Korg Monotron Oscillator Sync

    I developed this mod specifically for the Montron 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 the 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

Korg Monotron Duo PCB Component Labels

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

Korg Monotron PCB Component Labels

     I've been interested in modifying a Korg Monotron. I found a labeled PCB image at Tim Stinchcombe's site here. I found the red labels very difficult to read though, so I did a little editing in Gimp. Here's the resulting image.

more legible PCB labels

Korg Monotron Delay PCB Component Labels

Even though it's old-hat, I'd like to try modifying the Korg Monotron Delay.
Korg was nice enough to share their schematic, but not the board layout. Also, the board itself isn't labeled.
I need to know what I'm working with, so I used a multimeter, and some educated guesses to draw the following. I labeled the rough sections of the circuit, so that it's a little easier to find what you're looking for. Be warned that it's not perfect, but should be a decent starting point.

Korg KR-55 Recap Guide

I'm repairing my second Korg KR-55 drum machine. They both had a ton of failing electrolytic capacitors that needed replacing. In the first machine, the capacitors had leaked and damaged the board. In the second, one had failed and shorted, causing the snare, hihats, and cowbell to not work.

Recommending "recaping" as a general fix, is something of meme now. It's no joke on the Korgs though. I advise replacing all the electrolytics in them, so I made a the following images and lists to help.

The logic board only has a few capacitors. The 470u are the large, axial type, and don't seem to be an issue. They're listed for completeness.

Name	Value	Voltage
C21	.33u	50V
C1	10u	16V
C22	470u	25V
C23	470u	25V

Here are the relevant capacitors on the sound board. They're color coded by value.
Red = 1u / 50V
Org = 10u / 16V
Ylw = 100u / 16V
Grn = other

Here are the footprints on the bottom, to help with desoldering. These are not coded, because they should all be removed.

Here are the types and totals of capacitors:

Value	Voltage	Total
.15u	50V	2
.22u	50V	2
100u	16V	5
10u	16V	14
1u	50V	14
47u	16V	1

Here are the individual capacitors, and their function in the circuit. This can be helpful for debugging.

Name	Value	Voltage	Role
C59	.15u	50V	Low Conga
C60	.15u	50V	Hi Conga
C49	.22u	50V	HiHat
C61	.22u	50V	Tom Noise Audio
C50	100u	16V	Cymbal Power
C52	100u	16V	Hat Power
C106	100u	16V	Power
C107	100u	16V	Power
C123	100u	16V	Metal Power
C20	10u	16V	Bd, Sn Power
C21	10u	16V	Bd, Sn Power
C51	10u	16V	Cymbal Power
C71	10u	16V	Tom/Conga Audio
C72	10u	16V	Tom/Conga Power
C73	10u	16V	Tom/Conga Power
C98	10u	16V	RS/Clav/CB Audio
C99	10u	16V	RS/Clav/CB Power
C100	10u	16V	RS/Clav/CB Power
C101	10u	16V	RS/Clav/CB Power
C103	10u	16V	Mix Audio
C105 (C05)	10u	16V	Power
C112	10u	16V	Noise
C114	10u	16V	Noise Power
C2	1u	50v	Bd Trigger
C5	1u	50v	Bd Audio
C10	1u	50V	Sn
C13	1u	50V	Sn Audio
C16	1u	50V	Sn Body Audio
C18	1u	50V	Sn Audio
C54	1u	50V	Low Conga
C55	1u	50V	Low Conga
C64	1u	50V	Hi Conga
C65	1u	50V	Hi Conga
C90	1u	50V	Claves Audio
C95	1u	50V	Cowbell
C108	1u	50V	Noise
C111	1u	50V	Noise
C19	47u	16V	Bd, Sn Power

Korg Nanokontrol Schematic + New Firmware

Ages ago I drew a schematic for Korg's original Nanokontrol. I finally decided to dust it off and do something with it.

From the outside the Nanokontrol is just a USB control surface, but inside it hides an AVR microcontroller. It even has the ISP (programming) header broken out, though the pinout differs. With this it seems pretty natural to write a new firmware for it.

I wanted a simple project that demonstrates interfacing the potentiometers, LEDs and buttons. The idea of a CV step sequencer sprang to mind. In order to get nice clean voltage output I decided to add a DAC. The problem with adding to this setup is the general lack of IO pins. The serial pins TX and RX are conspicuously unused, but I chose to leave them free for possible MIDI communication. I don't plan on using USB MIDI, so I freed up IO by removing the USB chip.

PDIUSBD12 USB chip

The USB chip actually serves as the clock source for the AVR, so a replacement clock must be supplied. Step one is to flash the AVR fuses such that it can accept a crystal clock source. This has to be done while we still have an external clock. After that the USB chip can be removed and a crystal can be attached to the XTAL pins of the AVR. Conveniently the USB chip has an external crystal footprint that we can repurpose.

Now most of IO PORTD is free to use. I used one external interrupt pin for the trigger input. This makes it fast and easy to step the sequencer in response to an external clock/trigger. This leave plenty of pins for the DAC (MAX528). It gets its own clock and chip select lines, but it shares the data line with the LED shift register since it's clocked separately.

This is what the schematic looks like after the modifications:

Modified Nanokontrol

This setup works, but the CV output only spans about 0-4V. To broaden this, the DAC can operate on +12V while still responding to TTL level control. It just needs the supply rail and reference voltages changed to 12V. For convenience sake, I also added a 5V regulator. This makes it easy to run the entire setup off of 12V.

MAX528 12V configuration

Now all of the hardware modifications are done. It's time to write some code. The only trickiness comes from interfacing the multiplexed IO. Here is an overview of how to read/write them:

Read Buttons:

1. Pull PC0-3 low one at a time to enable one column
2. Pull PC4 low to enable buffer
3. Read PINB for button statuses (low = pressed)
4. Repeat steps 1-3 pulling each column low

Read Pots:

1. Set PA0-2 to select pot from Mux
2. Read Mux on PA4 and PA6
3. Read single pots on PA5 and PA7
4. Repeat steps 1-3 selecting each set of pots

Write LEDs:

1. Set PC7 high to disable LED output
2. Write first bit to PD7 (low to turn on)
3. Set PA3 high, then low to clock in bit
4. Repeat steps 1&2 for all 8 bits
5. Set PC5-7 to select column and enable LED output
6. Delay to allow LEDs to shine
7. Repeat steps 1-6 selecting each column

The code and schematics can be found on my github here. The code is pretty barebones and simple. It just serves to demonstrate interfacing the Nanokontrol hardware. There are some obvious upgrade and features that can be added, so this is just a starting point.