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