My HP LaserJet 4 all of a sudden started printing black over the whole
page. Beneath the black I can see that the image I'm printing is
transferred onto the page, but it is mostly obscured by the toner
stuck onto the page all over.

My (very limited) understanding is that this may be caused when the
corona wire doesn't properly charge the drum. Is this correct? Is it
something else?

Is this problem caused by a broken corona wire? Is the corona wire
part of the toner cartridge, and if so, will replacing the toner
cartridge fix the problem? Is there someplace else to look for a
broken or disconnected wire? Is there a possible fault in the circuit
that applies a charge to the corona wire?

This LaserJet 4 has been a faithful and reliable workhorse for about
15 years. I'd like to find the problem and fix it, if possible.

Can anyone give me some insights as to what might be the cause and
solution for this problem?

Thanks,
Juan