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Beomaster 3000-2 Distortion issues

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Krolroger
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London, UK
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Krolroger posted on Mon, Oct 8 2018 11:31 PM

Hello all,

I'm at the tail end of a project that was the subject of a thread (BM3000 Tuner Malfunction) which was solved thanks to much help from here.  In the course of this, I have changed out more components than I needed to (zeners, detector diodes), taken the switchplate off, and removed the radio board.  I have changed all electrolytics and nearly every tant, bar a couple.  The three big caps I have left alone as they measured and look ok.

The amplifier board had idle current problems on both sides - one (intermittent) traced to a cracked leg in TR47, and the other required complete replacement of the left channel idle circuit.  A couple of noisy transistors (TRs 37 and 38) also went in the bin.

I now have a fully functioning unit with excellent radio reception.

However, I have the following symptoms:

  • With volume slider at zero, I am getting low level audio through both speakers and it is slightly distorted.  I would expect silence.  The potentiometer is located correctly.
  • At higher levels (>1) there is no discernible distortion on FM input
  • There is some very slight distortion on Tape (CD) input

The slight distortion on CD input I will have to solve by tracing a signal through the pre-amp which is where I think the problem is.

But any ideas why I am getting distorted low level audio out of both channels with the volume control at zero?  (The poti has been blasted with Servisol, by the way).

Thanks,

Simon

 

 


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solderon29
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Have you set the idling current too low perhap's?

Otherwise,is the "test" switch at the rear of the unit intermittent.They become oxidised with age,and of course the "usual suspect",the function select switch could be culpable too!

Nick

Krolroger
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London, UK
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Thanks Nick and hello again.

The solution was far easier (and more surprising) than I was anticipating.

I was confident the idle current was set correctly.

It occurred to me that the problem might be to do with unwanted radiated AF signal being picked up on the other side of the volume/balance/loudness network.  Sure enough, a couple of inches of aluminium foil shielding the brown/brown white wires downstream of C421 and C511 silenced the unwanted audio I was hearing with the volume at zero.

(This may be because I replaced TR37 and TR38 with BC182B both with higher hFE than the noisy ones I removed.  The originals had green paint on them which suggests they may have met some parameter unknown to me).

I'm no audiofool, but I think the sound is cleaner for this mod which to my surprise has also cured the slight distortion on tape input.


Krolroger
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Just a postscript to this project which is now finished apart from some cosmetic improvements and one of Martin's preset covers.

I would like to make some new perspex sliders as mine are yellow.  Does anyone know where I can get 4mm perspex CNC milled to shape?  I've tried a couple of places and they weren't interested.

Looks like there are a number of B&O devices of this era that would benefit from the availability of new plastic.


Dillen
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Dillen replied on Wed, Oct 10 2018 7:56 PM

Krolroger:

Just a postscript to this project which is now finished apart from some cosmetic improvements and one of Martin's preset covers.

I would like to make some new perspex sliders as mine are yellow.  Does anyone know where I can get 4mm perspex CNC milled to shape?  I've tried a couple of places and they weren't interested.

Looks like there are a number of B&O devices of this era that would benefit from the availability of new plastic.

Like these:
https://beoparts.com/2018/07/19/beomaster-4000-4400-beocenter-3500-sliding-pointer-windows/ 

and these:
https://beoparts.com/2015/08/24/beomaster-5000-beolab-5000-sliding-pointer-windows/

Currently working on the ones for Beomaster 3000 etc.

 

Martin

solderon29
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U.K.West Midlands
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Most interesting,thanks for sharing.I'll put a note in my manual about that effect for future reference.

Regards,

Nick

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