Question about Bang and Olufsen Beocenter 9300 Shelf System

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Bang & Olufsen beocentre 7007 stereo music centre

Hi there, i've aquired a B&O 7007 and after connecting the speakers and activating the unit all came to life and the tape played fine, also heard some of the music being reproduced via the speakers.However, there is a 50Hz hum from both speaker units and the balnce, treble, bass controls have no impact on the sound itself, volume works fine as do all other function's.When the volume has reset i can stil hear a 25Hz harmonic through the speakers, i have not checked the set yet and will do so soon.I just wondered if anyone else had a similair problem with one of these as i never worked on a B&O unit before and this may cut the fault finding time down , i must say they are of an interesting design to say the least.
Thank you all in advance for any assistance or advice suggested.
Kind regards Rob. .

Posted by on

  • VintageValve May 22, 2008

    Thanks very much for your solution, i;ve finnaly had some time to connect my scope to it and way and behold a blockshaped wave apeared amongst the normal audio waves and this was clearly visable when fading from the left to the right channel.I've since replaced all 3 powersupply capacitor's (the originals where 4700 mf but i put 2x 3300 mf parralell per original cap, so we now have 6600mf per cap)and i must say it sounds amazing, the only problem i have now is that i need a new mmc4 phono cartrigde and the cheapest i've seen sofar is a american sound master copie of £79 not bad though as the original's go for well over £200.

    Kind regards.

  • VintageValve May 25, 2008

    Thanks for all of your help.

    I've replaced the rectifer bridge with some decent power dioides & it makes all the difference when powering up as the original bridge wouldn't have lasted very long.

    I really appreciate the time & trouble you've taken to help me out.

    I'm now working on a Panosonic quad amp which seems to use a total of 8 x 2sd217 bypolar transistors of which 4 have been replaced (not by me!) with the 2n3055 & so that particular stage is blown.

    This stage is the rear left & right output stage. I found some nice alternatives on Ebay but they are still quite expensive. Any ideas on an equivilant transistor (80v onwards), or any cheap suppliers?

    Many thanks, Rob

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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
amplifier.

Martin

Posted on Apr 24, 2008

  • Martin J Stadler
    Martin J Stadler May 23, 2008

    Now that you have upgraded the filter caps from
    4700uF to 6600uF, you may want to double check
    the ratings on the rectifier diodes, to make sure they can
    handle the increased surge current when you power up the
    beast.

    It's probably not a problem, but better safe than sorry.

    Martin


  • Martin J Stadler
    Martin J Stadler May 25, 2008

    Start a new thread, that way I get the credits.

    You way want to check the datasheets for the two transistors,
    (google them online)

    The 2N3055 is probably not the best replacement for the 2sd217.
    The 2N3055 was optimized for very high output current at low
    frequencies and relatively poor gain (hfe). It also has a lower
    collector breakdown voltage, compared to the 2sd217 and
    a poor power bandwidth. The low gain will cause more current
    to be drawn from the driver transistors, which is not a goos thing.

    I have no personal experience with the 2sd217, but the 2N3055
    is most often used in lo freq or DC applications, such as power
    supply regulators, sweep circuits or massively parallel arrays,
    such as the lens control in electron beam microscopes. It's
    not the best solution for audio hi-fi.

    For complimentary power amplifiers I prefer the Motorola MJ802,
    MJ4503 matched pair, if you can still get them.

    If you can fore-go the TO-3 package and use plastic transistors
    istead, I suggest the Texas instrument TIP-30, TIP-40 Series, which have better gain and better bandwidth. They are also much
    cheaper then the TO-3.

    What you need to consider:
    ====================
    HFE > 50
    Power bandwidth > 100K
    Vceo > power supply (+/- rail to rail)
    I max >> SQRT (2 x Output power (RMS) / 4 Ohms)
    P max > 2 x output power (with infinite heat sink)

    Heat sink thermal resistance such that:

    30C (ambient) + Power (RTH_junction to case +
    RTH case to heatsink, incl. insulator +
    RTH heatsink to still air)
    <
    100C

    If the amplifier uses a complimentary output (i.e. PNP/NPN pair)
    the transistors should be matched.

    Martin





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