the bottom is all the way up, it’s letting a lot of bass into the feedback loop. The amount is “limited” by R13, so I wonder if that resistor is too low of a value? A bit of a wild guess.
 
Is R13 the correct value? Just glancing at the schematic my impression is that when the bottom is all the way up, it’s letting a lot of bass into the feedback loop. The amount is “limited” by R13, so I wonder if that resistor is too low of a value? A bit of a wild guess.
Thanks for the response. Resistor values are good. I double checked them all back when I first built it because of a different problem. At this point I suppose there is a practical solution to the problem (just rollback a knob or two a tiny bit), but for learning purposes I'm interested in what theoretically could be happening. I find it interesting that guitar input level triggers it, and what exactly is the "it" that could be happening? I feel a little bit douchey for doing it, but maybe I'll try sending out the @Chuck D. Bones signal and see if he answers the call.
 
Hard to say from the clip, you would need to use an oscilloscope to really figure out what’s going on. I’m wondering if the hard clipping is chopping the wave so much to cause the sputter because the input is so large. Only the oscilloscope can tell us that.
 
Hard to say from the clip, you would need to use an oscilloscope to really figure out what’s going on. I’m wondering if the hard clipping is chopping the wave so much to cause the sputter because the input is so large. Only the oscilloscope can tell us that.
Maybe this is a good excuse for me to pickup a scope of some kind. I'll tell my wife that you said I needed one :) This is an interesting thought, but I'm not sure if that fits the symptoms. The stuttering tone continue even after guitar is silent. I was also able to accidentally create the problem when I shorted pins 7/8 of IC2, introducing 16v into the signal path. Once the pedal is in this state, only adjusting one of the previously mentioned knobs can stop it, the guitar level is no longer in play. I find the whole thing quite curious. Thanks for your suggestions!
 
Oh I see. So self oscillation is at the top of the list of suspects! Weird that the guitar input would trigger it but what you found is pretty solid proof. Also go ahead with that scope but don’t tell your wife who I am! :)
 
I think the next plan is to start checking around the coupling caps before and after the problem starts to see if I can find suspect DC leakage. But I also might just be throwing out random words that I've read without really knowing what I'm doing.
 
Alrighty, so I got myself one of those cheap digital scopes and figured it was time to dig this one back up again. Using a scope is entirely new to me, so if I'm making a rookie mistake somewhere feel free to post a meme about it. Here's a short video with the scope attached to the output while I'm playing a chord gently and then pounding on it:


Here's a picture of what the scope sees on the output:

PXL_20220212_161648543.jpg

With it is actively oscillating here is what is seen on the inputs/outputs of the opamps in the order of how they appear in the schematic

IC1.1
Pins 3, 2, and 1 - all show nothing
PXL_20220212_164832052.jpg

IC2.2
Pin 5
PXL_20220212_164854600.MP.jpg
Pin 6
PXL_20220212_164931541.MP (1).jpg
Pin 7
PXL_20220212_161625313.jpg

IC1.2
Pin 2
PXL_20220212_164944740.jpg
Pin 7
PXL_20220212_161546735.MP (1).jpg

IC2.1
Pin 6 - This one was bouncing up and down a bit since I didn't press hold before taking the picture and the photo actually caught it, so hence the artifacts in the image.
PXL_20220212_165011748.jpg
Pin 1
PXL_20220212_161556729.jpg

Just like before turning any of the Drive, Comp or Bottom knobs will make the oscillation go away. I find it curious that the oscillation survives the power cable being unplugged and plugged back in again. Does shorting the caps one at a time to see if there's one in particular that makes the problem go away make sense? Something else I discovered is that it takes less force when picking hard to trigger the oscillation the further up the neck I go. Like I said before I can avoid the problem with a little knob tweaking, but I'd like to understand what's going on here so I can be more smarter :)

Robert
 
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