Monotron Delay, a few mods

I’ve given plenty of thought for all kinds of mods for Monotron Delay, but the amount of possibilities confuses me to the extent that I’m unable to nail down a solid approach at once. So instead of trying to come up with a single decent set of mods, I figured I’ll just start with something basic, add / test mods one by one and see what I end up with.

Korg Monotron Delay

Just to sum up what I have in mind, my main idea is to build some kind of a detachable ‘expander’ box that will dock with the Delay. This box will house some additional controls and connectors that would be otherwise impossible to fit inside the original casing. What these added bits will be, remains to be discovered.

Start of sorts

Soooo, first mods.. The most obvious one is of course a VCF resonance potentiometer. Kevin of Picsynth has a very good description about adding one, so I won’t go into details with that. Just check the link. I’m going to install this to the extender box, running a loose potentiometer outside the synth isn’t exactly a lasting solution.

To begin implementing the extender, some space needs to be cleared for adding a connector to interface with Delay. With this in mind, I chose to mod the volume dial in a upright position. It’s not very sturdy (like on the picture below) and would likely break very swiftly in use  as-is. However cutting a snug-fit slot on the top panel adds sufficient enough support.

Volume dial installed upright, supporting pins near the top bent.

If going for a similar install one detail worthy of noting is, that the two supporting pins on the dial end need to be bent flat. Otherwise the part won’t slot in neatly with the hole on top cover.


A simple VCF mod that fits inside the original casing is a switch for rerouting LFO output. A second rather obvious target for the signal, VCF cutoff of course (fuck yeah, let’s wobble). As the LFO goes up to 395 Hz, this also allows the VCF to be used as a oscillator.. And it gets more fun with the resonance mod installed.

For the LFO signal location, I chose near vicinity of IC2. There’s sufficient space there under the top panel and it’s also where the pitch CV summing mixer resistors are located. Removing R7 from its solder pads provides decent connection points, but it can’t be omitted altogether. For my testing setup I chose to replace it with a standard through-hole part. Soldering directly to the pads is way too flimsy a solution though, so I still have to figure out something else for final installation. I actually managed to break one signal trace and had to repair it with a piece of wire-wrap.

Testing a SPDT switch for rerouting LFO output.

For connecting LFO to the input of cutoff CV summing mixer (IC4B, pin 6), I currently have a 10k series resistor installed. Whereas 22k was too “toned down”, the 10k sounded like a good enough solution. I will most likely test something smaller like 4k7 too.


Now then, about the extender connections. As you very likely already figured out from the pictures above, I chose to use bog-standard double pin headers. These have a wide enough pin spacing for easy soldering and positioning over the PCB. The double row configuration allows the header to be inserted with one row on top and the other on bottom side, making it more sturdy. I didn’t really plan the amount of pins to be installed, so let’s hope what I have now is enough!

The way I have the headers installed at the moment is to use some of the GND through-holes to anchor the parts down.  I also added high-temp (mylar) masking tape over the edge to steer clear of possible short circuits.  The GND through-holes are wide enough to allow inserting a piece of component feet through the board. Soldering this to header pins on both upper and lower sides creates a pretty solid anchor point.

Top side header pins

One additional idea with using double rows is, that if a original signal path needs to be disconnected on the Delay PCB it can be routed so, that vertically aligned pins will carry the in / out points for a given path. This way, when the expander is disconnected, the Delay can be used in its stock configuration by bridging pin pairs with jumpers. Computer floppy / hard drive connectors might do as well, but their use depends on how firmly I can secure the headers to the PCB.

If you look at the picture above, you’ll notice I have one jumper installed. This pin pair carries the oscillator output.  To reroute the signal, I relocated series capacitor C12 next to the bigger pin header / headphone jack. More options to play around with this way, as the oscillator can be disconnected altogether from the audio mixer opamp (IC4D, pin 13).

Bottom side header pins

The smaller header has currently only the resonance potentiometer signals wired up. More to come, rest assured ;)

Still needs a cutout for the LFO routing switch.. And almost looks like a stock Delay up front.

