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The basic layout is the same for both the G-303/G-808 and the G-505. The main difference is the kind of trimmers used for the cards. The G-303/G-808 has slightly larger trim pots than the G-505. However, the trimmers for the G-505 are arranged in a row. The G-303/G-808 has staggered and slightly offset trimmers.
The G-505 has holes in the access plate allowing the player to adjust the trimmers without removing the back plate. For G-303/G-808 users, the rear electronics cover must be removed to adjust the trimmers. The only other difference between the two cards is the method used for mounting the controls. On the G-303/G-808, most of the controls "float" on stiff wire, supported by brackets, rather than being attached to the board. This is to accommodate the carved top of the G-303/G-808 . The G-505 controls are mounted directly to the electronics card. The G-202 board is in many ways a simplified version of the G-303/505/808 board. The hex fuzz uses a single gain stage, and the circuit board itself uses a crude dual-layer design. |
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Ibanez IMG-2010 Electronics:
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The IMG-2010 improves on the standard Roland package in three ways: first, the IMG-2010 has control voltage buffering circuits for control voltage stability.
Secondly, the trimmers in the IMG-2010 adjust the output string level from 60% to 100%, rather than the 0% to 100%. By limiting the control range, there is more precise control over the string output level. Finally, the IMG-2010 does not use the fragile ribbon connector found in every Roland guitar. Instead, individually insulated solid core wires connect the electronics board with the 24-pin connector. The Roland ribbon connectors become brittle and easily crack with age. |
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Roland G-33/G-88 and G-77 Bass Electronics Card:
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The early G-33/G-88 card looks much like the sibling G-303/G-808 card. In addition to the essential synth controls, the card features a sweepable equalizer designed for bass guitar.
The G-77 card, also on the rare Steinberger bass, has a simple tone control knob plus a pickup balance knob for blending the output of two pickups. In addition, notice that on the last new product in the GR-series controllers, Roland switch to solid core wires for the synth pickup, similar to the Ibanez IMG-2010. |
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G-303/G-808/LPK-1 Version "B" and "C" Electronics Card, and Rare Version "A" Card:
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I was contacted by a G-303 player in the States and a G-808 player in Norway both using the first run, early "prerelease" version of the guitar electronics. The cards are labeled as version "A." The published Roland documentation supports version "B." These rare cards are unusual in several ways: the component layout is very different and the outputs trimmers seem to be arbitrarily placed on the board. You can also see jumper wires soldered across the board. In addition, the standard 1/4” output jack solders directly to the PCB, rather than the ribbon connector.
The first time I tried to repair a failed op-amp in a G-303, I realized that the pin-out documentation was wrong on the schematic. The pin out information was correct on the G-505 service manual schematic, which uses almost the same circuits. All the G-303s and G-808s that I have checked have op-amp pin outs consistent with the G-505. Which made me wonder if the op-amp information in the original G-303/G-808 service manual refers to the very rare "version A" board. It is difficult to see in the photos, but on the standard cards, version "B" and "C", three op-amps, IC1, IC2 and IC3 are used to amplify the signals to line-level for the GR-300. These op-amps are the top three 4558 op-amps in line with the 24-pin ribbon connector. IC4, IC5 and IC6 op-amps are used to create the hex fuzz sound. This is consistent with the G-505 schematic. The line-level amplifiers are surrounded by resistors for a simple gain circuit, and the hex fuzz amplifiers have the network of diodes used to create the fuzz sound. From looking at the version "A" photo, it appears the one op-amp is used per string to both amplify the signal and create the hex fuzz sound. If you look at IC6, at the top of the version "C" card, you can see resistors just to the left of the chip creating gain in the negative feedback loop, and additional diodes just to the right side of the chip for generating fuzz. This is consistent with the G-303/G-808 schematic. If anyone has any additional information on this card, I would love to learn more about this very rare electronics assembly. The larger photo came from a G-303 purchased from Dave Crocker at Fly-By-Night Music in Neosho, Missouri. |
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Two Hex Pickup Standards: Wide or Narrow
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Recently, it has come to my attention that there are actually two different standards for the hex pickup. Most obviously, one pickup is wider than the other, 11 mm wide versus 10 mm wide. It is a pretty small difference. However, after conducting resistance tests, it is now clear that one pickup has ten times the impedance of the other! The "narrow" pickup measures around 80 ohms per pickup, while the "wide" pickup measures around 900 ohms per pickup. Quite a difference! Based on my experience, the "narrow" pickups seem to be found in earlier model guitars, and the "wide" pickups in later guitars. I have worked on G-303 guitars with both pickups installed.
