Synthesizer Extra's No. 2: JOYO Instrument Tuner Hack.

This article explains how to connect a JOYO Instrument tuner to the VCO('s) of your modular synthesizer.

Inspired by LookMumNoComputer's Preformance VCO with built in tuner I bought three of these small Joyo tuners on eBay to see if I could adapt them to work with the synthesizer. I managed to do it and it's not difficult to do at all. Many people seem to do this too going by the Facebook reactions I got on this hack. :-)
The JOYO instrument tuner is a very useful bit of kit that you can buy on eBay or AliExpress etc. for around 5 US Dollars including shipping. It's pretty easy to hack these things so you can connect a jack socket to them and so connect it to the VCO's in your synthesizer. It makes tuning VCO's or Sequencers very fast and very accurate.

Now, before we begin I want to mention that the method below is just the way I did it. I've had lots of reactions to this hack and people do it in all sorts of ways, for instance connecting a jack straight to the piëzo wires without a resistor divider and not having the piëzo mic connected. Using a capacitor instead of resistors and it all seems to work so this is just my way of doing it but it's certainly not the only way, or the best way, to do it! Just so this is clear.

Here's what we do. You'll need to carefully open the tuner case by unscrewing 4 little screws at the back and then take out the two screws that hold the little circuitboard in place and now you can lift out the circuitboard. Leave the actual LCD display safely inside the case and make sure not to get fingerprints all over it. On the back side of the circuitboard, you'll find a very thin Piëzo-electric disk-microphone. Carefully push the white display lighting out of harms way and carefully solder a thin wire to the earth connection, which is the solder blob on the outer part of the Piëzo electric microphone. Solder an other wire to the center of the Piëzo disk microphone. Do not disconnect the red wire from the center of the microphone otherwise this won't work. 
Now make some small notches in the side of the plastic tuner case so you can lead the wires outside. Put the little circuitboard back inside the case first, to make sure where the notches need to go, then mark the right spot and take the circuitboard back out and make the notches in the case. Assemble the tuner and put everything back where it was. Now you can solder on the resistors following the diagram beneath. Solder them straight onto the input socket. Now solder the wires to the input jack socket (3,5mm) and the resistor and then hot-glue the input socket, with the resistors and wires attached, to the side of the tuner case. Make sure when glueing that you don't go past the bottom of the tuner case so the underside stays straight and flat. I myself then hot-glued the tuners to my synthesizer case by putting hot glue on the battery cover, which covers about 3 quarters of the backside, so the tuners are held in place by the battery cover. I later carefully drilled a 2mm hole through the covers into the wooden panel of my synthesizer and put a small screw in each one, that sits flush with the battery cover inside surface. That way it won't fall off at some point.

Powering the tuners:
I soldered two wires to the battery connectors. The spring connector in the middle of the battery compartment is the negative pole and the metal contact at the side is the positive pole. I have a 5V power rails in the powersupply of my DIY synthesizer and I connected the wires from the tuner(s) to that. I put three 1N914 diodes in series into the positive voltage rail to get the voltage down to just over 3V which is what the tuners need.
It's much better to give them their own internal power source because if you run them on the batteries the screen will flicker and vary in brightness. This way is all nice and stable.

SCHEMATIC DRAWING:

Here is the way I wired up the audio inputs. I believe the input of the Joyo tuner is capacitive so a resistor voltage divider like this will work fine and there will be no need to use a capacitor. This resistor voltage divider brings signals with an amplitude of 10Vpp down to 2Vpp.

This is the little circuitboard lifted out of the case. The white panel on the left is the LCD back-light. The gray and pink wires are the ones I soldered on myself:

Here's one of the finished hacked tuners without the battery cover because that is glued to my synthesizer. I blackened the glue with a permanent marker. As you can see I cut off the mounting eye for the clamp and made sure it was all sandpapered nice and flat. The fact that this particular model of tuner has a straight back and sides is why I chose this one. There are models out there with curved sides which make mounting more difficult. Bear that in mind when you buy one of these tuners.

Here are two of the three tuners mounted on my synth above two VCO's:

And finally a little video showing the tuners in action. As you can hear at the end, I was really pleased with the result. It looks very cool too on the synth I think.:

Oh and one final thing. You can use the left over clamps, from the backs of the tuners, to make a so called "Third Hand" to help with soldering :)

Leave the little screws in there and take some thick electrical copper wire (about 50 Centimeters long) and bend an eye at each end and mount the clamps to them with the little screws going through the eyes. Then hot-glue them in place and glue and/or tape that to a heavy enough metal base and with the copper wire you can bend them any which way you need them. And there you have it. Super useful! ^___^

EDIT: 6th of May 2022: Today I did two more of these hacks to have some tuners for my Eurorack system and they work fine :)

These were of a different make but they turned out to be even easier to hack. Here's a picture. I did use a lot of hot-glue to secure the sockets in place but it all works fine. In these I used a 120K resistor and a 6K8 resistor. which reduces a 10V input signal to a 0.54V output signal going into the tuner, making sure it's not damaged by the high synthesizer voltages.

In this case I left the clamps on the tuners because I can then clamp them to the powersupply wire of my Behringer Go Eurorack case. I drilled some holes in that case and connected the powerbrick straight to it so it won't dangle off the case which is very annoying.

Finally I want to show how one of our Facebook members (Martin Nyborg Sørensen) turned this idea into a very cool Eurorack Tuner module. He buffered the in and output so he can route signals through the tuner without any problems. I think you'll agree he did a very professional job.

The other side:

Okay, that's it for this one. If you have any questions please put them in the comments below or post them in the FaceBook Group for this website.
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