Atari CX2600A parts...

What's the parts availability like for Atari CX-2600A consoles? I've been offered one locally which the owner says "has a missing knob" - at the moment I'm guessing that means one of the four control levers on the top has been snapped off (but I'm awaiting clearer info). I'm totally unfamiliar with these machines; if that is the case, I don't know if that also means there's likely to be PCB damage or if it's more of a cosmetic issue? They also say they can't find "a connector" to hook it up in order to test it - that's a bit light on detail, too! Again, I'm totally unfamiliar with these critters - do they have a built-in RF modulator and just use a standard co-ax cable to the TV? Or are they composite video output? Or something else entirely? What about PSU; is this internal, or an external brick? I expect its value in current untested state is peanuts (which might end up being a problem if the seller equates age with value) - even tested, complete units with a similar amount of games don't seem to sell for very much. It'd be nice to get a handle on cost to fix any external issues though (setting aside any electronic problems which might be lurking - are there any gotchas in terms of these beasts being known for bit-rot?) cheers Jules (never an Atari owner!)