You've lost SD reception (SD being 625-line PAL), but not HD,
right? As far as I know, SD and HD use separate tuners. (I think.)
I could be that the SD tuner has gone out.

** The RF stages do not change with SD and HD.

Do you mean just the front end, or the IF section, too?

Given his description, it's hard to believe it was caused only by a change
in signal level.

We'll have a better when he swaps the sets.