I have a Panasonic PV7520 VCR, at least five years old, that a few
days ago stopped playing back in hi-fi stereo, but only tapes recorded
by it in the past few days. Tapes recorded earlier still play back in
hi-fi, and recently recorded tapes play back in hi-fi on other VCRs.
Changing cassettes and recording speed make no difference. This
isn't just a matter of the VCR's hi-fi/normal indicator not working
because I can hear the difference when I manually switch between hi-fi
and normal audio.

I tried cleaning the VCR by hand again, but it didn't change anything.
I clean it about twice a year, previously about three months ago, by
holding the leather swab stationary and rotating the cylinder back and
forth.

Could this be the mode switch? I thought that it wouldn't matter
since hi-fi/normal is switched electronically, isn't it?