Funky issue with MB Edgelord !RESOLVED!

[peccary]

Hello everyone,

I originally posted this over on the PedalPCB forum but this is the more appropriate place for it, so I hope you all don't mind. Here is what I said in my previous thread:

I'm having some trouble with this build and I'm hoping someone can point me in the right direction.

The pedal works well in Wah mode and Synth 1, but on Synth 2 I am getting very low volume as well as some scratchiness.

I went through and measured the voltages and was getting some wacky readings on the voltage regulator as well as a few pins on the LM 13700. I also adjusted the trim pot to the correct 7 volts.

On the regulator (LM317LZ) I'm getting I - 9.33 (fine) O - 7 (fine) and on the ground I am getting 5.76. I don't know what's going on with the ground because according to the docs it ought to be at 0, so this one is weirding me out.

On the LM 13700 most values are pretty close but there are a few that are pretty far out of whack: 1 reads 0.96 (should be 1.12), 5 reads 2.19 (should be 5.9), 7 reads 2.18 (should be 5.9), 10 reads 2.02 (should be 5.82), and 12 reads 2.07 (should be 5.83), 16 is 0.96 (should be 1.15).

I'm hoping that this is a bad regulator or chip that I can replace and that someone can tell me that based off this information, but if I need photos I can grab some. The 4558's and transistors all align somewhat well with the reported necessary voltages, though there is some variance. the ones I listed above were way off. I haven't had a chance to go through it with my probe yet, but I won't have time for that for a few days, at least, so I thought it couldn't hurt to ask here. It seems odd to me that it would work perfectly in two of the three modes, or at least that makes me think that it's something specific and that someone might be able to point to something based off this info.

Here's a photo of all of the values I got written out on the build docs that list the correct voltages if that's helpful: https://photos.app.goo.gl/RhLxVyPRG7RX2oo76

Here is the component side of the board: https://photos.app.goo.gl/jD3heK2awRG2jrDQ7

I know that I committed the cardinal sin of boxing it up, but it's no trouble to pull it out and get a photo of the bottom side if that's what is needed.

I did not sub any parts out on this build. I did have to roll a few of my own resistors, but that's it and all measured fine.

I guess I am wondering if the voltage regulator is faulty, or, if not, what else would give me a voltage reading on the ground pin of the regulator when there shouldn't be any (and why should that affect just the synth 2 mode, or are these somehow separate issues?). I don't want to start pulling parts out until I can get a little bit of a direction. Looking at the schematic I am having a tough time following/untangling the three modes, but I feel like I can fumble my way through it with my probe on Wednesday.

Thanks in advance for any advice or suggestions, I really appreciate it.

[madbean]

That's a typo in the docs on the LM317. I'll need to fix it. It should not be zero and what you measured is probably correct.

Your measurements on pins 1 and 16 of the LM13700 are most likely fine. But your readings on pins5/7 and 12/10 are significantly off. I'm not sure why this would be the case. Those are internally generated in the IC but I'd have to look at the datasheet again to remember how it works out.

Based on the description of the problem then I'd say something has gone wrong with that particular LM13700 or maybe something is up in the little distortion circuit used in the Synth settings (IC3A).

I don't have time ATM but later on today I'll look at your top pic and see if I find any wrong resistors, etc.

[peccary]

That's a typo in the docs on the LM317. I'll need to fix it. It should not be zero and what you measured is probably correct.

Your measurements on pins 1 and 16 of the LM13700 are most likely fine. But your readings on pins5/7 and 12/10 are significantly off. I'm not sure why this would be the case. Those are internally generated in the IC but I'd have to look at the datasheet again to remember how it works out.

Based on the description of the problem then I'd say something has gone wrong with that particular LM13700 or maybe something is up in the little distortion circuit used in the Synth settings (IC3A).

I don't have time ATM but later on today I'll look at your top pic and see if I find any wrong resistors, etc.

Thank you for such a quick reply, I really appreciate it.

Knowing that the regulator is fine helps quite a bit. I will try swapping out the LM 13700 when I get home today and see if that solves or changes anything since I have multiples (I assumed it was something with the regulator giving me those values which is why I didn't swap it out when I initially had the issue).

[peccary]

I replaced the LM 13700 and am still getting the same issue: the volume of Synth 2 drops dramatically.

Today I was playing through my mixer and there is nothing scratchy, just a significant volume drop (previously I was using a practice amp with the nasty power in my garage). The other modes work perfect as do the controls.

I didn't measure the voltage on the new IC since it was the same issue. I'm guessing I've got the wrong value resistor somewhere, I'll spend time tomorrow going through them. I should be able to probe it on Wednesday.

[peccary]

I spent some time today checking resistors and going through the circuit with my probe. I didn't discover the issue but I wanted to share what I found in the chance that someone can maybe point me in the right direction.

The resistors in the circuit around IC3 A are all good. I checked a good number of the other resistors as well and didn't see any issues but have more to look at.

