These are really freaking cool and test a lot of NES items. I have owned 5 of these so far, have 2 right now in fact. Each tester has a test cartridge inside, a NTF2 System Test Cart. There are 2 known versions, one white, one black and they are both in a Famicom style cartridge oddly enough.
The testers allow you to test controlers, fourscores, ROB's, Zappers, Powerpads, RF Swtichs, AC Adapters, RCA cables, game carts, and NES systems. They are super handy if you are a seller. You can test a huge pile of accessoires within minutes and know for 100% if they work or not. I recently just found an AC adapter that would power on a NES and make it work. I plugged it into the test station and it was failing the test. After closer inspection it had a short in it and could have burned down someones house had I not found it.
They also some with at least 2 different types of TV. The orignal TV type is a Panasonic. If you had to get a replacement one it was an RCA model.
They also have ports to plug in a SNES test station on the right side of it. The SNES ones aren't nearly as cool as the NES ones though, not nearly as many features.
The price on these is very odd. I have seen them sale for as little as 150 bucks, and for as much as 4000. I have sold 2 of them for nearly 800 bucks for just the test station. The test stuff is probably my favorite stuff for the NES, very cool items. If you want to know anything else just ask.
~~NGD
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Proud replier of post #1000 in the infamous Joel thread