Here are three ideas to double-check before spending money on another box.
First, did you go through your TV "Source" (or "input") menu to see if it might list a second HDMI port? If it does, it might be in an obscure location. (My main TV has a block of ports in back, but there is also a separate set of ports for plugging in from the side and could easily be overlooked. The TV it replaced had most of its ports in back but had one in front. On the other hand, my most recent TV purchase, for the bedroom, had all the ports in one spot in back of the TV. But in all three cases the "Source" menu listed all the ports.) It's at least worth checking if you hadn't already done so.
Second, does your cable box have a HDMI-input port? Though not likely, it is worth checking to see if, for example, it has an option to pass the HDMI-in to the HDMI-out. Well, it is worth a quick look and, if it is there, it may be a simple keypress on the cable box remote.
Third, there might be a different (non-HDMI) output that produces a HD signal for the TV. The two likely candidates are:
- Cable-out / Cable-in: If your cable box produces a HD/digital signal on the cable output, and your TV has a digital tuner, you may be able to use a short run of cable to run from the cable box to the TV. However, not all HD boxes with a cable-out produce a HD signal on the cable (the DVR I have doesn't), and a number of TVs manufactured between 2004 and 2007 were "HD-ready" or "HDTV monitors" and lacked the digital tuner, so a HD digital cable signal wouldn't be of any good to the TV.
- Component video: Generally a component video connection consists of five RCA connectors: Y (green), Pb (blue), Pr (red), Audio Left (White) and Audio Right (blue). (The Audio-out connectors may be positioned over at the composite or A/V port, but the three video connectors are almost always next to each other.) The capabilities vary on the device: 720p (HD), 480p (progressive, enhanced definition), 480i (interlaced, standard definition). (My DVR can be configured for any of those three.) If you have or can download the manuals, it may be worth seeing if the cable box can produce 720p on the component-out and your TV can accept 720p on its component-in, especially if you have to spend money to buy cables.
and Composite Video
) are standard definition (480i) only. Standard composite (A/V) ports have 3 RCA connectors: video or composite (yellow), audio left (white), audio right (red), and the same stereo audio out connectors are more often than not used by both composite and component video ports. The S-Video port uses a 4-pin mini-DIN connector and usually also requires a separate pair of cables for stereo sound. (However, one variant modulates the sound on the S-Video cable and I had ran one piece of electronics to the TV with both sound and video through the S-Video cable.) Therefore you don't need to check out S-Video, Composite video, or A/V ports if you want the cable box to feed a HD signal to the TV.
Anyway, those are three possibilities to check out before spending good money.