I have a digital converter box which allows me to use this old analog TV.
If the digital converter box has an unused set of inputs (like RCA red/white/yellow) you can use those - then you will have to use the remote that came with the converter box to switch to that input.
Not all converter boxes have alternate inputs, in which case you need to examine how the converter box is connected to the TV.
1) If the converter box has a cable that connects directly to the antenna screw on the TV (this is a 'spade' connector) then you need to get an "RF modulator switch" such as this:http://www.amazon.com/RCA-CRF940-Modula ... =pd_cp_e_2
With this device you can switch between both the converter box and the Roku. There are less expensive ones - this one accommodates 4 inputs so you could hookup a DVD player and Tivo along side the Roku as well - but all you need is one with 2 inputs.
2) If the converter box has a colored cable (like red/white/yellow) going into your TV, then you may need a simple composite (or RCA) switch like this:http://www.amazon.com/RCA-VH911-Video-S ... _e_title_1
This one also supports 4 inputs - but you could get one that only switches between 2 devices for a little less.