Valley People 610 Hum problem

Hello

I picked up a second hand Valley Audio 610 compressor recently
I like the sound and features of the unit but it has an annoying hum problem.
Its pretty much unusable when fed from unbalanced inserts of my chilton desk - its reduced slightly when using cables from rane note 110
But even when part of an entirely balanced signal chain whether it be
- straight in/out of my edirol interface
- or after a preamp and into the desk
there is always a hum - not as bad a with unbalanced cables but still annoying
The Hum disappears completely when the bypass on the compressor is pressed - both channels behave the same
I recorded a bit in logic - its seems to be around 100hz
My studio setup is far from perfect with regards to grounding but feel its more a problem with the unit - none of my other gear behaves like this
Any ideas of what I could do to fix this?

Thanks

Tom

Does it hum with 1:1 transformer in series with input.

May be bad solder connection for one side of the differential input.

JR