Updates: November 2017
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Roland BC-13 Bus Converter

Roland BC-13 Bus Converter

Features and Specifications:

  • Converts 24-pin signals to 13-pin signals
  • 24-pin input
  • 13-pin output
  • Guitar input
  • Dimensions: 162 (W) x 29 (H) x 29 (D) mm
  • Weight: 170 g
  • BX-13-MICRO - Modern BC 13 Clone
Roland BC-13

Introduction to the Roland BC-13:

In 1989, towards the end of the original 24-pin guitar synth series, and the dawn of the 13-pin synths, Roland produced the BC-13. The BC-13 was specifically designed to allow 24-pin guitars to work with the new Roland GR-50 guitar synthesizer. The name is short for "Bus Converter 13." This is a neat little box that converts the 24-pin guitar signals into 13-pin signals.

There are three connectors on the BC-13: a 24-pin input connector, a 13-pin output connector, and a 1/4” guitar jack. Operation is simple: plug your vintage Roland guitar in the 24-pin connector, then use a C-13 (13-pin) cable to connect the BC-13 to your favorite 13-pin device.

Unless you were using a GK-1, you also needed to run a regular guitar cable from the 1/4” jack on the G-X0X guitar to the 1/4” input jack on the BC-13. Electronically, the BC-13 does several things: the high-output vintage GR signals are conditioned to match the lower level signals found in the new Roland GK-2/2A/3 style pickups. Similarly, the filter knob (CV#1) output is adjusted to match the new Roland standard, and several digital transistors are used to adapt the Mode switch and the resonance knob (CV#2) to new switching functions.

Guitar Controls and the BC-13:

In brief, you loose most of the controls that make a vintage GR-X0X guitar so exciting when using the BC-13! On the other hand, this should not be so surprising, since the current Roland guitar synths have basically nothing more than a volume knob and program switches on them. Anyway, it was the dawn of the nineties, and digital programmability was in, analog knobs and tweaking were on the way out. As a result of this "philosophy" change in design, the BC-13 adapts the vintage Roland control knobs, such as the filter (CV#1), resonance (CV#2) to do more mundane tasks. Here is a breakdown of how the BC-13 translates the controls:

BC-13 Controls

This diagram shows the controls on the Ibanez IMG2010 and various Roland guitars. Notice the layout is basically the same. Also, note the IMG2010 does not have touch plates to activate modulation. The vibrato (or modulation) knob, CV#4, is always active on the IMG2010.

Guitar Knob/Controller Original Function BC-13 Function
Volume Knob Volume Not Used*
Balance Knob Guitar/Synth Balance Not Used
Filter Knob (CV#1) Filter Volume
Resonance Knob (CV#2) Resonance Guitar - Guitar/Synth - Synth
Whammy bar* (CV#3) Pitch Bend Not Used
Modulation Knob/Touch Pads (CV#4) Modulation Not Used
Mode Switch "UP" Mode III Switch #1
Mode Switch "MIDDLE" Mode II Not Used
Mode Switch "DOWN" Mode I Switch #2

* The only guitar ever to use the whammy bar, CV#3, is the Ibanez X-ING IMG2010 guitar. Roland notes CV#3 on the schematics for the GR-700 and GM-70, but they never built it into one of their guitars.

Volume Level and 13-pin Synths

A look at the schematics of various Roland guitar controllers reveals that no two are wired exactly the same. As noted on the G-707 page, some G-707 guitar shipped with buffered outputs for the synth and volume controls, and some did not.

Likewise, the internal circuitry of a Roland GR-55, VG-99, GI-20 and GR-20 differs with each unit. As a result, maximum volume levels can change with different guitars or different synthesizers. This is as a result of an interaction between the volume output voltage and the input stage of the synthesizer, a phenomenon known as "loading."

As a result, some modern synths used with the BC-13 may max out with a volume level of 93, or 96, instead of the full level of 99 or 100. Modern 13-pin guitars have pull-up resistors, or voltage regulation circuitry, to prevent this problem.

Guitar/Synth Balance and the BC-13:

GK1-MIX.jpg

The BC-13 uses the Resonance Knob (CV#2) in a way that is very similar to the Mode switch on the vintage GR-series guitar. But rather than having a three-position switch, the knob has three "zones." The first zone is guitar only, the middle zone is guitar and synth, and last zone is synth only. Confusing? Yes. And you will notice the old Guitar/Synth balance knob is ignored.

The Extra Cable Connection*

Roland BC-13- Connections

Finally, unless you are using a GK-1, you actually need two cables to be able to control the volume of the guitar with a BC-13. You need both the special Roland 24-pin cable and a regular guitar cable from the 1/4” jack on the G-series guitar to the 1/4” input jack on the BC-13. With this extra cable in place, you can use the master volume knob on the guitar to control the volume of the direct (guitar pickup) sound. Otherwise, turning the volume knob on the guitar does nothing, the guitar is always on, at maximum volume.

With the Roland GI-10 this does not matter, since the Roland GI-10 does not have a regular guitar output. With the Roland VG-88 you can program the volume level of the normal guitar. But with the Roland GR-33 you have the guitar sound always loudly playing with your synth sounds.

The diagram at left shows the single cable used to hook up a GK-1 to the BC-13, and two cables for the G-Series guitar and the BC-13.

Comparison with GK-1

Roland BC-13 Cable

The GK-1 does basically the same thing, by using a short, 4” cable from the 1/4” output jack on the guitar to a miniature input jack on the GK-1. This way volume for the GK-1 direct (guitar pickup) level is controlled by the passive volume knob on the "host" guitar.

Why do you need an extra cable with a G-series guitar? Originally, Roland designs used a VCA inside the guitar synth (GR-100, GR-300, GR-700 etc.) to change the guitar volume by responding to voltages from the vintage guitar synth controller. The BC-13 ignores all these voltages.

Photos:

Roland BC-13 Roland BC-13 Roland BC-13
Roland BC-13 Roland BC-13 Roland BC-13
Roland BC-13 Roland BC-13 Roland BC-13
Roland BC-13 Roland BC-13 Roland BC-13
Click on any image for larger view.

Videos:

Schematics - Repairs - Service Bulletins:

There are no know service bulletins from Roland addressing any BC-13 problems.