Depending on what type of connections your cable box has available you can do the following:
1) If yo only have the standard VIDEO & L R AUDIO connections you can use the DTV connection on the back of the V2500. If your cable box also has an S-VHS connection this would be the same connection to use.
2) If you have COMPONENT VIDEO connections on your cable box then you would still use the DTV connection. Looking at the back of your V2500 the regular DTV RCA connections are just to the left of center. Farther over to the left are the COMPONENT VIDEO connections. They are labeled Y, Pb, and Pr. These allow for the higher needs of HD TV transmissions. If your cable box has these connections then making use of them would allow for the best quality for your picture.
The following website has your manual available for download in case you need it:http://www.devicemanuals.com/guide/Audio%20and%20Video/Yamaha/Yamaha-RX-V2500-Owner%27s-Manual-TVRBek9UVT.html
Please let us know if you need any further assistance with your issue.