On edge of the board between the USB port and the power socket are the connectors for the bed and hot end. The bed is next to the USB, the hot end is next to the power. If Repeiter-Host is not throwing up any errors, check those connectors.
I had one that had got hot (due to a poor connection...resistance = heat), and melted the plastic connector, which interrupted the power.
If you don't find a connector problem, you can use a voltmeter to check those transistors (MOSFETs I think); one each for bed and hot end. Be careful not to short anything out. If the previous owner was goofing around, it's possible they blew the transistors. It's cheap and easy to get replacements and solder them in. Also visually check the traces on the board for any damage, etc.
A lot of people talk smack about the Printrboard, but I've had good luck with mine. Any problems I've had have been self-inflicted. I've even modded both of mine for g-code fan, etc. Once you get it going, there are numerous mod and upgrade paths.
Once properly calibrated, and with a few tweaks, the SD4 can be a reliable printer with excellent output. I routinely run prints of 6-9 hours endurance, and my record is a 32-hour print. Prints from my SD4 look better than the majority of what's on Thingiverse!
