>A few years ago I used to work beside someone whose old CRT
>monitor had a very noisy flyback transformer. The 15kHz noise
>drove me absolutely nuts yet no-one else seemed to be able
>to hear it. I was always turning the sqealing thing off when
>she wasn't around.

That typically happens when a component has "sorta failed" and the output
stage waveform isn't exactly what it should be.

>I haven't had any problems like this for a couple of years.
>I'd like to think it's because most monitors are LCD these
>days, but more likely as I've got older my hearing doesn't
>stretch to 15kHz any more.

We have an LCD monitor around here that does it.

The problem with this sort of noise seems to be components in a flyback
stage which either have a combined component tolerance across the whole item
which makes the operating waveform non-optimal, or a component fails in a
manner which allows the circuit to still operate, but again the operating
waveform is non-optimal, but not badly enough out of spec to cause major
failure of other components. Almost anything that does high voltage
conversion seems to be susceptible to this - CRT output stages, LCD
backlight units and switchmode power supplies all seem to be capable of
doing it.


But Jinxs' original problem (or more correctly his friends problem) could be
caused by bolt corrosion on the tower rectifying RF, and producing either an
audio sound, or having a high enough voltage across the corrosion to have an
arc, although I suspect the latter would require such an RF field that any
person in the vicinity would have a distinctly warm feeling all over.
Another alternative may be metal joints within her house (- does it have a
galvanised iron roof?) doing the same thing. I have heard of bed springs
rectifying signals from AM radio stations, as well as metal tooth fillings
causing problems when cracked. The last one should be easily verifiable by
going to another place with high RF fields - get her to visit you Jinx and
go for a drive along scenic drive through the ranges behind you, past
another BCL tower ;).

And finally, it is not something silly like the noise of the wind blowing
through the tower (any nearby trees cut down recently to allow this to
happen)? I doubt this, as I get the impression the noise is constant, and
this would vary according to wind speed. It would also be lower frequency
than suggested, but still worth a consideration, ad be eliminated as a
source.


Another possibility - is it really coming from the tower? It could be coming
from the fuse board, and be due to a ground fault condition, causing ground
currents from neighbouring consumers to come down the neutral and then to
ground through her fuse board. Especially if there is a 3 phase consumer
nearby (I get the impression she is in the country, so a neighbouring farm
may well have 3 phase for a milking shed or other plant) and that can create
a 600Hz buzz which could sound like she suggests. It could even be the power
to the transmitter site itself, that would almost certainly be 3 phase, and
could well be higher than the 230V, which might mean arcing on the
insulators in wet weather ... you lot have been having an inordinate amount
of floods etc recently.

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