Before you invest a bunch of money, you need to
absoultely sure that your dish hasn't moved due to wind or other factors. Check the obvious things like elevation & skew settings and ensure that clamps on the dish and LNB are tight. If you have
any doubts, it would be smarter to start from scratch, trying to find the signal as if you were doing a new install, than to buy a new receiver or new LNB only to discover that they weren't the problem.
Signal strength on channel 712 it is at about 10
Hmm...this is kinda inconclusive......it appears that you're getting power from the receiver to the dish, since it's not zero. But I'm surprised it's not higher than 10. Background noise will usually raise the meter to about 20, but you should be getting this level on 299 also, and you're not.
Anyway, I'm presuming you don't get any audio/video on 712, or any other channel in your package...?
If you could beg/borrow/steal a satellite meter you could put it in line at the dish and see if you get a signal. But you would need one that is self-powered and does not rely on power from the receiver, and they're not cheap.
I don't really know how to isolate if it's the LNB under these circumstances. It's pretty tough to check the operation of the LNB if the receiver isn't operating properly. A new LNB costs about $60. I bought a spare for just such circumstances as this.
You could buy a new DSR-209 for the interim period until the "promised/rumored" new receivers come out. But if you buy a 505 now for retail price, you will still have to also pay full price for the new 6xx, being a southern sub. At least with a 209 you will have a temporary low-cost solution to SDef while waiting for the new line to come out. And, it would help to isolate the source of your problem.
Also, I'm presuming as a southern sub, you don't have a friend whose receiver you could substitute for yours to isolate the problem?