If the Hydra has got an AUI port, just buy a generic TP transceiver (<$10). Any type should do. Set the jumpers to 10Base5 or AUI and you're set.
If there's no AUI, you can look for a BNC/TP media converter or simply an old hub with TP and BNC. The latter you can probably pick up for free somewhere or little money on ebay. I've probably got one somewhere if I look hard.
Watch out for the somewhat non-standard BNC ports on the Hydranet: if yours has two connectors you'd connect one to the hub and put the terminator on the other. On the hub you use the standard T-connector plus terminator - as you would on the Hydra with only one connector.
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