With the probe I poked and prodded around quite a lot. I was having trouble starting at the beginning of the circuit so I started at the end and worked my way backwards in multiple modes. The main consistency was that Synth 2 was quiet everywhere. Every point I tested from the out back through IC3 A was quiet in Synth 2 mode except for R 29 and R15. The other modes were all loud.

I also swapped out IC3 with a different 4558 just in case but it made no difference.

Something that I found interesting/peculiar while probing was that R3 was loud in both Wah and Synth 1 mode but was quiet in Synth 2 mode. I found it odd because it is so close to the in, so it stuck out to me.

I'm hoping that what I did today was productive/helpful and that someone with more experience can point me where to look next. I'm determined to suss this one out.

[madbean]

I actually reviewed your build pic this morning and I also did not find any misplaced resistors. So, I think we are good there.

The base of Q1 getting quieter on the Synth2 mode is curious and troubling. There's no reason I can think of for that to happen. That is the input biasing resistor for Q1 base and R3 is the only contact point for the VD supply rail that powers it in the entire circuit. I did notice you used two resistors in series for the 9k1. Do you remember which ones you used? I can't make it out from your pic.

Try this instead: audio probe the output of the Q1 buffer in each mode and report what you find. That where the emitter of Q1 and R4 are in contact. You can find it by referring to the trace diagram in the build doc.

Other point to audio probe is IC3A pin1 again in both synth modes. If it's still quiet on that pin in Synth2 then we have narrowed down the problem area to the input buffer  or IC3.

[peccary]

I actually reviewed your build pic this morning and I also did not find any misplaced resistors. So, I think we are good there.

The base of Q1 getting quieter on the Synth2 mode is curious and troubling. There's no reason I can think of for that to happen. That is the input biasing resistor for Q1 base and R3 is the only contact point for the VD supply rail that powers it in the entire circuit. I did notice you used two resistors in series for the 9k1. Do you remember which ones you used? I can't make it out from your pic.

Try this instead: audio probe the output of the Q1 buffer in each mode and report what you find. That where the emitter of Q1 and R4 are in contact. You can find it by referring to the trace diagram in the build doc.

Other point to audio probe is IC3A pin1 again in both synth modes. If it's still quiet on that pin in Synth2 then we have narrowed down the problem area to the input buffer  or IC3.

Thank you for the info, and thanks for taking a look at the resistor values. I should be able to try this out in the next day or two and will report back with what I find.

The 9k1 resistor is actually two 18K resistors in parallel and a 100R resistor in series .

[madbean]

The 9k1 resistor is actually two 18K resistors in parallel and a 100R resistor in series .

In actual practice, you could have just used a 10k and be done with it. In this application, it's not that important to be that precise. Not that I would expect everyone to "just know that". But, bias voltage is often quite forgiving.

[peccary]

The 9k1 resistor is actually two 18K resistors in parallel and a 100R resistor in series .

In actual practice, you could have just used a 10k and be done with it. In this application, it's not that important to be that precise. Not that I would expect everyone to "just know that". But, bias voltage is often quite forgiving.

I nearly used a 10k but, like you said, I just wasn't secure enough to know it'd be okay. I have all of these tiny 1/8w resistors I've gotta use up so somehow, anyways 

[madbean]

I nearly used a 10k but, like you said, I just wasn't secure enough to know it'd be okay. I have all of these tiny 1/8w resistors I've gotta use up so somehow, anyways 

FWIW, always feel free to ask before you start to build. It's impossible to account for or document every potential question in these docs so some things I kind of leave for people to figure out. And, I do want to stress the importance of testing your build before boxing. Not a criticism - simply good advice from someone who has built many non working circuits!

[peccary]

I nearly used a 10k but, like you said, I just wasn't secure enough to know it'd be okay. I have all of these tiny 1/8w resistors I've gotta use up so somehow, anyways 

FWIW, always feel free to ask before you start to build. It's impossible to account for or document every potential question in these docs so some things I kind of leave for people to figure out. And, I do want to stress the importance of testing your build before boxing. Not a criticism - simply good advice from someone who has built many non working circuits!

Yeah, I for sure got cocky after not having any issues for a good run. I don't mind being called out on it 😉

[jimilee]

If you don't mind me jumping in here, do you have continuity between ground and the ground of the regulator?


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[madbean]

If you don't mind me jumping in here, do you have continuity between ground and the ground of the regulator?

The regulator doesn't have a ground point, as I mentioned earlier in the thread. I made a mistake in the voltage chart (IOW, I forgot to actually read the correct voltage on the regulator!)

[jimilee]

If you don't mind me jumping in here, do you have continuity between ground and the ground of the regulator?

The regulator doesn't have a ground point, as I mentioned earlier in the thread. I made a mistake in the voltage chart (IOW, I forgot to actually read the correct voltage on the regulator!)

My bad, it would help if I would have looked at the schematic, too.


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[gordo]

A sad statement on my level of addiction:  I just "happened" to have the 9.1's in my stash...