The main voice is generated by a rectangle oscillator. No fancy sinusoidales. The supply voltage VCC is 12V.

When pin OUT is at 12V, capacitor C1 is charged through a resistor of the keyboard. When the voltage at C1 reaches the control voltage Vcontrol applied at pin CV, pin OUT flips to 0V and discharges C1 again down to Vcontrol/2. Pin OUT switches to 12V and the cycle repeats. The frequency is set by selecting different resistors by keypress on the keyboard.

The pin DIS mirrors the state of pin Out but is an open collector output with R3 as a pull-up. This pin is used as output for the rectangle voice.

If no key is pressed C1 is not charged and R2 discharges C1 to 0V. No rectangle and sound is generated.

The frequency can also be changed by appliing a different voltage than 8V to pin CV. An emitter follower applies Vcontrol to the pin CV. The transistors Q1 and Q2 together form a pseudo darlington. The Darlington configuration is needed to drive the low ohmic load resistor R1.

The low output impedance of the emitter follower is needed to force Vcontrol over the internal voltage devider of the NE555.

The voltage shift of 600mV added by the emiterfollower is blissfully ignored.