Vortex Sound Research

Battery noob here. The batteries in my 606 ran out and the ones I got to replace them aren't working. I read something about how Um-2 C cells aren't common anymore so I got a pack of these: Duracell Plus C Alkaline 1.5V LR14 MN1400.
Nothing. The old batteries still power the thing up so it's not a fault with the unit. I have noticed that the replacements I bought a quite a bit heavier than the old ones. What should I be looking for as replacements if not the above?