I see a lot of old set-top DVRs going for cheap ($20-$30) locally; most of them don't support HDMI but I'm not concerned about that. Is there anything preventing them from being used as a generic component+s-video+composite capture device? Are they locked to the TV networks' signal somehow or have some stupid DRM that prevents copying videos off the HDD? I don't really know anything about these so just looking for advice before I plunk down the cash for one.

( Before anyone suggests "just get an EasyCap of Ebay/Amazon!!" I've tried that - more than once - and had no end of problems with them. If you actually manage to get one that isn't fake it's a total crapshoot whether the one you get will even work with the only available Linux + Mac unofficial drivers. Not going through that again. I'm looking at DVRs specifically because I want a standalone device. )