Yusynth dual gate delay missing C 7/8

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dummyplugconspiracy
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Yusynth dual gate delay missing C 7/8

Post by dummyplugconspiracy »

I just finished this build and the PCB has a C7 and C8 location that are not mentioned in the bill of material or in the schematic. They are pictured in one of the board diagrams, but not in the wiring diagram. Do I just leave them on populated? Thanks!
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Re: Yusynth dual gate delay missing C 7/8

Post by Paul Cooper »

They look to have been +15V de-coupling caps. U1 and U4 have a single de-coupling cap each across the +/-15V supply rather than to ground so unsure as to whether C7/8 became redundant or not. Try it without C7/8 and if it works then great but just wondering if these became late additions and didn't make it into the documentation. I can see no drawback in fitting C7/8 if you have a couple handy.
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Re: Yusynth dual gate delay missing C 7/8

Post by dummyplugconspiracy »

OK thank you. What would I stick in there if I wanted to populate them? I am wiring it tonight so maybe I can get a test by tomorrow.
Thanks
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Re: Yusynth dual gate delay missing C 7/8

Post by Paul Cooper »

The PCB shows 100n which is the default standard but shouldn't hurt to go a preferred value up or down from this.
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Re: Yusynth dual gate delay missing C 7/8

Post by dummyplugconspiracy »

I powered this up and it seems to be mostly working, but I think I'm doing something wrong still. When I plug gate into input one, the delay one knob has no effect and the delay one light does not flash. I get no Signal out of the gate slew or gate output on channel one, but I do get output from both of them on channel 2. I presume this is because of the Normaling jumper that goes across the gate inputs. Correct so far? I don't have a second gate input available in my test rig, but if I plugged it in to gate 2 input at the same time gate 1 input was populated, is it likely it will work on both channels? I will test that part when I get a chance, but is that what it's supposed to do? Thanks!
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Re: Yusynth dual gate delay missing C 7/8

Post by dummyplugconspiracy »

I did some more testing. The LED is always illuminated on channel one even when there's no signal present and nothings plugged in. When I plug into channl one I see results on channel 2 but never on channel 1. When I plug into channel 2, I see results on channel 2. So channel 2 seems to be working perfectly. I checked my wiring and I wired both channels exactly the same. Is it possible this is a symptom that might be caused by those missing capacitors? Would it only affect channel one? Or would it make the whole thing not power up? Not sure what to try next here… I am tempted to cut that jumper across the two inputs and see if it behaves any differently. But I see no reason that would change anything.
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Re: Yusynth dual gate delay missing C 7/8

Post by Paul Cooper »

dummyplugconspiracy wrote: Thu Jun 16, 2022 8:07 pm Is it possible this is a symptom that might be caused by those missing capacitors?
Highly unlikely.

So by now and we have been with you through many of your builds you should now be at a stage where you should be looking first and asking later.

How it should operate is explained on the YuSynth

"This module has two separate units that can be triggered either by one single GATE signal or by two separate GATE signals. Each unit provides two output signals :
a delayed GATE signal (0-10V) with delay that can be set between 1ms and 1s;
a log rising control voltage (0-10V).
This module is useful for building up complex cannon sequences, trigger delayed events or to drive a VCA for creating a delayed vibrato.

The output GATE signal is on only if the input GATE signal is still on. The output GATE signal returns to 0V as soon as the input is off.

The output SLEW signal starts rising when the input GATE signal is set, and stabilises at 10V. It returns to 0Vas soon as the input is off."

How the circuit works is also explained on the YuSynth site. So, in the first instance, do some fault finding with your scope as this is an ideal module to gain fault finding experience on as it is nice and simple and there are two identical circuits.

Come back with your findings once you have done some basic checks. You can do it ;)
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Re: Yusynth dual gate delay missing C 7/8

Post by dummyplugconspiracy »

You were right about this being an ideal learning opportunity. Tonight I compared how the two circuits behave when I introduce the same pulse input signal. First I looked at what the Chip was doing. Pin 3 (+ input if I'm reading the schematic right) gave me a pulse of 0 to +15 V on the working side but gave me a slew pulse of 10 to 15 V on the nonworking side. Pin 1, which is the output if I'm reading the schematic right, was giving me a -15+15 V pulse on the working side but a stable +15 that was not pulsing at all on the nonworking side. Realizing that my problems began on input, I went to the beginning of the circuit and started comparing what each component was doing. Following the flow of the schematic I got as far as R7 on the nonworking side before I found anything that was behaving differently than the working side. On the working side that resistor is giving me a stable 0 to+15 V pulse on one end and a very slightly pulsing +15v on the other end. But on the non-working side i got a very slightly pulsing +5 V and the other end is giving me that same very slightly pulsing +15. Luckily this is just a 1 K resistor and I had some extras, so I replaced it, but that did not have any effect. I'm quitting for the night, but that seems to indicate my problem is somewhere between Q4 and R7 unless it has something to do with the potentiometer. Q7 behaviors matched in both working and non working circuits though. The fact that the LED on the nonworking channel is always illuminated even when there is no signal present is confusing. Any thoughts so far? Thanks!
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Re: Yusynth dual gate delay missing C 7/8

Post by Paul Cooper »

A good start, comparing a non-working circuit against a working circuit is a very valid technique in fault finding especially when you are new to it.
The next stage is to see if you can identify the functionality of any sections of the circuit. In this case look at the op-amps as these are two classic arrangements.

U1b is a simple non-inverting voltage follower (buffer with a gain of 1) where whatever voltage is at pin 5 non-inverting input will appear at the output pin 7. The high impedance non-inverting input does not load or alter the voltage across C3 but buffers it so it can drive as here an output to another module. Without the buffer, the circuit operation which in this case is the voltage across C3 which relates to timing – more on that below would be affected.

U1a is a comparator, you will note that there are no feedback resistors so the gain of this op-amp is its open loop gain (differential voltage gain > 1 million). The inverting input has a fixed voltage on it based on the R8/R9 voltage divider which from Ohms law is about 10V. What all this means in practice is that if the voltage on pin 3 exceeds the 10V on pin 2 by even just a mV or so, the output of the op-amp will go to maximum positive voltage and if it is below 10V, the output will go to maximum negative voltage. This gives a fast rise / fall time on the output even if the input voltage is changing slowly (which it is).

The circuit is all about delay and for that you use a CR circuit which is C3(R1+R7). Q3 and Q4 form a half-bridge circuit where when one transistor is on, the other is off and vice-versa. This is used to charge C3 slowly via R1 and to discharge C3 rapidly when Q3 is on. Q3/Q4 are controlled by the common Q1 which is a simple NPN switching transistor circuit to create a voltage at its collector of 0V (ish) and 15V depending on the state of the input gate signal. This 0V/15V signal drives Q3 and Q4.

I’m holding your hand here hoping this will aid you in finding the cause of the problem. Armed with the above description, can you carry out further tests with your scope. I would suggest using both scope channels, channel 1 on the input gate (with a gate frequency of 20Hz), set the scope to trigger on this channel and use channel 2 to check out the circuit operation. Set the pot to minimum initially and at certain points in the circuit you should see a time differential between the gate pulse and the delayed gate which varies with R1.
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Re: Yusynth dual gate delay missing C 7/8

Post by dummyplugconspiracy »

I had to look some of those terms up, but I appreciate the help! Here I thought that "always on" LED was going to be the smoking gun that took us straight to the likely culprit. No such luck, huh?

I have to work out the triggering feature of my new scope. When I do those additional test, is there any specific anomaly I should be looking for? Or am I really just looking for behavior on the non working side different form on the working side still?

THANKS
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