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

Controlling the CK9561 alarm IC (16 Song Music Box)

     A friend made me aware of the CK9561 (aka KD9561) alarm IC. I wanted to understand it better so I could use it in my projects. I started by studying a popular kit that uses the chip. It goes by multiple names, but usually includes the following keywords "16 Song Music Box". Here is a link to one of them.    

16 Song Music Box

    The circuit uses switches to select one of four pitches, and one of four "presets". Together you have 16 combinations. There exists a schematic for the board, but it's not very revealing. I redrew it so I could make sense of it.

16 Song schematic

    The pitch circuit uses CMOS switches to connect different combinations of resistors across two pins of the 9561 chip. This results in different effective resistances that set the rate of the internal oscillator.

    The preset circuit uses some TTL and Diode-Resistor-Logic to select the different alarm types. The 9561 is odd because it requires pin F1 to be "open" (floating, hi-z, etc) to select one of the modes. This means the pin has to be disconnected from both power and ground. Another CMOS switch is used to connect and disconnect this F1 pin.

The New Control Circuit

    I took what I learned from the 16 Song board, and started thinking about how I could control this chip with CV instead.

    I came up with a totem of comparators that would select the "presets". Their logic is very similar to the TTL/DTL circuit of the original. A CMOS switch is used identically. The main difference is that a single control voltage can be used to select from the four alarms.

    The pitch control is very different though. I realized that a clock signal could be injected into one of the pins for the pitch resistor. I found this easier than voltage controlling the internal oscillator, so I selected a common chip that includes a VCO: the CD4046. Two transistors form an exponential current sink that gives the expected voltage to pitch relationship.

    Now it's possible to voltage control the 9561 alarm IC.

New CV control schematic

BEAM Bird Sound Generator (CMOS Bird Song circuit)

     A friend showed me this sound effect circuit from Wilf Rigter. It produces surprisingly nice bird sounds with good variations, while using very few components.

Original schematic

    I wanted to understand the circuit better, so I started by drawing it in Eagle.

Redraw

    I then rearranged the circuit for clarity. Starting at the top left, we have an LFO (IC1B) modulating the pitch of another oscillator (IC1C). IC1C also has a Light Dependent Resistor that affects its pitch.

    IC1C modulates the pitch of the main oscillator formed by IC1E and IC1D. It appears to be a take on the textbook transistor astable-multivibrator. Its output drives a speaker via IC1F.

    At the bottom left we have one more LFO that serves to turn the main oscillator on and off. This keeps the oscillator from droning on. You instead get bursts of sound.

Rearranged

    Finally I made a rough board layout. It was easy to route with a single layer, and seems like a good candidate for stripboard adaptation.

Board layout

Chinese PT2399 Echo/Delay/Reverb Board Schematic

    I purchased one of the most common PT2399 boards from Amazon here. It's a generic Chinese board that's sold under many names. This one happens to be listed as "Youngy PT2399 Microphone Reverb Plate Reverberation Board No Preamplifier DC 6V-15V". Maybe that will help people find this page.

The PT2399 board in question

    I mostly wanted to figure out what configuration the board was using. I made a component for the PT2399 and started drawing a schematic.
    The board turns out to be the basic "Echo" design from the datasheet. Since I'm not able to easily measure things like the unmarked capacitors, I've taken all the component values from the datasheet. The component numbering matches the board though. This should make it easier to modify and experiment with the board.

Schematic

Board