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Beomaster 4000

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Krolroger
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Krolroger posted on Tue, Nov 6 2018 4:34 PM

I probably paid slightly over the odds for this non runner but it's in nice cosmetic condition with rosewood veneer.  It seems to be an earlyish version of this unit with only one idle current trimmer per channel in the output stage.  No RCA sockets either.

It's clean internally too, though the switches needed a good blast of Kontakt 60.

The right hand channel was defective (practically no volume) and is now partly sorted, due in part to a defective TR40.  But I still have a lot of hiss over the audio which I think is down to TR39, a lockfit BC159B.

My question is whether I can use BC559B as a substitute in this position?  Lockfits don't seem to have the best reputation.

Also, there is some evidence of plasticiser migration as some of the internal wiring is sticky.  I don't think the physical integrity of the wires is compromised.  Is there any fix for this: such as cleaning with ipa or meths then some sort of clear coating?

Thanks all,

Simon


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Dillen
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Yes, you can use BC559B - I suggest replacing in both channels.
And there is only one idle current trimmer in each channel in all Beomaster 4000s.

Martin

solderon29
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The'4000  is( obviously )a development of  the '3000,but with a different output stage,which employ's Darlington transistors,and sound's argueably better too?

You don't seem to have replaced all the caps on the output stage pcb(yet?)Those yellow jobbies need to come out,even if they read ok!And the presets of course.

You'll have the same problems with the function select switch,and you will have noticed that the radio section is the same as that used in the '3000.You have plenty of experience with that area of course!!!

Interestingly,the '4000 has an "ambio/surround sound function too,where if you connect an extra pair of speakers,you can achieve an ersatz surround effect,that's quite good.

A nice receiver,well worth some tlc!

Good luck with it Simon.

Nick

 

Krolroger
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Thanks, Martin, BC559B nailed the problem.

I took out the bias setting Darlingtons TR43 and TR54 in order to remount them.  They are SPS 5418.

I can't find a data sheet for them so I can't tell if they've gone low gain though they seem to test ok on my Peak tester.

Should I replace them with MPSA 13 as a matter of course (which show an hFE of approximately 10 times as much)?

Cheers,

Simon

Krolroger
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Hi Nick,

Work in progress!  Replacing trimmers/caps as we speak..

Dillen
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Don't worry too much about hfe for those transistors. Unless they are proven faulty, I would gladly leave them alone.
High Hfe is not better than low. Particularly when we talk about amplifiers like this one.
The risk of self-oscillation rises as you fit "to eager" components like higher-Hfe transistors and lower-ESR capacitors.
Try aiming for something close to what's fitted originally.

And I agree on replacing the remaining capacitors on the board - the tantals too.
- And replace tantals with tantals.

Martin

Krolroger
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Hi all,

Work progresses on the amplifier board but a couple of things have come up.

The service manual states that from set number 177100, the emitter resistors were changed to 0,33 ohm and the (external to the board) 8,2K/8,2K ohm voltage dividers have been changed to a lower value.

Well, my set is numbered 177418 and there is no evidence of any change.  Curious.

I remounted the original TR43 and TR54 as per Martin's advice (fiddly job).  The original TO-3 transistors on the back panel seem to have reasonably fresh (not dried out) thermal paste on them, so I left them alone.  Heat transference from them into the back panel seems fine so I'm not overly concerned.

I needed to set the idle current so I thought I would measure the voltage drop over a single emitter resistor.  The prescribed current is 80mA and the emitter resistor is 0,15 ohm.  I should therefore measure 12mV across it.

I set the idle current to 12mv each side but was alarmed to see the output transistors get uncomfortably hot in a short period.  I then reverted to the method advised in the manual and reset the idle current to 80mA, down from an indicated 100mA or so.

However, I now see only 9,5mV over the resistors but nothing is getting too hot.  I probably need to whip them out and check they haven't drifted in value.

This got me thinking about the process of inserting a cheapish (or even not so cheap) Chinese DMM into the circuit - a Uni-t 61e in my case. Its shunt resistance is 10.4 ohms on the mA range which seems on the high side and must make a small difference to the measured value.  Measuring an emitter resistor is clearly a tidier way of doing this if you know the exact value of the component.

Or maybe the way to go would be to place a precision 0,5 ohm resistor into the circuit and measure across that.

Anyway, it's ok for now but I will revisit in due course.

Onto the tants..

Regards,


Krolroger
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Evening all, and a Happy New Year.

The Beomaster 4000 is coming along nicely - if rather slowly.

