Roland GM-70
A guitar-to-MIDI interface that shipped with a factory correction sheet longer than most owners ever saw.
Overview
The GM-70 is Roland's guitar-to-MIDI interface. The factory MIDI implementation chart lists it as version 1.00, dated Nov. 11 1986, and the chart itself is stamped MAR. 1987. So the firmware spec was frozen in late '86, and the printed documentation caught up the following spring.
The surviving service documentation shows a unit Roland kept revising after it left the factory. A service bulletin lists three distinct production revisions, each tied to a serial number break. Units prior to serial 740870 shipped with one set of behaviors. From 740870 up, the primary fuse was deleted and replaced with a jumper, and the fusing function moved to the fuse in the power transformer. From 752450 up, the unit got a software reset fix. The bulletin describes the original reset feature as unreliable, though the OCR truncates before the full description of what was unreliable about it. From 753118 up, Roland reworked the hardware to meet VDE and FCC requirements. That meant adding a capacitor with reconnected IC pin arrangements, eliminating the EMI board and ferrite beads (F1-6 and F8-16 removed and replaced with jumpers), and relaying out the main board to take a new 24-pin connector assembly, part number 7936317000.
That last change is the one that matters to anyone repairing one today. The service documentation is explicit that the replacement part for the EMI board with 24-pin connector, as fitted to units below serial 753118, is supplied as the 24-pin connector assembly only. When replacing it, the patterns on the main board must be reconnected as shown in the bulletin's Fig. B. The board layout changed mid-production, so a repair on an early unit means following the rework procedure rather than dropping in the later assembly.
The MIDI implementation is thorough, the way late-'80s Roland documentation tended to be. The GM-70 transmits system exclusive data in four selectable groups: entire memory, the first half of the 128 patch memories (patches 11 through 88), the latter half (patches -14 through -88), or system memory. Each patch consists of 80 bytes and is sent by a block including 160 data bytes. System memory consists of 32 bytes and is sent by a block including 64 data bytes. The format is Roland's One Way Transfer with a 21-bit logical address, and the model ID 0001 0001 identifies the GM-70. The checksum rule is stated plainly: the summed value of all bytes between Command-ID and EOX must be 00H, including both the Command-ID and the EOX itself.
One detail for anyone working with the unit: the documentation notes that under usual performance conditions, the exclusive message is neither sent nor received. Dump and load only happen when selected through panel operation. The GM-70 is not constantly talking in the background.
The receive logic checks incoming data at every stage. The Roland format must start correctly, the received Device ID must equal the Control Channel, the next logical address must be correct, the checksum must check out, and EOX must follow at the end. Fail on any of these and the GM-70 shows DATA ERROR in the display and returns to playing mode. If fewer blocks arrive than expected, it waits. If more arrive, it ignores the excess. Even if loading goes wrong midway, the data received so far is stored into memory, and loading can be aborted at any time by pushing any button on the panel. The Control Channel resides in system memory, so it does not change until the system memory block is fully received.
The foot volume issue would have annoyed working guitarists. On units prior to serial 740870, MIDI Control Change data representing the maximum value of the foot volume connected to the GM-70 sometimes fails to reach 7FH. A receiving unit set up to recognize 7FH as a switch-on condition would fail to toggle. The pedal goes all the way down and nothing switches. Roland fixed it in the 1.01 firmware revision at serial 740870 and up.
The same revision also corrected choking behavior in Poly mode. If only one string is plucked, choked via MIDI Bender Change, and then muted at a high pitch, a receiving unit could have a sound whose pitch falls toward the original during the release period. The longer the release time, the more noticeable the artifact. The cause, per the documentation, is that Bender Change for the original pitch is transmitted alongside the Note Off when muting. A second, cosmetic fix in the same revision: the Control Assign display for C94 showed "SELES DEPTH" and was corrected to "CELES DEPTH."
That's the honest framing for the GM-70's era: the engineering was serious, the documentation was meticulous, and the fundamental problem of tracking a guitar string cleanly was not yet solved.
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