Starchoice tech showed up on Saturday, and I described losing the horizontal channels. He seemed quite impressed that I had narrowed it down to that. Told him I had a bit of help!

I already had a ladder set up, so he immediately went up there and took down the 8 month old quad LNB. He said it was hot to the touch. So he changed it and the missing channels reappeared.

At that point I was happy, but he made a big song & dance about the dish being misaligned and started fooling around with it. This in spite of the fact that the dish was professionally installed, solidly mounted, and had been working fine for many years. When he was finished, the signal strength was the same as when he started, so I dunno.

Anyway, swapping the LNB solved the problem with the channels, and the second receiver eventually stopped working as predicted. Thanks to all who helped in this thread, I learned a few things along the way. :cool: