Bang & Olufsen beocentre 7007 stereo music centre
The frequencies do not sound right, unless you are in Europe.
Low frequency hum almost always results from:
1) stray input pickup = Damaged cables, poor shielding,
Missing ground connections,
or ground loops.
This pickup occurs at the power line frequency:
50 Hz in Europe and Asia
60 Hz in North America
Harmonics (multiples) may occur as Fourier components
from signal distortion:
50, 100, 150, 200 .... in Europe
60, 120, 180, 240 .... in North America
Sub-harmonics such as 25 Hz are mathematically
impossible to derive from power line frequencies unless
there is some kind of weird inter-modulation, mixing
or frequency beating going on.
2) Defective power supply most often occurs at twice the
power-line frequency because of full wave rectification.
100, 200, 300... In Europe
120, 240, 360... In North America
Except for switching supplies, like those in a computer,
which can produce a high freq whistle at any frequency.
3) Amplifier feed back and other malfunctions can produce
other low frequencies, often by rectifying and detecting
the envelope of very high frequencies in the tens or
hundreds of megahertz. This is called motor boating
and it sounds more like a revving motor boat than hum.
(This kind of behavior is unlikely in a well designed amplifier)
By correctly identifying the noise frequency and
the environment, it is possible to narrow down the culprit:
1) Have you actually measured or observed the frequency
on an oscilloscope ?
If not, do you have the equipment to do so?
2) Where are you, and what is your power line frequency?
If you have line frequency noise, possibly stray pickup,
check the cables, shields, make sure the turntable is
grounded and check for ground loops created by long
runs of parallel shielded cables running multiple grounds
between two auxiliary devices.
If you have twice the line frequency noise, check the
power supply filter capacitors and voltage regulator
chips and/or transistors within the power supply.
Power supply filter capacitors (electrolytic type) can dry out,
degrade and die during long periods of not being used.
They need to be periodically charged to regenerate the
dielectric insulation coating.
Also if you are actually getting line frequency noise
from the power supply, then check the rectifiers.
Full wave rectifier => twice the line frequency
Half wave rectifier => line frequency
A full wave rectifier with a blown diode behaves like half-wave.
From the description of your symptoms, hum is not affected
by input switches nor controls, I would suspect the the
problem is in the power supply feeding the final amplifier,
or perhaps in a broken feed-back loop within the output
amp, causing excessive gain and motor-boating.
A burned out power transistor in the output amplifier
can also cause bizzare oscillations, as the negative
feed-back circuit is trying to balance the DC operating
point of the amplifier but it can't.
Forget the voltmeter. If you have an oscilloscope, check
the power supply ripple, and scope key points in the output
Apr 24, 2008 |
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