Orbit III Synthesizer

Overview

The Orbit III is a self-contained monophonic analog synthesizer that swings up from the organ console for service. It has its own master oscillator (VCO), power supply, and amplifier. Introduced in 1971, it was bolted identically onto Wurlitzer models 4026, 4027, 4037, 4373, 4573, 460, 465, and 555.

A Wurlitzer with Orbit III synthesizer. The Orbit III mini-keys sit above the upper manual with preset tabs to the left and variable controls to the right. Photo: synth-db.com.

Close-up showing the 25-key Orbit III keyboard (top), colorful preset tabs, and the dual 44-key manuals below. Photo: synth-db.com.

Circuit Boards

The Orbit III contains six main circuit boards, each handling a specific function:

Board	Function
Master Oscillator (Issue 1 & 2)	VCO — the free-running voltage-controlled oscillator
Divider-Steering and Hi-Lo Gates	Octave dividers from VCO + note priority logic (highest/lowest note)
Xylo-Vibes-Glock and Chimes	Percussive preset voicing (attack/decay shaping)
Preset Voicing	Sustained voicings (Reed, Brass, String — filter networks)
Switching and Special Effects	Wah-Wah filter, LFO (Modulator), Second Touch, Slide, Delta Pitch
Amplifier and Power Supply	Dedicated amp and power rails for Orbit III

Signal Flow

graph TD
    KEYS["25 Mini-Keys<br/>(dual contact)"] --> LAD[Resistor Ladder<br/>Network]
    LAD --> CV["Control Voltage"]
    CV --> VCO["VCO<br/>(voltage-controlled oscillator)"]

    VCO --> SINE["Sine Wave<br/>Tab Selection<br/>16' · 8' · 4' · 2' …"]
    VCO --> PRESET["Preset Voicing<br/>Filters ×10"]

    SINE --> WAH[Wah-Wah Filter]
    PRESET --> WAH

    WAH --> VCA["VCA<br/>Attack / Sustain"]
    VCA --> LFO["LFO Injection<br/>(Modulator)"]
    LFO --> DELTA["Delta Pitch<br/>(detune / chorus)"]
    DELTA --> VOL[Volume]
    VOL --> SPK["🔊 Leslie · Stationary"]

    style KEYS fill:#92400e,color:#fef3c7,stroke:#d97706
    style VCO fill:#92400e,color:#fef3c7,stroke:#d97706
    style SPK fill:#78350f,color:#fef3c7,stroke:#d97706

Sine Wave Footage Tabs

The footage tab labels come from pipe organ tradition — non-octave intervals that allow the player to build harmonic stacks from a single key press:

Tab	Footage	Interval from 8’ unison
16’	Low	One octave below
8’	Unison	Concert pitch
6-2/5’		Major 3rd above 8’
5-1/3’		Perfect 5th above 8’
4’		One octave above
2-2/3’		Octave + 5th
2’		Two octaves above
1-3/5’		Two octaves + major 3rd
1-1/3’		Two octaves + 5th

Note

These non-octave footage stops (6-2/5’, 5-1/3’, etc.) add harmonics at intervals that reinforce the overtone series of the fundamental. Engaging multiple stops simultaneously builds a richer timbre — the same principle that makes a pipe organ sound full.

Preset Voicings

Ten preset voice filters shape the VCO output:

Sustained: Reed, Brass, String, Banjo, Harpsichord, Electro Piano

Percussive: Vibes, Xylophone, Glock, Chimes

The percussive voices use attack/decay envelope shaping on the Xylo-Vibes-Glock board — the note decays naturally after key press, mimicking struck instruments.

Second Touch (Dual-Contact Aftertouch)

Each mini-key has two stacked switch contacts:

• Contact 1 (light press): Note ON — triggers VCO via CV ladder
• Contact 2 (deep press): Modulation — routes to LFO or Slide depending on button selection

This dual-contact system predates MIDI aftertouch by a decade. For the MIDI conversion, each Orbit III key contributes two digital inputs (50 total for 25 keys).

Caution

The Orbit III uses proprietary Wurlitzer ICs. If these fail, replacement is only possible from donor organs — there are no modern equivalents. Handle the Orbit III boards with care.