Got one of these working at last. Some re-wiring, a good scrub, remove some of the mods and a bit of guesswork.
I can only assume this isn't how it left the factory. The rest of the Arp is beautifully wired, but this is shit.
slightly less shit
I assume this is an external input jack, it had been rewired so turning the switch connected +15V to ground. Decided it was not a useful mod and removed it. If anybody is able to supply photos or a schematic showing how this is connected, please comment.
Again I don't know if this is standard or mods.
This is the side panel on one keyboard. The pot and switch are not connected to anything. The other pot & circuit put a voltage onto the external jack, but are not connected to the keyboard other than sharing power supply. It seems unfinished or half-baked, bit of a theme with this synth.