Rhythm Section

For Players

The rhythm section is the 555’s built-in drum machine. Press a button — Waltz, Latin, Swing, Rock, or March — and the organ plays a repeating percussion pattern behind you. A tempo knob sets the speed. The Toy Counter adds extra percussion fills, and Dancing Chords automatically plays accompaniment chords in time with the rhythm. None of this uses a computer or digital memory — every beat pattern is hard-wired into the circuit using diodes, like a music box where the pins on the cylinder are soldered connections on a circuit board.

Overview

The 555 includes Wurlitzer’s Swingin’ Rhythm system — an analog pattern sequencer that first appeared as the standalone Model 5020 drum machine in 1969 and was subsequently built into Wurlitzer’s organ line through the 1970s. The 5020 was designed by Harold O. Schwartz, Peter E. Maher, and John E. Holt at Wurlitzer’s De Kalb Division.

The rhythm section on the 555 encompasses four related subsystems, all controlled from the left panel below the manuals:

Subsystem	Function	Manual Page
Swingin’ Rhythm	Five preset rhythm patterns	14
Toy Counter	Additional percussion voices and fills	14
Dancing Chords	Auto-accompaniment — chords played in rhythm	15
Repeat	Attack percussion on organ keys	13

Note

The owner’s manual dedicates three pages to the rhythm features (pp. 13–15), covering Repeat/Attack Percussion, Swingin’ Rhythm, and Dancing Chords. The rhythm section is entirely independent from the organ’s tone generation — it has its own clock, voice circuits, and output stage.

Console Controls

The features diagram from the owner’s manual shows the rhythm controls on the left side of the console:

Console layout from the 1973 owner’s manual. The Swingin’ Rhythm Controls, Toy Counter Controls, Dancing Chords, and Repeat Control are all on the left panel below the upper manual. Image: The Wurlitzer Company.

Swingin’ Rhythm

Available Patterns

Five preset rhythms, selected by pushbuttons:

Pattern	Time Signature	Character
Waltz	3/4	Bass on 1, brush on 2 and 3
Latin	4/4	Syncopated with block accents (bossa/cha-cha feel)
Swing	4/4	Shuffle feel with snare backbeat
Rock	4/4	Straight eighths, strong bass and snare
March	4/4	Military-style snare pattern

A tempo control adjusts the master clock speed, and a tempo indicator light flashes on the beat.

Percussion Voices

The Swingin’ Rhythm generates five analog percussion sounds, each with its own synthesis circuit:

Voice	Synthesis Method	Character
Bass Drum	Low-frequency damped oscillator	Deep thud — a capacitor discharged through a tuned circuit produces a decaying sine wave
Snare	Filtered noise with envelope	Broad-spectrum burst shaped by a fast attack/decay envelope
Brush	Super-regenerative noise circuit	Softer, sustained noise — the “sizzle” sound
Block	High-frequency damped pulse	Sharp, clicky transient (woodblock/rimshot character)
Cymbal	Band-pass filtered noise with shimmer	Longer decay than snare, with amplitude modulation for metallic shimmer

Note

These are analog approximations of acoustic drums — not samples. The bass drum is a ringing LC circuit, the snare is shaped noise, and the cymbal gets its shimmer from a low-frequency modulator. The imperfections are what give vintage analog drum machines their musical character. The same synthesis principles appear in the Roland CR-78, Korg Minipops, and every organ rhythm unit of the era.

Architecture

The rhythm section follows the standard 1970s analog drum machine architecture: a clock oscillator drives a binary counter chain, and a diode matrix selects which counter states trigger which percussion voices. Each rhythm pattern is a different set of connections in the diode matrix — literally a hard-wired read-only memory.

graph TD
    CLK["Clock Oscillator<br/>(tempo-controlled RC)"] --> CNT["Binary Counter Chain<br/>(divides clock into beats)"]

    CNT --> DM["Diode Matrix<br/>(pattern ROM)"]

    DM -->|trigger| BD["Bass Drum<br/>Circuit"]
    DM -->|trigger| SN["Snare<br/>Circuit"]
    DM -->|trigger| BR["Brush<br/>Circuit"]
    DM -->|trigger| BL["Block<br/>Circuit"]
    DM -->|trigger| CY["Cymbal<br/>Circuit"]

    BD --> MIX[Rhythm Mixer]
    SN --> MIX
    BR --> MIX
    BL --> MIX
    CY --> MIX

    MIX --> VOL["Rhythm Volume"]
    VOL --> AMP["Main Amplifier Bus"]

    SEL["Pattern Select<br/>(Waltz · Latin · Swing · Rock · March)"] -.->|selects matrix row| DM

    style CLK fill:#92400e,color:#fef3c7,stroke:#d97706
    style DM fill:#78350f,color:#fef3c7,stroke:#d97706
    style AMP fill:#065f46,color:#d1fae5,stroke:#10b981

How the Diode Matrix Works

The diode matrix is the heart of the pattern generator. It’s an elegant analog solution to the problem of storing repeating sequences — essentially a crossbar switch where each intersection is either connected (diode present) or not.

