Roland MKS-20 (1986)
A rack-mount piano module whose circuit diagrams are dated June 1986 - and whose ROM board will still drop straight into an RD-1000.
Overview
The MKS-20 is a rack-mount electronic piano module. The paperwork that survives with it is unusually detailed for a box this size. The service documentation carries a circuit-diagram date of June 1986. The same drawing set lists the MIDI implementation as Version 1.0, dated December 20, 1985, so the firmware was locked down months before the hardware drawings were finalized. The manufacturer's own parts lists catalog the unit plainly as "MKS-20 Electronic piano module."
A lot of the MKS-20 is shared with Roland's flagship digital piano of the period. The parts list notes that the CPU B board (7935909002, PCB 2292529102) is the same board used in the RD-1000. The ROM board (7935905000, PCB 2292528800) is compatible with the RD-1000's ROM board as well. The only difference is that the MKS-20's board uses longer wiring, so it can be swapped into an RD-1000 but not necessarily the other way around. The MKS-20 is not a cut-down budget module built from leftover parts. It is the RD-1000's architecture repackaged into a rack chassis. Collectors who have chased RD-1000 parts will see why MKS-20 boards get scavenged.
The engine is a hybrid of digital and analog. The tone generation runs through a ladder resistor network current source, and the service manual reproduces the pin configuration and truth table for the converter stage. The effects and equalization section is built around a pair of CEM 3360 dual VCAs, a μPC1252H2 bipolar analog IC, and BBD (bucket-brigade device) delay lines for the chorus. The chorus is not a digital effect. It is a genuine analog BBD circuit with a clock that has to be trimmed so the right channel matches the left, and the service manual devotes an entire adjustment procedure to it. The effect board carries separate trim pots for BBD clock, BBD bias (one per channel), compressor, and MID Frequency, all set with an oscilloscope in test mode.
The equalizer section is more sophisticated than the "bass/mid/treble" labeling suggests. The MID EQ is a state-variable bandpass filter. Its center frequency is set by the MID FREQ control, and its Q is switched by the MID WIDTH control turning Qs 35-37 on and off. The bandpassed signal is routed through two VCAs (IC45a and IC45b) driven in opposite directions by ICs 46a and b, so the same band can be either boosted or cut depending on the MIDDLE LEVEL setting. Roland's own circuit description admits the obvious problem: changing EQ parameters drastically alters the sound energy, so the same note at the same velocity comes out at different perceived volumes depending on where the controls sit. The fix was a third VCA (IC31) fed an EQ LEVEL control signal that is inversely proportional to the resultant parameter setting. That is automatic gain compensation in the analog path. Most manufacturers of the era simply ignored the problem.
MIDI implementation is functional rather than generous, and the manual's chart is blunt about it. The MKS-20 receives on channels 1-16 (memorized), recognizes Mode 1 and Mode 3 plus POLY and OMNI ON/OFF, and responds to Note On with velocity from 1-127. Note Off is recognized but velocity is ignored. Pitch bend and aftertouch are not implemented, and the chart marks both with an ×. Control change support covers just seven numbers: foot control (CC4), volume (CC7), expression (CC11), damper (CC64), soft (CC67), tremolo (CC92), and chorus (CC93). There is no system exclusive, no song position, no song select, and no tuning. Program change is recognized across a range of 0-63, but the manual notes that the assignment of received program changes to the internal bank is fixed, with the mapping spelled out in a table. Owners report that the chorus control changes can be ignored entirely depending on how the unit was powered up. Holding the VOICE 1 switch while switching on disables that response, which helps if you want the module to behave predictably under sequencer control.
Note numbering catches people out. The recognized range is 15-113, and the manual is explicit that note numbers outside that window are transposed to the nearest octave inside it. If a controller sends the full 0-127 range, the extremes fold back rather than drop out. It is a quirk of the era, not a fault.
The power supply came in three line-voltage variants - 100V, 117V, and 220/240V - and Roland's service documentation notes that the three versions are identical except for the fuses. The 100/117V primary fuse is a 1.25A SD6, the 220/240V primary is a 315mA CEE, and the secondary fuses differ by voltage as well (1.6A and 1A for 117V; 1.6A and 1A BET-T types for 220/240V). The transformer itself (22455442U0) is a multi-tap 100/117/220/240V unit, so in principle a tech could rewire a unit for a different mains voltage by changing the fuse values and the tap. The manual warns, though, that replacement boards may be supplied from a different voltage category, so the fuse rating must be verified before powering up.
There is also a documented production change. The CPU-A board went through a revision. From serial number 662750 to 674199, a 10kΩ current-limiting resistor (R38) was added between the backup battery and diode D2. From serial number 684200 onward, R38 was changed to 1.5kΩ to reduce the voltage drop across it on power-off, which prevents possible data erasure in the RAM IC8. Early MKS-20s are therefore more prone to losing their memory when the battery gets tired, and the fix is a resistor swap that a competent tech can do in an afternoon. If a unit's user memory behaves erratically, check this first.
The rest of the parts list reads like a snapshot of mid-1980s Japanese electronics: HD63B03RP CPUs, three R06-series gate arrays, TMM2764D EPROMs, MB8416 and TC5517 static RAMs, and an 8MHz crystal on the CPU-A board paired with a 12.8MHz crystal on the CPU-B board. The manual notes that the high-speed MB8416A-12P-SK-G (or HM6116) can substitute for the slower 20LP-G and TC5517APL parts, but not the reverse.
Mix (January 1986, p.80) caught the MKS-20 at AES, where it was shown for the first time, and described it as very impressive while noting the hype surrounding it — a fair summary of a module that arrived with the RD-1000's reputation attached. A decade later, the used market had priced it as what it always was: a rack-mounted RD-1000. Sound on Sound (November 1996, p.260) listed an MKS-20 in its classifieds at £450, described parenthetically as "RD 100 in a rack" — which is precisely the shorthand the module earned for itself.
The MKS-20 is not a general-purpose synth module. It is a piano module with a fixed set of voices, an analog chorus and EQ section that give it a character most digital pianos of the period lacked, and a MIDI spec that will feel cramped next to modern gear. It is also the cheapest way into RD-1000 tone generation, and its boards are the same ones people pull when an RD-1000 dies.
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