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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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Craig
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Craig replied on Fri, Mar 29 2019 5:46 AM

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

milva1
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milva1 replied on Sat, Mar 30 2019 11:31 AM

Hi

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

https://www.diyaudio.com/forums/power-supplies/326670-cure-buzzing-chokes-transformers.html

 

Milton 

Krolroger
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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

solderon29
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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

Krolroger
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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.


Krolroger
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This is the data sheet for the new transformer.  Terry Monaghan at Canterbury Windings is very knowledgeable and answered my questions.  He will design and construct to your specification.  It took five weeks from order to delivery.

I paid GBP 101 including UK delivery.

Rudy (Beolover) says he paid USD 450 for a BM 4000 toroid in 2016 but that included two primary windings one of which had an additional tap as per the original specification which enabled use of the original voltage selector.

Krolroger
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The metalwork is now finished and I have installed the new tray with the same clearance the old transformer had relative to the bottom pan.  This will give adequate space for the lead outs from the toroid.  With the new big caps installed and their retaining bracket flattened to the new narrower profile, I now have minimum 2mm clearance left to right, and considerably more front to back.  The new transformer will be secured with a countersunk M6 bolt, and sits on a 3mm neoprene foam pad to minimise any mechanical noise.  The left hand sidewall is also lined with neoprene.

I reused the old transformer support brackets after drilling out the spot welds, so that has saved a bit of labour.

I removed the old mains lead and have replaced it with 0.75mm 2 core flex with standardised colours.  Getting it out was a bit of a faff, but using a razor saw as close to the back panel as possible and slicing through the retaining grommet did the job.  I was able to reverse the grommet and reuse it.  It needs to be temporarily compressed with thin wire to enable it to pass through the back panel.

Purists should read no further, but I can assure them that no irreversible changes have been made to this Beomaster and no holes drilled - apart from decommissioning the voltage selector.

I had a number of concerns about the old mains switch and decided to use a power relay to switch the 275VA load of the transformer and 5000uF smoothing cap.  I am using an DPDT Omron MY2 relay with a 240VAC coil.  It has a 10A current capacity and can switch an inductive load of 2A.  It can be easily inspected and replaced in a matter of minutes if required as connections will be made with faston type connectors.  It is mounted on the internal rear wall of the unit on an thermally coupled aluminium plate that will carry the new rectifier.  This means the old switch will only be handling the (inductive) load of maybe 10mA drawn by the relay coil.  I am hoping a snubber won't be necessary.

No idea as yet whether a 240VAC relay coil is going to introduce any noise but it's as far away from signal inputs as practicable.  It may be necessary to shield it.

This is an experimental project though I hope for a decent outcome.  If anyone has any suggestions that will improve my chances of success, please post them.  In particular, I am wondering about screening and shielding of any mains borne noise, and fusing arrangements.  I plan to use a 1,6 AT fuse for the transformer and 40mA F for the relay coil, using the two fuse holders on the back.

Regards,

Simon


Krolroger
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Krolroger
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The relay, fuse board and transformer primary side are now secured in place and connected (temporarily in the case of the new toroid).  There is no mechanical noise at all, even on switch on.  The relay switches quietly and there is no evidence of arcing on switch off - though it will not be under load until I connect up the secondary side.

The voltages on the secondary side are 53 VAC and 24 VAC which will come down under load.  The toroid is rated at 230 VAC.  Were I to do this again, I would specify a 240 VAC primary.

I rewired the fuse box to retain use of the two holders.  One fuse is rated at 50 mA and protects the relay coil (current draw of 4.9 mA) and switch wiring, and the other is a 1.6 AT rated fuse for the transformer.  I have installed a standby led on the top of the fuse box which is connected to the changeover contacts on the relay.  It is a reminder that while the unit is plugged in, the fusebox is live.  The pilot light extinguishes on switch on.


Krolroger
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And the fuse box.  The control side wiring is 22 awg silicone sleeved.


Krolroger
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This modification seems to work well.  There is zero mechanical noise from the toroid and no induced hum in the audio signal.  The relay arcs slightly on make but I can't see anything on break.

The photo shows a bit of a lash up with the wiring as I was anticipating having to rotate the toroid to minimise any induced noise but this has not proved necessary.

It remains to tidy the transformer connections and wire in the 105 degree thermal cutout.  I will use appropriate crimped and faston connectors which will enable easy removal of the transformer and relay.  I will check rail voltages - not that there is much I can do to change them - and the idle current.

I have fitted a heatsinked Vishay GBU 8A bridge rectifier in place of the previous B80 C5000 lump.  This is much smaller and can withstand higher surge currents - which is probably just as well as I think the new transformer is slightly overspecified.  (The main smoothing cap of 5000uF would suggest a smaller current requirement than the 5.85A rms my unit is capable of delivering)??

 


Søren Mexico
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Good Job and solution I like it Yes - thumbs up

Collecting Vintage B&O is not a hobby, its a lifestyle.

sonavor
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Yes, I am with Søren. Very nice work. Thanks for documenting the restoration with photos as well as description.

-sonavor

solderon29
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A very nice job sir,and thanks again for sharing the details too.

It's not only restored another beauty to full operation,but must have improved the spec too?

I'm not sure about the blue power light btw,praps it's just me?

Nick

Krolroger
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Gentlemen, thank you for your kind words which I greatly value.

Nick the standby light is red, not overly bright and hidden from direct view by the metal grille at the back.  I did think instead about running 1.5mm fibre optic cable to a new hole in the front panel but decided that would ruin the aesthetic.

Anyway, I guess this job is absolutely do-able with a good quality toroid and some patience and some aluminium sheet.

Will post again when I have wrapped it up.

Simon

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