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

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

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

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

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.

Krolroger

The plasic cover just slides off the switch assembly, its not the best fit...in my limited experience....but a bit of a careful tug may be required. It's also fun getting it to sit correctly on replacement too.

Craig

Hi

In a final attempt to save the old transformer maybe you can try the following trick to reduce or even eliminate buzzing:

link

Milton 

Craig, thanks for that.  I think that removing the plastic shroud will likely sacrifice the switch, so for the time being I will leave it.

Milton, thanks.  Unfortunately the original transformer is already potted within its box, so the addition of varnish is unlikely to help with loose laminations.  The only possible solution might be to bake it at a lowish temperature but that may produce unwanted side effects.

Simon

Thanks for doing and sharing the groundwork that you have done with this project Simon.Presumeably,Canterbury Windings could produce  more of these  transformers to order as they will have the spec.on file now.I imagine that a similar but lower power version could be made up for the BM3000-2 now  also?

The mains transformer and function select switches are the two "Achilles Heels"of these otherwise robust designs.

Nick

One thing's for sure and that is that you can't neatly change the smoothing and output caps on this unit unless you remove the mains cable.  With a bit of controlled force the plastic shroud came off the switch and I was able to disconnect and tuck the cable out of the way.  Nine or so ground connections to the negative of the smoothing cap (Vishay 256 series) required a bit of jiggery pokery and heat shrink.

Smaller dimensioned caps will give me the extra few millimeters I need for the toroid.

Fixman double sided tape is useful for securing caps in position.