[sdiy] GEM / GeneralMusic SX-3 service manual

[Rutger Vlek]

Hi guys,

I'm working on a Gem (GeneralMusic) SX-3 workstation and have a malfunctioning display (vertical lines shown). I have been working from the service manual of the S2/3 synthesizers, as they are similar in architecture, but I've come to a part where they are really different from each other. So I was hoping someone here had access to the service manual for a SX2 or SX3, or at least schematics. Anyone?

Any additional knowledge sharing on the display control system is also highly appreciated. I believe the LCD itself is working fine, but I can see little activity in the Y-scan clock, or in the enable signal (E) for the display driver (SED1330). The latter may be too short a signal for my scope to register, although the S2 service manual says it should be at least 220nS in duration (which I can measure, but don't see in this SX3). As with most S or SX-series, this unit has suffered from battery leakage, so I'm focussing on the affected area, and signals running through this.

Additionally, another question for GEM conaisseurs: there's a pedalboard input (looks like a DIN-variant plug) that seems intended to be used with bass pedals. Anyone know which type of pedals could connect to this? Or what the control protocol is? I'm suspecting it's either a straight diode-matrix-scan input (but don't think there's enough lines to make up the required notes), or communication line to some kind of multiplexer or shift-register internal to the pedals. I have pedals from another brand here, with straight switch-outputs and I'd love to modify it to work with the Gem.

Regards,

Rutger