Well, no one replied so I will just give you general help but not specific.
Somewhere on that modem, it should mention what voltage your device take most likely on the rubber tab area where you plug your power in. But some won't say it so you will have to search for your device's manual on the net and go from there.
Also I doubt that the modem this old (most of us runs high-speed connections), many of us will have the same hardware so it would be hard to get help here.
But if someone still runs 33.6 modem, please post.