On the rear side, things start looking a bit different :)

Besides cutting a (too wide and off-center) slot for the volume dial, both top and bottom halves of casing needed openings for the headers. The plastic is thin and soft enough to be cut with a hobby knife, so no problem there.

Next up, I’ll give the LFO routing switch a permanent install and test some other simple mods..  When I have some additional signals routed to the headers,  testing all kinds of ‘single part’ mods should prove a snap!

If you missed my initial post about Monotron Delay, go read that here.


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7 responses to “Monotron Delay, a few mods”

  1. jon dhonson says :

    cool, I think Im gonna beggin to modmy monodelay this week, any chance to take a look to someschematics about lfo to cutoff mod?? ;)
    cheers mate!

    • arto says :

      Will write a bit more about it in the next Monotron post, so if you have any specific questions in mind let me know and I’ll try to include those.

  2. gdhj says :

    im a little bit confuse with lfo vcf routing could you explain a little bit more about this mod, the pics doesnt make it clear.
    do the red jumper conect the middle pin into the lfo switch??

    • arto says :

      Yes I’m working on a new post with more pictures. In the meantime, the mod should be pretty self-explanatory if you can read the schematics.

  3. Emil Alexander Hammarlund says :

    hey, nice work. looking forward to your upcoming posts! What I personally would like the most is CV/Gate inputs, for external control! :)

  4. jack says :

    Hi! I tried to swap the op-amp and destroyed the monotron. Would you be so kind to tell me how you did it? Did you use a square tip? What size? Thank you!

    • Arto says :

      I often use a slightly questionable method, that needs practice and can damage the part or the board. You definitely want to have a adjustable soldering iron with replaceable tips and whatever no-clean solder for this. Separate soldering flux is a plus, as it helps lessening the amount of mess.

      For this method, I use a flat soldering tip (the wider the better) and set the temperature between 250-300°C. The latter depends on the board/part to be worked on; if the part has a lot of pins or the board has large copper areas that draw more heat, a higher temp might be required. Practice helps building up ‘the eye’ for this.

      What I do is, I add some amounts of non-ROHS (= leaded) solder on one side of the chip, basically bridging/short-circuiting the row of pins. The solder adds fresh flux to the existing joints and the added mass helps retaining the heat a while longer. I then sweep back and forth along the row of pins, distributing the heat / keeping the solder in a flowing state, whilst gently prying the chip from either end. Once the row of pins nudges off the board the slightest, I switch to the other row and repeat the process.

      I often use a small flat-bladed screwdriver for prying the part, as rotating along its longitudinal axis allows using the ends of the flat blade to generate the prying force in very fine amounts. Generally, you want to use something flat with slightly rounded (worn) edges as this equals more even pressure distribution on the board surface. Anything with very sharp edges creates more ‘needle-like’ contact point with the board and thus carries a higher risk of damaging the board.

      After both rows are covered in solder and slightly nudged off the board, it’s then all about repeating. Alternate between the pin rows, slowly inching the part further from the board. At some point, the part will come off and then you can move on to clean-up and installing the new part. If the solder starts looking as if majority of the flux has been burned away, add some either by adding fresh solder or flux. The latter is of course better, as you’ll end up with less solder to clean away.

      Important: When switching sides, ensure that the entire blob of solder is in a flowing state before applying the prying force. Failing to do so will transfer the prying force to the solder pads on the board and very likely tear some off.

      If you decide to try this method, I highly recommend practicing on whatever broken piece of consumer electronics first. If you don’t have that crapped-out DVD player (or whatever), go ask a local electronics shop if they have some scrap to donate :). Especially the nudging requires very diminutive movement at first, and you need to have the feel for ‘physics’; how much heat or force to apply etc.

      As the board you’re working on is very likely ROHS (= unleaded solder used), you need to be very thorough with the solder clean-up. Mixing leaded and unleaded solder on any permanent joints is not recommended. This results in mechanically weak bonds, that are prone to fracture over time.

      As a side-note: If the board is something that will get serviced by someone else later on, you should use only single type of solder. So: if the board is ROHS, only use unleaded solder for permanent joints. Despite it’s easy to recognize leaded and unleaded solder by their surface texture, it’s simply a good practice not to have other people guessing what horrible wrong-doings might’ve transpired ;)

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