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Old 4th Dec 2023, 10:38 am   #1
agardiner
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Default Strange stereo problem - B&O 900

Hi,

I am hoping someone will be able to explain what is going on so that I can troubleshoot a B&O 900 stereo receiver.

It has previously been recapped and working fine. There is now a very strange problem when receiving a stereo FM broadcast; mono works fine.

The stereo broadcast is working and in stereo, but sounds odd - the mid range frequencies are all lifted; higher end appear fairly normal, bass is very much reduced and the overall volume is higher than normal. It sort of sounds like a phasing problem with one channel perhaps out of phase. However, disconnecting either speaker does not make it sound any better.

The obvious problem would be in the stereo decoder which is a separate PCB as it was an option back in the day. However I have tried another decoder board with the same effect, and I have tried the original board in another unit and it works just fine. As already said, mono sounds perfect, including on a stereo broadcast where the mono switch is engaged.

Can anyone explain what might be going on and point me in the right direction?

Thanks.
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Old 4th Dec 2023, 11:49 am   #2
stuarth
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Default Re: Strange stereo problem - B&O 900

I had a problem donkeys years ago with a B&O unit which appeared to be in the stereo decoder. On those early B&O sets, the switching transistors in the decoder which do the stereo demuxing are biassed through the "Stereo" bulb. A failed bulb gave odd results, depending whether the stereo pilot tone was present or not.

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Old 4th Dec 2023, 9:25 pm   #3
Mr 1936
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Default Re: Strange stereo problem - B&O 900

Hi

From the audio quality you describe, it sounds like both channels are producing predominantly the Left minus Right "Difference" signal. Even with stereo recordings, Bass tends to be very similar for left and right, so there won't be much difference. Presenter's microphones will be mono, so again very little difference signal to be heard.

In the decoder, the difference signal is produced by mixing 38 kHz with the supersonic audio components from 23 to 53 kHz This is then "matrixed" with the correct amount of the "sum" audio signal from 0.05 to 15 kHz. It sounds as though there isn't enough "sum" signal getting through, possibly a coupling capacitor upstream of the decoder has gone low value which is cutting the level especially at the bass end. The supersonic components appear to be getting through well enough.
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Old 4th Dec 2023, 11:43 pm   #4
agardiner
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Default Re: Strange stereo problem - B&O 900

Thanks for that excellent possible explanation, I will take a look at coupling caps. Could a low level IF signal cause such an issue? I presume that it would result in an insufficient level of the 38KHz signal, such that the decoder circuit would not operate at all.
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Old 5th Dec 2023, 3:17 pm   #5
Mr 1936
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Default Re: Strange stereo problem - B&O 900

Hi

In the broadcaster's stereo encoder, the stereo difference signal is modulated onto a 38 kHz carrier which is then suppressed, i.e. DSBSC modulation is used. This results in a pair of sidebands in the supersonic range 23 to 53 kHz, which is added to the sum signal along with a low level 19 kHz "pilot tone". These 3 components are then modulated onto the RF carrier.

In the listener's decoder, the 19 kHz pilot tone is doubled to re-generate a local version of the original 38 kHz having exactly the right frequency and phase. This 38 kHz is then mixed with the modulated difference signal to bring it back down to audio frequencies.

The 19 kHz pilot tone must be above a certain threshold level for the "stereo beacon" to light up and the decoder to be brought into action.

With FM, provided we aren't talking about very weak and noisy signals the IF level has no effect on the level of the recovered audio.
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