I've now replaced all electrolytic caps including tants (bar a couple in hard to reach places) and installed multi-turn trimmers on the amplifier board, and a number of failed transistors as described earlier.  Tants for the most part were in spec, but were showing elevated ESR compared to replacements.

I remounted the power transistors with new mica/paste and have done quite a lot of reflowing.

On FM it sounds great over headphones.

But I am getting quite a lot of hum on Phono with an open input with the volume slider at 50%. (Just at the moment, I have nothing to plug into it).  Tape 1 and 2 input exhibit no hum, just some background noise which is slightly worse on Tape 2.

Is this to be expected, and will it quieten down with what shielding is provided by the top and bottom covers when in place and a Beogram plugged in?  Or should I be looking elsewhere, like signal ground connections?

 

 

Krolroger
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The phono hum on an open input seems to have disappeared with the installation of the steel bottom plate which is a relief, but it did prompt me to remake all the ground connections on the pre-amp board.

So, onto the finishing off.  I've now replaced every tant including the ones in the front end and detector modules and loudness board, the last of which was a bit of a faff.

The ambio/stereo switch contacts looked somewhat oxidised so I took ithe switch off and cleaned it up.  Ideally I should have dismantled it to do the job thoroughly, but I 'll see how this goes.  Will be quite interested to hear what ambiophony sounds like.

 


Krolroger
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Having wrapped up this repair and recap, I now find that this excellent Beomaster is all but unusable on account of transformer hum/buzz.  This was not so pronounced until I wound up the idle current to 80mA and put it to work.

The chances of finding a donor transformer are slim, so I am contemplating installing a toroidal transformer which will be made to suit.

I know that Rudy and Martin have both done this conversion and I have learned much from their comments.  However, I am aiming to do this inexpensively (around GBP 50) and in a manner which is reversible and above all, safe.

My proposed specifications are as follows:

  • Single 230v primary and a 
  • 22v secondary winding rated at 4.5VA and a
  • 47v secondary winding rated at 275VA 
  • Thermal re-settable fuse

I will bypass the voltage selector box on the back and install an in-line 1.5AT fuse.  I'm not sure whether this would survive the inrush current so it might have to be a higher rating.  There are ways of limiting inrush current with thermistors or relays but they would add complexity and heat.

Having functionality for more than one input voltage would serve no purpose in this usage other than to make use of the existing fuse holders, so I propose a single primary.

I understand that radiated noise is an issue so the new transformer will have an EMI/RFI shield around it made of silicon steel.

I lack a 3D printer to make up a baseplate so I'll see what's possible with acrylic or aluminium sheet.

I'd be very grateful for your advice and comments.

Simon

 

 

 

solderon29
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It will be neat to keep the design as original as possible Simon,but you could use a separate small transformer to supply the tuner?

This could make the choice of main transformer easier,and you might be able to find an "off the shelf" device too,rather than having to have something custom wound,with attendant cost.

I have a BM4000 with a "singing"mains transformer too,so I'll be following your progress with interest.

Nick

W.Frankenberg
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hi Nick

I found this on ebay germany

 

https://www.ebay.de/itm/Bang-Olufsen-part-Beomaster-4400-mains-transformer-voltage-setting-switch/143125501341?hash=item2152f1d59d:g:RHwAAOSw2EJcYAVq:rk:1:pf:0

should be the same as in the BM 4000

wilfried

solderon29
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Thanks for the link sir,but the '4400 main transformer is quite different from the '4000,although it's tempting to purchase it for future use.

Regards,

Nick

Krolroger
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Hello Nick,

Thanks for the encouragement!

I'm now finding a manufacturer for the replacement and the names that keep coming up are Airlink, Canterbury Windings and Toroidy in Poland.

Their prices seem pretty reasonable as well.

Simon

 

 

Krolroger
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The new toroid has arrived from Canterbury Windings in the UK.  It is finished in black acetate tape and has a 105 degree thermal cutout and GOSS band.  It's smaller than the old transformer box and over 1.2 kilograms lighter.  I will make up a 1.5mm aluminium mounting plate that uses the old screw holes.

I opted for a single 230 volt primary winding.  This means the voltage switch at the back will be redundant but I will keep it to avoid changing the look of the device.

The toroid is rated at 280VA and I am wondering whether a soft start circuit driven off a small auxiliary transformer might be desirable to limit inrush current.

Can anyone advise me how to remove the power cord grip (needs new cord) and the power switch?  The switch has a metal shroud which comes off easily enough, and then a plastic shroud, but I am unclear how to remove that for fear of breaking something.  The parts diagram is no help.

Any advice gratefully received.

 

 


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