Zinwell ZAT 970A issues; it deleted some of my channels and I can't get them back. Help?

We used a Zinwell ZAT 970A digital converter box to get TV at home. The reception is usually bad but recently it was completely black on several channels. I decided to try restoring it to factory settings and had it rescan for channels, but now I am missing about 2/3s of my old channels! No NBC, ABC, Fox, or many others. Now, I'm thinking that the problem may have been that the signal for those channels was down and that's why we are having trouble, but I have no place to check.

I've tried manually searching for stations, searching frequencies, and even tried upgrading the box but nothing has helped. Is there some resource where I can see if digital signals are broadcasting in Los Angeles at any given time? Anyone else ever dealt with this problem?