I AM TRYING TO HOOK UP A ROKU TO AN OLDER TV USINGMODULATOR. i GET BLACK LINES. nO PICTURE? WHAT AM i DOING WRONG

Forgive me I am out of touch with modern techno stuff but I think this is some media streaming gizmo if so then your TV would need possibly HDMI or at least a 15 pin svga port found on flatscreens. Most older TV's would have an aerial socket, scart and phonos of one sort or another (with the odd BNC and DIN type thrown in).
Now if this is so, I doubt you will manage it at all your ROKU is chucking images out at a rate that is far beyond any old system to work with. A modulator say those £30 thingys that have a scart or phonos can only cope with basic video or rgb.
Your ROKU would have to "slum" it down to RGB or COMPONENT level before you could get that converted down to say a modulator with basic video by then its quality would be very gone. To understand deeper look for the word Scaler in up and down this is a device to alter images from one format to another found in say TV systems, though for you I would get a more modern flatscreen that has suitable sockets to match your ROKU plenty of say older models with less features than their newer cousins and have still older style connectors for older secondary equipment (video) to work with.
I am working a bit by guesswork over what you have there to work with, some convertions are easier less complex than others.
Hope this helps a little.......