I cannot find any published specifications that clearly note this difference. But there are some differences in the circuit design. It appears that electronics cards designed with the lower impedance "narrow" pickup have a gain ratio of 330:1. The G-505 schematic shows a 330K resistor in the negative feedback loop, with a 1K resistor to ground. And the original G-303/G-808 schematic shows a 330K resistor in the negative feedback loop, with a 1K resistor to ground.
Likewise, electronics cards designed with the higher impedance "wide" pickup have a gain ratio of 33:1. The G-707 schematic shows a 330K resistor in the negative feedback loop, with a 10K resistor to ground. A similar ratio is found with the LPK-1 card, which has a 33K resistor in the negative feedback loop, with a 1K resistor to ground.
However, there is no reason that there should be any consistency with these standards. I worked on a LPK-1 card which had the 330:1 gain structure rather than the 33:1 design shown in the LPK-1 schematic. When I installed the correct "wide" pickup for the 33:1 circuit, the LPK-1 would work with the GR-300, but the internal trimmers were very sensitive. After a quarter-turn, the gain would be too high for any kind of dynamic control. The solution was to replace the 330K resistors with 33K resistors. After this change, everything worked perfectly. I also checked the gain settings with the more sensitive GM-70, because the ladder LED display gives a much more accurate reading than one red LED on a GR-300.
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Replacement Resistors to Change Gain Structure:
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GR Hex Pickup Test:
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A vintage Roland GR pickup consists of the divided pickup plus an attached ribbon cable. Be careful when working with these pickups, as the ribbon cable will become brittle with age and is easily damaged. There are 12 contact points on the ribbon cable. The first seven are for the hex pickup, and they include a common or ground point, plus the six string outputs. Reading from the long edge of the ribbon to the solder points, the outputs are strings #4, #3, #1, #2, ground (common), #5 and #6. Refer to the diagram below from the service manual.
The next five contact points are actually for attaching wires to the top panel guitar electronics. Reading in the same direction, they are the p-touch, p-touch lock, the guitar pickup output, ground, and the normal guitar out. Refer to the diagram below from the service manual.
To test each element in your pickup, attach the ground lead from a multimeter to the ground (common) contact point for the hex pickup, then move the positive test lead to the various pickup outputs. Depending on whether you have a wide or narrow pickup (see above), a working pickup element should read 80 to 90 ohms for the narrow version, or 800 to 900 ohms for the wide version. If you read between two pickup output points, you should see 160 ohms or 1.6K (depending on the version of pickup). This is double the reading of an individual pickup. If you get a reading of 0 ohms, or an open circuit reading (infinite resistance), then you likely have a damaged pickup.
Comments:
Surprisingly enough, there are many variations in the design of the internal electronics of vintage Roland guitar synthesizers. Most notably, there is no hex-fuzz on the Roland G-707, STK-1 and Ibanez IMG-2010 guitar. The STK-1 kit is almost identical to the G-707, with only some minor changes to components.
"Buffering" means that there is a simple circuit between the control on the guitar and the electronics in the synthesizer. The circuit buffers the voltages so that the synthesizer does not "load down" the voltage. For example, the maximum filter voltage should be around 14 volts, but by loading down the circuit, the actual voltage is only 12 volts. Roland BC-13 users are familiar with this problem. If they are using the Roland BC-13 with a G-505 guitar and a Roland GI-20, they may find that their MIDI volume only goes to 119, instead of 127.
This problem continued for Roland, until a few years into the new, GK-2 standard, where they started installing a "pull-up" resistor to the +7 volt power supply on the synth volume control. This guaranteed that the synth volume would hit 127. The problem with this "pull-up" design is that it reduces the control range of the volume knob. It has been my experience with 13-pin equipment that you reach maximum output with the volume on the guitar at "7." Moving the volume knob from 7 - 10 does not incease the volume. A properly setup system, without using the "pull-up" resistor, should result in a smooth increase in volume across the range of motion of the volume control.
EMI Filter on the GM-70 and GR-77B:
For a brief period Roland installed EMI filters on some of the guitar synth products. Some GR-77Bs and BAK-1 kits have a modified 24-pin ribbon with 16 1K ohm resistors installed to control EMI signals. And a few GM-70s, early models, had a more elaborate system with similar resistors, and some additional filters, to control EMI signals.
Only the power supply lines and ground connections are unaffected.
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Summary of Guitar Controls:
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