Introduction to the Roland BAK-1 GR Bass Guitar Controller Assembly:
Roland sought to increase the user base of their guitar synthesizer players with the release of the STK-1, LPK-1 and BAK-1 retrofit kits. Up until this point, the only guitars or basses that worked with the vintage Roland 24-pin synths like the GR-300 or GR-33B were produced by the Fuji Roland partnership.
The BAK-1 is a kit consisting of the electronic components from the Roland G-77 bass guitar synthesizer controller, plus additional knobs, guitar back panel hardware, templates for guitar routing and miscellaneous wiring.
In all my years of searching for vintage Roland GR gear, I have seen only one BAK-1 kit offered for sale on ebay. They are rare.
Links to more information:
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Schematics Repairs - Service Bulletins:
There are no known service bulletins from Roland addressing any BAK-1 problems. The BAK-1 shares the same electronics as the Roland G-77 bass. Download the G-77 service manual (belowe) for BAK-1 schematics.
Download the G-77 Service Manual (pdf).