The Resonance Mod

This was my first Monotron Mod :

The Korg Monotron Delay is missing the filter resonance control, here is how to add it back again ! The resonance sounds amazing and was a sad loss from the Monotron Delay.

Note that this procedure is not for the faint hearted and involves 1 very delicate surface mount soldering joint and 2 easier joints to connect to the Monotron's miniature circuit board. Use thin single core wire (eg a strand from telephone cable) to make the delicate connections and a fine tip on your soldering iron. You will need a 10K linear potentiometer for the resonance control and a knob to fit.

In the Monotron Delay Korg replaced a 2.2k resistor and 10k resonance pot with a single 3.3k resistor that feeds from AUDIO back into the filter. I wanted to add back the 10k pot, there is no need to change the 3.3k resistor, in fact I thought this combination sounded better when oscillating. (If you want it exactly like the orginal you can connect an additional 10k resistor from slider of pot to top of pot which will then effectively reduce the 3.3k to 2.2k like in orginal Monotron)

Korg Monotron Delay Resonance Mod

Step 1: Solder a wire to top of volume control. This is point marked "AUDIO" in the schematic (easy solder point).
Step 2: Solder a wire to one end of the 3.3k resistor (R78) shown (I will call this "the purple wire"). The resistor is a tiny surface mount type marked 332 note carefully it's position near the "time" knob. The wire connects to the junction of the 3.3k resistor and 5.6k resistor as shown in the schematic. This is a very tiny joint to make indeed - I suggest you practice it first on a discarded surface mount board. Put a tiny blob of solder on the top of the resistor first, do not apply too much heat otherwise you may dislodge the whole resistor. Then coat 0.5 mm of copper at the end of your wire with solder. Finally touch the two together and apply a small amount of heat. You may need to do this while looking through a magnifier. Note the other end of the resistor is internally connected to the point marked AUDIO in the monotron - connecting the other wire here however is more risky then connecting to top of volume control in step 1.
Step 3. Solder a wire to negative battery terminal (very easy solder point). With the three wires connected you can close up the monotron case - be careful plugging in the keyboard ribbon it is delicate. It is a good idea to test your Monotron Delay after this, the three wires should have made no difference to the working of the Monotron.
Step 4. Connect the pot making sure "the purple wire" connects to the middle connection (the slider of the pot). The other two connect to the outer points. The wire connected to the negative terminal should be the minimum setting of the pot. That's it - enjoy twiddling! The next project is to make a connector (see The Red Connector) that allows you to plug in various mods including the resonance potentiometer.