They were similar to Series 1 electronics, but closer to Anniversary in my opinion. The Hyak instruments did not have single coil pickups. Also each pickup had preamps and filters for each pickup that were summed together. With Series electronics, they shared the same card, with both channels using the same hum cancelling coil. Very different. Also the Hyaks were not 18 volts, but 36. They took 4 nine volt batteries!!!!
Back in the day, Hyak sold electronics packages just like Alembic does. So, there are Hyak electronics floating around somewhere without an instrument. I bet that would be hard to find though!
Michael