Korg Monotron Oscillator Sync

I  developed this mod specifically for the Montro Duo, so that one oscillator could sync to the other. Since all three models of Monotron share the same oscillator design though, the mod is applicable to them all. Here we'll look at the original model.

The crux of syncing is resetting an oscillator, but not every oscillator lends itself to being reset. I also wanted to avoid heavily modifying the Monotron board. That means limiting things like cutting traces, lifting legs, removing/replacing components.

Here's the VCO we have to work with, taken from the Monotron schematic. It's a saw-core that uses a couple schmitt triggers and a transistor for the charging phase, plus a current sink for the discharge phase. Q2 is constantly discharging the timing capacitor, C11. Meanwhile, the schmitts watch the voltage of C11. When it gets too low, they recharge it through Q5. The built-in thresholds of the schmitts define when they start and stop this recharging stage. 

This "recharge" is a method of resetting. So, the oscillator already has an internal reset signal and reset mechanism.

Monotron VCO

Korg's Sync Circuit

We need look no further than the LFO to find an actual sync circuit (though they call it "reset"). The LFO is essentially the same as the main oscillator, just with this sync circuit added.

Monotron LFO with sync circuit highlighted

I've created a version of the LFO in Falstad's Circuit Simulator, link here. I've renamed the input to "Sync", and the internal pulse to "Reset Pulse" on the trace below.

When the sync pulse goes high, Q8 pulls the input of the first schmitt low, as if the capacitor's charge was low. This forces the circuit into the recharge stage. Q7 pulls the second schmitt low, and delays the recharge until the sync pulse goes low. This is enough to sync the LFO.

Simulated LFO

While this is promising, there is a catch. The reset circuit inserts resistors R33 + F1 before schmitt one, and R32 between the schmitts. This would require some fiddly trace cutting if we were to apply it to the VCO.

My Sync Circuit

I took a different approach, and tried to discharge the capacitor using the sync pulse. This can be done with just an NPN transistor. We have to be careful about how much we discharge the capacitor though. A naive approach might discharge the cap below the point that it should begin charging again, changing the pitch and amplitude.

We can prevent this overshoot by checking the output of the second schmitt. While it's high, we're safe to discharge. Once it goes low, we've hit the lower threshold, and it's time to stop. If we AND this signal with our sync pulse, we can safely limit how much the capacitor will discharge. We also get a free sync enable/disable input if we use a three-input AND.

Here's the Falstad link.

New sync circuit

This works well in practice, but there are limitations. 

If the sync pulse is too long, it will extend past the capacitor recharging stage, and force it low again. This will keep it cycling rapidly until the sync pulse stops, and produce a high pitched tone.

If the sync pulse is too short, the capacitor won't fully discharge before the pulse ends, and the oscillator won't reset.

So, the ideal length for the sync pulse would be exactly long enough to discharge the capacitor.

Keep in mind that the capacitor is always being discharged based on an ever-changing pitch voltage. The capacitor will also be charged to an arbitrary voltage when we try to reset it. This means that the discharge time is a moving target, and there is no one perfect sync pulse length.

We can fix this by adding a set-reset latch that will catch the edge of of sync pulse, stretch it out to the right length, and end it once the capacitor is fully discharged. Here's an exaggerated trace to demonstrate the concept.

Latched sync pulse example

Here's is the Falstad link for the latched version

Latched sync circuit

The sync pulse sets the latch, which starts discharging the capacitor. Once it discharges to the lower limit, the second schmitt will go high, resetting the latch, and allowing the capacitor to recharge.

While the non-latched circuit might be good enough, this circuit will work across a much broader range of frequencies.

Korg Monotron Duo modifications

    I've been experimenting with Monotrons since around the time of the original's release. I took an interest in the Duo model more recently though. The extra oscillator opens up some modulation options, and paraphonic possibilities. I thought it would be fun to digitally control as many parameters as possible. 

    After experimenting with using OTAs to control things, I determined that digital potentiometers would be easier. Unfortunately, there has been an ongoing shortage with the digipots that the project calls for. Because of this, the project has been shelved for some time. While working on this, I developed some modifications that don't require digipots though: 

• Oscillator sync
• XOR ringmod
• Squarewave anding
• Sub oscillator
• PWM 
• filter HP input
• VCF crossmod

    I'm posting now to at least keep the schematics from sitting on my hard drive. I hope to follow this up with a breakdown of each mod.

Full schematic

VTech Precomputer 1000 schematic

 I stumbled across the VTech Precomputer 1000, learned that it's based on a z80, and looked for a schematic. I couldn't find one, so I took a crack at drawing one myself. I haven't validated it, and there are some things still to figure out, so consider it a rough starting point.

DrumFire DF500 Schematic

    Here's one more analog drum schematic that I've redrawn. I worked from the drawing found here, and changed the layout for clarity.

 

Korg Monotron Duo PCB Component Labels

     I added labels to the last of the three Monotrons, the Duo.

NJD Dub Siren schematic redraw and PCB

    I have a bit of a fascination with dub sirens. This one, the NJD, is particularly iconic. A schematic is floating around, but it's not the most pleasant to look at.

    I redrew the schematic to make the different parts more clear. You have an LFO that's a pretty standard two-transistor astable, and a main oscillator that's very similar. In between you have some LFO shaping and an LED driver, along with two buttons.

    Dials at the top select different combinations of resistors that change the oscillator rates, along with switching modulation routing. 

    The LFO is not very interesting. It only outputs a squarewave. It can be reset via switch S1 and D3. Its output goes through D4 and R22 to change C5. This gives us a slightly more interesting, uneven triangle shape. The discharge rate is set by R23 and R24. Depending on the position of S3, R24 might be switched out via R23 being grounded. C5 can also be manually charged slowly via R25 and S2 or quickly via D4 and S1.

    S4 lets you select between combinations of the square LFO and the shaped LFO as modulators for the main oscillator. It can also force the osc to a fixed pitch via D1, D2, R8, and R9.

    The main oscillator is similarly square, though the pulse width will change depending on how the two halves of it are modulated relative to each other. The harsh highs of the square wave are rounded off by the cascaded lowpass stages before the final output at R30.

 

DD1912PA (XL6007) dual/split rail switching converter schematic

    I was looking for a solution for generating dual power rails from a 3.7V LiPo battery. I found this DP1912PA board that seems to do a good job generating ±5V. I drew a schematic so I can integrate it into a design.

PCB schematic

    Once again the design was mostly taken from the datasheet. Interestingly the datasheet shows the three inductors as part of a dual-secondary transformer. This seems to be a mistake.
 

Datasheet Schematic