Roland RS-505 (1977–1978)

A 49-note string ensemble with a genuine synthesizer section bolted on - and a service manual that reads like a physics lecture.

Overview

The RS-505 gets called a "string machine," and then it quietly embarrasses every other string machine in the room. The service notes explain why. The strings half is a proper divide-down ensemble, but the synthesizer half has a real 24 dB/oct low-pass filter with resonance, cutoff, LFO sensitivity, and envelope sensitivity. It is not a preset-tweaked afterthought. Owners report that the synth section alone justifies the footprint.

The RS-505 Service Notes (September 1978) document the architecture. A master VCO feeds a top-octave divider, which drives the tone dividers and gates. The VCO is an LC oscillator tuned by a varicap diode, with a maximum pitch shift range of one octave. Frequency stability is specified at within 15 cents at normal working temperature. That means tuning is a one-time job after power supply work or VCO component replacement, not a weekly ritual. The tuning reference is 1768 Hz at the "A" tone divider, adjusted with VR-1 on the OPH-28 board.

The part to look at in the ensemble is the ETH-7 modulator. It uses four sets of audio delay lines built around 2AD512D ICs, with clock generators running from 40 kHz to 250 kHz. Each of the four delay lines is modulated differently. Two get separate triangular waves, and two get anti-phase triangular waves summed with a sine wave. That is why the ensemble doesn't sound like a chorus pedal. It sounds like four separate string sections playing the same part.

The bass section is its own animal. Three 8-bit priority encoders (TC-4532) generate 6-bit codes from the bass keyswitches. TC-4042 ICs latch those codes, and the latched signal is decoded to select one of 24 bass notes. It holds until the next bass key is pressed, so the bass line doesn't glitch when a key is released mid-phrase. The bass AR envelope is triggered by the same priority circuit, so envelope tracking stays consistent no matter what the upper sections are doing.

The LFO is an R-C sine oscillator with a range of approximately 1 Hz to 15 Hz, set by the Rate knob. The Delay Time control sets how long the LFO stays stopped after a key is depressed before it starts oscillating again. When it restarts, the amplitude ramps up gradually instead of jumping to full.

The synthesizer VCF is a 24 dB/oct low-pass filter. Its control voltage runs through an anti-log circuit, and so does the VCA control voltage. The level-adjusting trimmer is mounted on each circuit board, so calibration is a per-section job rather than a global one. Players who have owned both the RS-505 and its descendants note that the filter's resonance is more aggressive than on later Roland string synths. The anti-log CV path gives the cutoff knob a different feel across its travel.

The control surface is dense. The strings section has Upper Strings and Lower Strings tablets, a 4'-8' Mix control (Upper/Lower), and an Attack control. The synthesizer section adds Upper 4', Upper 8', Lower 4', Lower 8', Bass 8', Bass 16', and Ensemble tablets. It also has Resonance, Cutoff Frequency, LFO Sensitivity, Envelope Sensitivity, Attack, Decay, Sustain, and Release controls. A Second Touch On/Off switch is there too. When second touch is active, the key trigger detector generates both key on-off pulses and separate trigger pulses. The bass section has Cello 8', Tuba 16', Contra Bass 16', and Ensemble tablets, with Attack and Release controls.

The pitch shift section has three modes: Auto, Off, and Manual. In Auto mode, the gate signal triggers the envelope generator, and the pitch rises to the level set by the Pitch Set control before decaying. In Manual mode, the trigger and Time are independent of the gate, and Pitch Shift sets the maximum shift voltage. That lets you dial in a fixed detune or a slow drift without a key press to start it. When the Ext Pitch jack is not connected, the Time, Pitch Set, and Normal/Down settings are invalid.

The RS-505 is 905 mm wide, 370 mm deep, and 145 mm tall, and weighs 14 kg. Power consumption is 14 W. The rear panel has output level switches for Synth/Bass and Stereo/Mono, and jacks for Stereo, Mono, Bass/Synth, Trig, Gate, Ext, Sustain, Pitch, and VCF. The keyboard is a 49-note C scale. The casing parts list in the service notes includes a music rack rail and bushing.

If you're servicing one, the ensemble modulator bias adjustment goes like this. Feed a 1 kHz sine at -20 dB into the Ext In jack and connect a scope to the Mono jack. Set the Ensemble switch to position I and increase the input level until the output is slightly distorted. Then adjust VR-71 for symmetrical positive and negative half-cycles. Test points TP7-301, -401, -501, and -601 let you check individual channels. If the ensemble sounds lopsided, check this first.

The service notes say the OPH107 circuit in the RS-505 is basically the same configuration as the OPH29 in the RS-505 Roland String Ensemble. So the pitch shift and envelope behavior are refinements of an existing circuit, not new designs.

The RS-505 was built while Roland was shipping a lot of professional keyboards, and the service documentation reflects that. The circuit descriptions are detailed enough that a competent tech can trace signal flow without a schematic for most sections. That's not common for 1978 consumer gear, and it helps keep these instruments serviceable today.