Fixing the random module

This Random module, with its noise generators and sample&hold, is one of the first modules I made in 2017.  It's based on a design by Yves Usson for his Yusynth.

For some time, I had been under the impression that the output of the sample&hold had no influence on the modulations I assigned it to.

After some investigation, it appears that the input jack is at fault.  This jack has a switch so that if no plug is connected then the internal noise feeds the sample&hold. When a plug is connected, it opens the switch and the input signal feeds the sample&hold function.
 
In this case, the jack is of bad quality and its switch no longer closes properly.
 

Access is not so easy

Once replaced, everything works again.
I even changed the clock input jack because I thought we were using the same internal/external function management principle. But it wasn't necessary: the switch is outside.

Fixed !

Sequencer

One of the simplest and cheapest CV/gate step sequencer is the Korg SQ-1.
 

Korg SQ-1 step sequencer

Powered via USB or batteries, the small machine is able to deliver one 16 steps or two 8 steps sequences in various of ways.

There are very few settings for CV out.  The SQ-1 can play linear, minor, major or chromatic scales with a 1V, 2V or 5V ranges (in V/Oct mode) or 8V range (in V/Hz mode).

Its biggest drawback is that there is no way to choose the root note of the scale.  So it's either C major/A minor (major mode) or C minor/Eb major (minor mode).   You'll have to transpose yourself by adding voltage (83 mV per semi-tone).  Good enough for fun, maybe not suitable for a full symphony.

Otherwise it's an enjoyable machine that I bought mostly for parameter automation.

I don't have a lot to control on the synth just yet.  But it's still possible to have some fun.

In action

Here above, the SQ-1 sequences Rings in sympathetic strings mode.  The sample and hold modules gives some randomness to the position parameter.  Odd output is untreated, while the even output is connected to the phaser.  So one every two notes is treated by the phaser and mixed a bit differently for a bit of variation.  Sequence evolution is me activating and deactivating the gates.  The SQ-1 has no pattern memory whatsoever.
All is synced to the tempo of the Korg Volca Beats, responsible for the basic drum pattern.
Additional effects : TAL DUB II for the delay, NI Guitar Rig as guitar amp simulation and Thomas Mundt LoudMax.

Yusynth's Random Module.

This is the story of my build of a Yusynth's Random Module in Eurorack format.

I learned a lot by building this.  Read :  "I made a lot of mistakes."

Finished module

I started from the schematics from Yusynth website.  It is clear and quite well explained.

Once again, I patiently captured the layout in DIYLC.  I used Yves' own layouts to guide me. 
I chose to make the two PCB version, one on top of the other, as I was sure one PCB would be too large for the Eurorack format.

I added two reverse polarity protection diodes.  And I chose Schottky diodes this time, to limit the voltage drop.

Anyway, I reproduced the mistake of having the power connector horizontally and not vertically.  Certainly due to the fact it was oriented that way in Yves'  layouts.
Here are the layout I used.

The layout of the two boards

I was mistaken in one of my part order and ordered some 2-Watt 10 Ohms resistors.   Nothing wrong done, as I had enough space to accommodate them, but they are total overkill.

Work-in-progress

As I was soldering the parts, I scrupulously followed each and every wire and parts, highlighting the schematics and my layout at the same time.

Despite those efforts, nothing worked at first.  Except the white noise output seemed to output ... some noise.

I quickly diagnosed that the oscillator did not oscillate.
Board inspection found nothing.  Without an oscilloscope, I was blind.  You cannot debug by ear a non audible LFO.

I finally bought a used Tektronik oscilloscope.  It confirmed the diagnostic, but still, no clue.
Oh, what's this ?  An unconnected pin on R5 ?
Soldered.  First problem fixed.

Oscillator debugging.  First success.

I began inspecting each output systematically and saw nothing at the random output.
It uses the pink noise output as reference.  And nothing on the pink noise output as well.
A wrong connection this time, due a misleading layout : because of the bad layout drawing, it was not clear which pin of the Q4 transistor to connect R20.  Of course, it was clear to me at layout time but I chose the wrong one at solder time.

Now white, pink and random outputs seemed to work.  More on this later on.

The sample&hold did not to hold.

Not holding properly

I couldn't understand why.
I reproduced the circuit on a breadboard with spare parts, so that I could better debug it and learn how it works.

Part of the circuit on a breadboard

I could reproduce the behavior : not holding well with signals from the noise module : white or random, while the circuit hold well with other signals. 
I switched capacitors, changed resistors, exchanged transistors to not avail.

Out of idea, I gave up and asked Google about it.
Finally, the solution was given by Yves Usson himself.  In a forum he detailed why it might not work :

Here are the possible causes for a holding fault :
- too high white noise signal (>8Vpp)
- a dead BF245C (Q5)
- a dead TL072 (U2)
- a dead diode (D3)

The last three could be excluded as the circuit behaved properly with another (about 1Vpp) input signal and I had the same behavior after having reproduced the circuit on the breadboard.  So dead components were out of the question.

Noise source was effectively way too high : about 20Vpp.  
In fact, noise saturated both output opamps so R34 and R27 should have been lowered.  I didn't want to lower my noise sources as much as 8Vpp, so I chose a compromise.
I chose R34 and R27 as 220k, both because hasty measure and crude calculation would give me about 16Vpp and I had some in my drawers.

As the circuit seems to be sensitive to the input voltage, I decided to add a passive attenuator on the input.

More debugging

Finally, my Random module works as expected.
It took some time and effort but the journey was worth it.

And it does a little bit more than blinking its LED...