Roland CR-68
A primitive analog drum machine, and one that Roland's own service documentation treats as a serious piece of computer hardware.
Overview
There is no single date to hang on the Roland CR-68. No production years surfaced in the Archive's sources, and no original price either. What survives is the machine's own service manual, which lays the CR-68 bare down to the silicon. It is a primitive analog rhythm unit - a drum synthesizer, not a sampler - but the design underneath is serious for a box this simple.
At the center of the CR-68 sits a uPD8048, an 8-bit parallel microcomputer on a single silicon chip. The manual describes it plainly: a 1k x 8 ROM program memory, a 64 x 8 RAM memory, 27 I/O lines, an 8-bit timer/counter, and clock circuits. Roland used a specific masked part, the uPD8048C-015, with the program and data for the CR-68 burned into the program memory. So the rhythm set is fixed at the factory and can't be changed.
The CR-68 runs that little computer in two modes. In STOP mode, the uPD8048 scans the panel switches continuously in very short periods, pulses of several microseconds. Once the START switch is pressed, scanning drops to once per measure, just before the measure is initiated. Panel settings are read through Port 2 (P24-P27) driving a 74LS138 binary-to-octal decoder, with the switch matrix returning to Port 1. The manual uses the SWING switch as its example. Depress it, the decoder pulls its output low, Port 1 reads the condition, and the computer identifies the setting. Repeat across the matrix and every switch position is known.
Once the panel is read, the matching rhythm data is pulled from ROM and fed out through Ports 1 and 2. Two 74LS138s (IC8 and IC9) run in parallel to form a binary-to-hexadecimal decoder for this job, with Port 1 switching to output duty. The MANUAL button gets special treatment, because the computer only reads data once per measure, and a press landing between reads would otherwise be lost. An RS flip-flop built from IC4a and IC4b latches the event. It resets when START/STOP is tapped, and a NAND of the inverted read pulse with the ALE line from the processor produces a clean reset.
Tempo comes from a master oscillator, a multivibrator built around Q101 and Q102, whose period is variable from 10ms to 200ms via the TEMPO control (VR2). The service procedure is specific. Connect a scope to the Q102 collector, turn TEMPO full clockwise, and adjust VR1 for 10ms between fall or rise of the squares. Then turn TEMPO full counterclockwise and rotate VR3 until the period is 200ms, repeating as needed. If the bottom of the waveform is insufficiently saturated, the manual says to replace Q101 and Q102 with a new matched pair of the same rank. A separate CR oscillator built from IC3e and IC3f generates the roughly 3MHz clock that drives the computer itself.
The clocking arithmetic is the clever part for a machine of this class. To send 8-beat and 16-beat clock pulses out the TRIGGER OUT jack, the master oscillator's output is divided by three and then by six, using four MC14013B D flip-flops (IC7a, IC5a, IC6a, IC6b). IC7a acts as an inverter to shape the waveform and keep the divide-by-3 stage from misbehaving. The signal is then divided to produce 16-beat pulses from IC6b, and divided once more to produce 8-beat pulses from IC6a. A foot switch circuit for START/STOP and VARIATION uses a CR time constant combined with a Schmitt trigger to reject contact chatter, the same basic arrangement on both functions.
Voicing happens on a separate board, the VG-12. Three 74LS175 flip-flops latch the rhythm pattern pulses coming from Ports 1 and 2, clocked through IC2d and IC2e, and cleared by the master oscillator so the latch output is a 5V positive-going pulse with the same duration as the oscillator output. Those pulses are then converted by Q25–Q35 into negative-going pulses with a +15V–0V swing, which is what the actual voice circuits want to see. Accent is handled by Q21, Q24 and VR14: the accent pulse passes through Q21, is differentiated and integrated into an envelope, and drives the gate of an FET (Q24). With ACCENT at 10, most of the signal flows into the accent potentiometer and Q24, and very little into R137, giving the high-level output that reads as an accent.
There is also a leakage sound killer, Q20 and Q23, designed to suppress the transient thump the voicing circuits would otherwise produce at power on and power off. Q20 will not function normally until C79 charges sufficiently relative to its emitter, and the gate voltage at Q23 drops faster than at the drain or source after turn-on, shutting Q23 off.
The manual's voicing chart lists the individual sounds and their adjustment targets. The BASS DRUM is set up for a 62.5 Hz frequency with a 100ms decay and a 1.4 V-pp amplitude. The LOW CONGA runs at 208 Hz with a 4.8ms frequency adjustment and a 100ms decay. The LOW BONGO sits at 400 Hz with a 2.5ms frequency adjustment and a 40ms decay. The HIGH BONGO sits at 600 Hz with a 1.66ms adjustment and a 40ms decay. There are separate HIGH and LOW cowbell settings (800 Hz and 555 Hz), a RIM SHOT at 1,480 Hz with a short 5ms decay and a 4.4 V-pp amplitude, and CLAVES. The manual notes that these figures are factory standard and may be slightly deviated for personal taste or to meet the frequency response of an amplifier being used.
Two board revisions appear in the documentation. The GL-10B (part 142-010B), viewed from the foil side, is marked for serial numbers 822000 and up. Roland notes that GL-10A and GL-10B share the same circuit diagram, since some components are attached on the foil side or connected in series in a pyramid arrangement on the GL-10A and accommodated on the GL-10B in place. The manual also carries the standard warning for the era: always handle MOS ICs while wearing an earth-grounded wristband, and earth-ground all test equipment, to prevent electrostatic discharge failures.
A September 1982 item in Modern Recording (September 1982, p.26) noted that Roland had announced two new, updated models to replace the popular CR-68 and CR-78 - the CR-5000 among them - and described the CompuRhythm line as one of the most popular brands of drum synthesizers.
The Archive has no production years, no original price, no dimensions, no weight, and no confirmed list of rhythm patterns beyond the individual voices visible in the service chart. For this machine, the service manual is the primary document.
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