# VFE Merman - Oscillation (?) motorboating at higher gain



## PangeaDestructor (May 7, 2022)

Hi, I have a VFE Merman that I built using the actual SPS board from VFE, just boxed it last night. I'm pretty happy with it, but when I finally had a chance to really crank it today I noticed that it starts motorboating the last 20% or so. I don't really plan to use it at full gain, but still to have the option and a properly functioning pedal. Also, it seems somehow related to the Compression knob - if I adjust compression clockwise, it can prevent the motorboating, but roll back the comp while gain is maxed, and it comes right back.

I was trying to attach a video but the file's too large, so here's an Imgur link with audio. I can post some pics shortly if nothing jumps out, there's a chance I knocked loose the circled cap on the first attachment but have to pull the board and check the bottom.

I just like saying "motorboating"


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## flemming (May 7, 2022)

I feel your pain.  I think it may be the same thing I posted a while back with my SeaMonk build:





__





						Seamonk glitches depending upon guitar signal level
					

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...




					forum.pedalpcb.com
				




I posted some pictures of what I saw with a scope but it didn't go anywhere.


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## PangeaDestructor (May 7, 2022)

flemming said:


> I feel your pain.  I think it may be the same thing I posted a while back with my SeaMonk build:
> 
> 
> 
> ...


IT'S THE SAME EXACT THING. So Frustrating.


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## Robert (May 9, 2022)

Hmm, that's interesting.   

Since the official VFE board is doing the same thing I suppose I can stop poking around at the Sea Monk layout for a solution.

Where did you guys get your TL072's?


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## PangeaDestructor (May 9, 2022)

Robert said:


> Hmm, that's interesting.
> 
> Since the official VFE board is doing the same thing I suppose I can stop poking around at the Sea Monk layout for a solution.
> 
> Where did you guys get your TL072's?


Mine are from Tayda. However, I have an interesting update. I fixed the problem by adjusting a voltage trimmer on the SPS switching board that VFE provided with the PCB. It took a few minutes, but basically adjusting the input voltage down got rid of the issue. He goes over the whole assembly here (



) but uses this function at the very end, and there's another video on the SPS schematic as well. Not sure if this is helpful, but there might be something to it for the Sea Monk.


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## Robert (May 9, 2022)

How low did you have to turn down the voltage?


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## PangeaDestructor (May 9, 2022)

Robert said:


> How low did you have to turn down the voltage?


The trimmer comes stock at noon, so following what he does on the video I turned it to all the way clockwise to 3:00 - no motorboating, but it sounded like shit and not much drive - then went counter-clockwise until I hit the skipping sound, then dialed it back clockwise slowly until I had a good drive sound with none of the bad stuff. I thought it was just to manage the noise floor, but apparently it has other important functions.

Here's the SPS circuit walkthrough:
https://www.youtube.com/ watch?v=sWtLXhE6kfc


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## flemming (May 10, 2022)

Interesting.  To answer the earlier question, I had the same problem with TL072s from Tayda as well as ones directly from TI.  The charge pump setup with the SPS is obviously different than what's in the SeaMonk so I don't know that trying to dial in the voltage to reduce noise applies here, but there could be something to that in regards to preventing the oscillation.  @PedalPCB what would be the easiest way to tinker around with this idea to see if it makes a difference on the SeaMonk?


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## Robert (May 10, 2022)

It's a little puzzling because the original Merman didn't use the SPS module, the charge pump was on the audio board with no adjustments.

I have one concern that I want to look into with the Sea Monk, but that wouldn't explain why the Merman PCB would have a similar issue... unless the two issues are similar but unrelated.


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## ianmarks (May 10, 2022)

Robert said:


> It's a little puzzling because the original Merman didn't use the SPS module, the charge pump was on the audio board with no adjustments.
> 
> I have one concern that I want to look into with the Sea Monk, but that wouldn't explain why the Merman PCB would have a similar issue... unless the two issues are similar but unrelated.


Yeah, the sps board threw me for a loop, too. I think I mentioned this in someone's dragon thread. It is not a fine tune control for noise and goes from 0-18 regardless of what input voltage you are throwing at it. Sweet spot is typically between 10 and 12v. Some get noisy above 12 and of course don't function right at lower settings. It's tough because unless you know what it is supposed to sound like you are kind of guessing and tuning to taste. You can probe at pads on the switching board but be careful, I jumped one of them with the probe accidentally and it fried the converter. The most current vfe power board design goes back to the original way with through hole components and does not use the inductor based voltage converter system.


