YuSynth LFO V2

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dummyplugconspiracy
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YuSynth LFO V2

Post by dummyplugconspiracy »

Hi -

Just finished (hah! apparently not!) the YuSynth LFO V2. It's giving me some trouble getting the waveforms to look right. I want to make sure I'm not overlooking anything obvious before I dig in:

1. The LED doesn't flash, but it does illuminate. When I read the description of the module again, it does mention brightness, but not blink rate. Am I correct that this version doesn't blink, but instead increases in brightness with higher frequencies? It was a bit hard to tell with the lighting on my bench, so I may have missed it.
2. On the wiring diagram there appears to be a 10n cap inline between the board and the tip of the Sync jack. But there is no other mention of it and the parts kit didn't include it, so I am sort of assuming it's either not needed or it's a mistake. Anyone know?
3. I am getting rough saw, square, triangle (sort of) and something that kind of resembles a sine if I tweak the heck out of the trimmers. But I cannot seem to get them all at the same time. Every time I tweak one, I lose the others. Is this typical of this module? Or is that an indication something is wrong? I feel like I am going to extremes trimming. And the T3 sine trimmer doesn't seem to do anything at all.
4. Lastly, I noticed as I was testing and trimming that my bench supply current meter started reading 0.04, which seemed about normal. When I glanced at it again, it was 0.12, which alarmed me a little. I could make it drop some by adjusting the frequency knob. At one point it read as high as 0.14 and I turned it off just in case. But performance didn't change during any of that. It was a little warm, but I also live in Florida and work in the garage. It's over 100F here. Does this seem odd? As I completed my testing it had settled back down to 0.03 and I have to wonder if my trimming was what made it rise.

Last time I had trouble with an LFO it was just a case of balancing the trimmers with some serious trial and error, but this seems like possibly something else going on. I will do a full inspection tonight.

THANKS!
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Re: YuSynth LFO V2

Post by dummyplugconspiracy »

I fixed one minor problem that has made this module almost work perfectly. Right now the only problem I have is the sine wave output. I cannot get it to look like a sine wave. The strange thing is that I'm supposed to be using the T3 trimmer in order to shape it. That trimmer has no effect whatsoever. Normally when you tweak them all the way to one extreme setting they click. This one doesn't seem to do that. Possible bad component? I also noticed that T2b trimmer has no effect on any waveforms either. The only design anomaly I see is that the wiring diagram tells you to connect a dedicated ground to the sine output and the schematic shows all outputs sharing the same ground. I tried it both ways with no change. Shouldn't matter anyway since they all reference the same ground trace on the pcb.

Any ideas to make that sine wave look right? The T2a trimmer does affect the sine output but doesn't make it look like a sine wave. The rest seems to work. It appears that IC3 is responsible for sculpting the sine wave. I probed around but couldn't find anything useful there either. Certainly nothing that looked like a sine wave.

Thanks!
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Re: YuSynth LFO V2

Post by Paul Cooper »

I think we have been here before. It is rare that someone can fault find remotely with little or no data to go on. We would need to know what actually works as well as what doesn't to be able to make a contribution to your fault finding.

Break down your circuits into sections that you can check for correct operation. T3 for example primarily controls the symmetry of the triangle waveform so does it in your case? The sine wave is derived from the triangle wave but that is secondary. D3 & D4 are used in the sine shaping, are they the correct polarity?

T2A and T2b are identical in terms of operation, each being switched in depending on the range switch. Simply measure the voltage at J3g which should change as you adjust T2b for one range switch position and for T2a in the other position. They will affect the sine wave because they affect the sawtooth which in turn affects the triangle which in turn affects the sine.
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Re: YuSynth LFO V2

Post by dummyplugconspiracy »

Ok - J3G is the positive pin of the sine jack. I do see voltage change there when I adjust T2a and T2b with the range switch in the respective positions. That's working and I can see the effect on the oscilloscope. The trimming instructions say that T3 is the sine shaping control though. That trimmer still has no effect on sine nor triangle. I checked the D3 and D4 diodes and they are correct. The frequency knob is affecting the sine output and the range switch is working. Cv control works too. I'll take pictures of the waveforms here with range on 1.0 and frequency at max. As you can see, ramp looks good, triangle isn't real good, pulse is fine, and sine isn't even close. And that T3 trimmer just seems wrong. Is it possible that component is bad? Wouldn't it have some effect, however small, on triangle and sine if any signal at all was running through it?
Thanks!!
Attachments
Sine
Sine
Pulse
Pulse
Tri
Tri
Ramp
Ramp
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Re: YuSynth LFO V2

Post by Paul Cooper »

Sorry no, J3g on the range switch is where you want to be measuring the voltage from T2as & T2b.

Is the ramp output also good in the x0.1 range as well? If you adjust T2a in the x0.1 range do you see the ramp change. Also do the same for T2b in the x1.0 range.

I am focusing on one issue at a time and starting with T2b as it has an effect on all waveforms. Make sure your scope is set for a dc input, not ac.
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Re: YuSynth LFO V2

Post by dummyplugconspiracy »

Oops. Ok I get it now. J3G goes to the R15 resistor so that's the middle right side pin on my switch. Or pin 5 if we count them off anti clockwise like an IC. that voltage does respond to T2a and T2b when the switch is in the respective positions. On the oscilloscope those trimmers definitely affect ramp.
Ramp looks great with switch in either position.
Oscilloscope definitely set to DC.
T2b is affecting all 4 waveforms.
What next?
Thanks!!
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Re: YuSynth LFO V2

Post by Paul Cooper »

Good so T2b does affect the waveforms so we can eliminate that from your original problem list.