The binary counter cycles through a fixed number of states (typically 12 or 16 steps per measure). Each counter state represents a beat subdivision. At each state, the matrix determines which voices fire:

Step	Bass	Snare	Brush	Block	Cymbal
1	x				x
2			x
3		x
4			x
…

Each “x” is a physical diode soldered on the pattern board. Changing the pattern means switching to a different set of diode connections — either a different section of the same board or a physically separate matrix, selected by the pattern pushbuttons.

Tip

This diode matrix approach was pioneered by Ikutaro Kakehashi (later founder of Roland) in 1967. His patent describes “a plurality of inverting circuits and/or clipper circuits connected to a counting circuit to synthesize the output signal.” The design became ubiquitous — Hammond, Wurlitzer, Korg, and virtually every organ manufacturer adopted it. The Wurlitzer Swingin’ Rhythm is a direct descendant of Kakehashi’s original concept.

Toy Counter

The Toy Counter is a secondary percussion circuit that adds fills and accent patterns on top of the main Swingin’ Rhythm. The name comes from its circuit topology — a counting circuit (binary counter) that sequences through additional trigger outputs, producing rhythmic subdivisions that complement the main pattern.

In theatre pipe organ tradition, the “toy counter” was the department of the organ that controlled physical percussion instruments (drums, cymbals, train whistles, bird calls). Wurlitzer borrowed the name for their electronic version — a counter circuit that plays toy-like percussion accents.

The Toy Counter has its own controls on the left panel and can be enabled or disabled independently of the main rhythm.

Dancing Chords

Dancing Chords is an auto-accompaniment feature that ties chord voicing to the rhythm clock. When engaged, chords played on the lower manual (or detected from the pedals) are automatically gated in time with the rhythm pattern — the chord sustains, cuts, and re-attacks in a rhythmic pattern rather than sounding continuously.

This feature bridges the rhythm section and the organ’s tone generation — the rhythm clock gates the keying circuit, turning the sustained organ tone into a rhythmic pulse. It’s an early form of what later organ manufacturers would call “auto-chord” or “chord memory.”

Repeat (Attack Percussion)

The Repeat control adds a re-triggering envelope to held organ notes. Instead of a note sustaining continuously when a key is held, the envelope repeatedly attacks and decays — creating a tremolo-like percussive effect synchronized to the rhythm clock.

This is distinct from the organ’s tremolo effect (which modulates pitch and amplitude with the LFO). Repeat is a rhythmic re-attack, creating the illusion of a guitarist or mandolinist re-picking a sustained chord.

Signal Path

The rhythm section’s output is entirely separate from the organ voices until the final mixing stage:

graph LR
    RHY["Rhythm Voice<br/>Circuits"] --> RVOL["Rhythm Volume"]
    DC["Dancing Chords<br/>Gate"] --> KEY["Lower Manual<br/>Keying"]
    RPT["Repeat<br/>Clock"] --> ENV["Note Envelope<br/>Re-trigger"]

    RVOL --> MIX["Main<br/>Mixing Bus"]
    KEY --> MIX
    ENV --> MIX

    ORG["Organ Voices<br/>(Upper · Lower · Pedal)"] --> MIX
    ORB["Orbit III"] --> MIX

    MIX --> FX["Tremolo · Vibrato<br/>Reverb"]
    FX --> AMP["Amplifier"]
    AMP --> LSL["Leslie · Stationary<br/>Speakers"]

    style RHY fill:#92400e,color:#fef3c7,stroke:#d97706
    style ORG fill:#92400e,color:#fef3c7,stroke:#d97706
    style ORB fill:#92400e,color:#fef3c7,stroke:#d97706
    style AMP fill:#78350f,color:#fef3c7,stroke:#d97706

The rhythm voices, gated chords, and repeat envelopes all feed the same amplifier bus as the organ and Orbit III. The rhythm has its own volume control so the player can balance percussion against organ tone.

MIDI Conversion Relevance

The rhythm section presents two opportunities for the MIDI conversion project:

MIDI-In (controlling the rhythm from external sources):

• Pattern selection via relay-driven switching of the pattern pushbuttons — documented in the Output Inventory as a binary target (confidence: Low)
• External clock sync — if the rhythm clock accepts an external trigger, MIDI Clock messages could synchronize the Swingin’ Rhythm to a DAW or drum machine. This is an open investigation item

MIDI-Out (sending rhythm triggers externally):

• Tapping the diode matrix outputs could provide MIDI Note On triggers synchronized to each voice — turning the analog rhythm into a MIDI drum trigger
• The tempo clock itself could drive MIDI Clock output for synchronizing external gear

Caution

Both MIDI integration paths require bench investigation of the rhythm circuits. The pattern select mechanism (pushbuttons vs. rotary switch vs. multiplexed logic), clock circuit topology, and voice trigger voltages are all unknown until the 555’s rhythm section is tested and traced. This investigation is on the maintenance queue.

Current Status

The rhythm section has not yet been tested on this particular instrument. It’s listed on the initial assessment checklist along with individual tab stops, reverb, and the cassette deck.

Given the instrument’s age and storage history, common failure modes to watch for include:

• Dried electrolytic capacitors in the voice circuits (causes weak or missing percussion sounds)
• Corroded pattern select contacts (causes stuck or intermittent pattern switching)
• Clock oscillator drift (causes unsteady tempo)

The Swingin’ Rhythm circuit is entirely discrete analog — no proprietary ICs — so any failures should be repairable with standard components.