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## benny_profane (May 10, 2022)

ianmarks said:


> The most current vfe power board design goes back to the original way with through hole components and does not use the inductor based voltage converter system.


The most current vfe power board is the SPS system. Are you referring to the madbean projects?


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## ianmarks (May 10, 2022)

No, I am refering to the truesoft sps system. The one from madbean is truesoft standard, the one above in the video is sps system then there is the truesoft sps that looks like the image below. Admittedly, I may have got confused on the release order.


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## PangeaDestructor (May 10, 2022)

ianmarks said:


> No, I am refering to the truesoft sps system. The one from madbean is truesoft standard, the one above in the video is sps system then there is the truesoft sps that looks like the image below. Admittedly, I may have got confused on the release order.
> 
> View attachment 26171


This is the system that came with the Merman PCB, although it's all SMD rather than through-hole components.


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## ianmarks (May 10, 2022)

PangeaDestructor said:


> This is the system that came with the Merman PCB, although it's all SMD rather than through-hole components.



I don't think so. The one above is like the original side mount boards. The SMD is an inductor based design. Either will work, technically, but they are different designs. SMD is inductor based to create the separate +/- voltages while the other two use a voltage inverter IC (7662)


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## benny_profane (May 10, 2022)

ianmarks said:


> No, I am refering to the truesoft sps system. The one from madbean is truesoft standard, the one above in the video is sps system then there is the truesoft sps that looks like the image below. Admittedly, I may have got confused on the release order.


Gotcha. We’re on the same page now. I’m not sure when that one was released—I just know it from the video and the design files. The last batch of pedals Peter made and the pcbs were (to the best of my knowledge) all with the SMD SPS board. Have you seen that board used in anything/built with it?


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## PangeaDestructor (May 10, 2022)

ianmarks said:


> I don't think so. The one above is like the original side mount boards. The SMD is an inductor based design. Either will work, technically, but they are different designs. SMD is inductor based to create the separate +/- voltages while the other two use a voltage inverter IC (7662)


You're probably right, actually.


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## ianmarks (May 11, 2022)

Havent seen one in the wild but would imagine it would be in the latest revision of the choral reef if it was (apparently it had issues with the smt sps) I have wanted to build one of those after the latest version used two parallel delay lines. Wish there was a demo of it.


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## benny_profane (May 11, 2022)

ianmarks said:


> Havent seen one in the wild but would imagine it would be in the latest revision of the choral reef if it was (apparently it had issues with the smt sps) I have wanted to build one of those after the latest version used two parallel delay lines. Wish there was a demo of it.


I have one build by Peter. I think I honestly prefer the earlier edition with the MN3008. 

I thought the Tractor Beam was the one that had issues? From the VFE website:


> *The Tractor Beam will come with an empty TrueSoft Bypass PCB that uses the TC7662B charge pump. The LFO in the Tractor Beam makes a quiet clicking noise when used with the SPS System. You'll also receive a pre-programmed 12F509 switching microcontroller.


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## ianmarks (May 11, 2022)

Gotcha. Will take your word for it! 

I was mistaken about the choral reef. It and the old school benefit from the sps smt due to the ability to provide the higher voltages allowing for more clean headroom, however, you can do the same on the originals by using a 15v power supply. Alpha Dog and Fiery Red Horse also mentioned around 12min mark, but really the whole thing is very informative.


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## PangeaDestructor (May 12, 2022)

I just got a reply from Peter, again I'm not sure how helpful this is for the other circuit but here's what he said:

"While that is certainly a unique symptom, it does sound like there is a DC bias issue. This could be caused by many things, but the most likely is an op amp that is either defective or doesn't play nice in that specific part of the circuit. A TL072 op amp should work well there. If you look at the schematic, the drive knob is part of a loop that connects the positive input to the negative input of the op amp. If the VB voltage doesn't have a capacitor to create a good enough virtual ground on the middle leg of the drive pot, some strange issues could also arise. But if everything else is the right part in the right location and direction, and the solder joints look good....my guess is that it's the op amp."


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