Now adjust T2a (x1.0) and T2b (x0.1) to give you a symmetrical ramp waveforms at the output jack, i.e. it should be around +/-5V. The actual voltage is not so important as the symmetry.

Now check the triangle output and adjust T3 so as to give a good symmetrical +/-5V at the triangle output.

Post back with pics of your ramp and triangle waveforms.
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Re: YuSynth LFO V2

Post by dummyplugconspiracy »

I can make my ramp waves look great in both range settings. Pictures attached. When I go to adjust triangle using T3, it has absolutely no effect even if I tweak it really far in either direction. I attached a picture of where the triangle starts after I get the ramps looking nice. If I adjust T2a and T2b I can make it look a little better, but the bases if the triangles never meet and I always have that gap between them. But T3 is doing nothing at all. Thanks!
Attachments
This is the best triangle I can get
This is the best triangle I can get
Triangle 0.1 range
Triangle 0.1 range
Triangle 1.0 range
Triangle 1.0 range
Ramp 0.1 range
Ramp 0.1 range
Ramp 1.0 range
Ramp 1.0 range
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Re: YuSynth LFO V2

Post by Paul Cooper »

What I can't see is symmetry on your sawtooth. I assume you have the trace aligned with the graduated graticule (the centre line on the x-axis on the screen) when the input is grounded. If so, then the positive and negative excursions of the sawtooth should be equal (+/-5V).

That is unlikely to be the cause of the triangle problems. T3 looks to be quite a coarse adjustment so it would shift the triangle wave severely up and down to the point where it would turn back into a saw or ramp waveform at the extremes of the T3 adjustment. This means you have a problem around the sawtooth to triangle conversion part of the circuit. This is based around IC3b and IC3c.

Check component values and solder joints in this area for anything obvious. I have built 300+ module this year and never had a component fault, any issues have always been a missing solder joint. IC3b is a precision rectifier so put your scope on pin 7 of IC3b and post a scope shot of what you get. To prove T3, put your DVM (or scope on dc) on R10, the end that goes to T3 and it should show a range of 0-15V as T3 is adjusted from its extremes. IC3c is just a summator adding together the original sawtooth, the negative going part of the sawtooth from IC3b and the dc offset from T3 what all combined gives the triangle waveform. We know that the sawtooth is working so the test at IC3b pin 7 and on T3 will help prove the remaining two signals in the summation.
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Re: YuSynth LFO V2

Post by dummyplugconspiracy »

When you say IC3a and IC3b … I know which IC is numbered 3 on the board. But what does the a and b designation mean? From the schematic these appear to just be different sectors of the same IC. right? I see the pin numbers there. But R10 connects to IC4, not IC3. Am I screwing up here? Or was that a typo? I attached a picture of pin 7 on IC3 if it helps.
My DVM and scope are both reading no voltage at R10. I connected the negative leads to the shared ground. Is that correct? I’m reading voltage other places so I don’t think I just grounded it incorrectly.

I’ll triple check soldering around the points you mentioned but please clarify the IC numbering for me. Thanks!
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Pin 7 of IC3
Pin 7 of IC3
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Re: YuSynth LFO V2

Post by dummyplugconspiracy »

By the way - I did link a wire across R34 as the directions said. It’s hard to see capacitor labels once installed but I’m certain my resistors are all correct values. I also reheated all the ones in the region you mentioned. No change yet. I don’t think I missed any notes, but would you find a picture of my PCB useful? I did make a small error on a couple resistors during the build, but the removal and replacement went really smooth so I don’t think that’s the problem.
Thanks!
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Re: YuSynth LFO V2

Post by Paul Cooper »

dummyplugconspiracy wrote: Thu Aug 18, 2022 10:27 pm When you say IC3a and IC3b … I know which IC is numbered 3 on the board. But what does the a and b designation mean? From the schematic these appear to just be different sectors of the same IC. right? I see the pin numbers there. But R10 connects to IC4, not IC3. Am I screwing up here? Or was that a typo? I attached a picture of pin 7 on IC3 if it helps.
My DVM and scope are both reading no voltage at R10. I connected the negative leads to the shared ground. Is that correct? I’m reading voltage other places so I don’t think I just grounded it incorrectly.

I’ll triple check soldering around the points you mentioned but please clarify the IC numbering for me. Thanks!
My mistake, it's IC3b & IC3c.

IC3 is a quad op-amp, it has 4 op-amps inside which are designated a, b, c & d. It is nothing more complicated than that.
Capture7.JPG
Capture7.JPG (36.71 KiB) Viewed 18 times
See the schematic above, R10 connects to IC3c and T3. It's the T3 end of R10 that you want to check the voltage at. The other end will always be 0V (remember the virtual earth we spoke about before).

That trace of IC3 Pin 7 is not right, IC3b is not functioning as a precision rectifier at all. You should see a flat top for half the period of the sawtooth waveform.

Okay, just grabbed a PCB from stock and bugger, yet another issue with accuracy of the YuSynth documentation! R10 going to T3 on the schematic is actually R24 on the PCB! There is another R10 (the correct one) on the schematic going to IC4, so no wonder it is causing confusion for you. Measure the voltage change for T3 on the end of R24 closest to T3.

We should also still be focusing on the build around IC3b so yes, post pics of both sides of the PCB. As good as quality as you can manage.
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