HMV Model 11 pick-up

I bought this years ago, but it never worked, so the other night I desided to rebuild it. I replaced all the rubber, as well as the radio leads which had also perished. It now works, but has a buzz coming through it. I've tried swapping the polarity of the lead wires around, but it made no difference. It doesn't have a ground wire, so I'm just not sure whats causing it, or how to try and fix it.
Any idea's??

Is this a magnetic pickup?

If so, the shielding may be missing. Or it may be that a strong hum source is present which was not there when the unit was designed.

Did you use a shielded cable? Or is it twisted pair?