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BM4000 from Mr Roast

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Craig
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Craig Posted: Tue, Jan 29 2019 8:34 PM

On his last visit Mr Roast gifted me this rather nice looking BM4000, I cant remember what he said about it other than it had issues, so stowed it away until now when my curiosity got the better of me Wink a click will open the full image


Craig
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Apart from the obvious slider missing and the BASS and BALANCE trims gone it really is a pretty example so I had a look inside


Craig
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Inside is very tidy too......there are footprints however betraying a previous repair...and I couldn't help but notice the odd configuration of the R15 ohm emmiter resistors, there are 8 when I'm thinking there should only be 4, I will look into this


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Craig
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Removing the slider bar complete with surviving sliders reveals the reason two are absent....broken tangs on the linear pots


Craig
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As chance would have it I am the proud possessor of a handful of miscellaneous spares and amongst them I found everything I require except one curser lens....but I know a man who has some of these Smile fortunately the BALANCE linear pot is good and its the BASS and TREBLE that are broken...I don't have a spare BALANCE pot and they are very odd animals


MrRoast
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MrRoast replied on Wed, Jan 30 2019 8:01 AM
It was too pretty to put in a skip Big Smile - glad its gone to a good home!
Craig
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Powered her up with the variac and had a look at the emitter current....both channels a little low at around 60mA so tweeked them up to 80


Craig
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Craig replied on Thu, Jan 31 2019 8:04 PM

Looking good so I connected a pair of speakers and was rewarded with a very persistent hum from the transformer.....this I suspect was the issue that led to it falling into Mr Roasts hands...…...I have heard of this noise from transformers before, but never heard it first hand 

sonavor
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sonavor replied on Thu, Jan 31 2019 9:40 PM

Yes, the buzzing/humming Beomaster 4000 transformer problem really ruins the experience. It makes it unusable for me. The low profile transformer is also difficult to replace. I know Rudy ordered a custom built toroid replacement for one of his Beomaster 4000 receiver projects. It was from a USA company I had also used for my first Beogram 4000 turntable restoration (the original transformer in my BG4000 had failed).  The custom toroid transformer is not inexpensive but if you want silent power that fits in the BM4000 cabinet it is the way to go.

-sonavor

Craig
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Craig replied on Fri, Feb 1 2019 9:24 AM

Sonavor

Yes I have read Rudi's blog chronicling the exercise...and very impressive it is too, but its a very expensive solution and as Rudi would attest requires a degree of "Beolove" few could afford to stretch too ;¬)

I do have a doner BM4000, from which I have stripped the linear slider pots, which boasts a transformer....I dont know it this transformer is silent or not so will carve it out and apply the variac to it.....hopefully it will be in good condtion.

Craig

chartz
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chartz replied on Fri, Feb 1 2019 1:38 PM

A real PITA this. My Beocenter 3500 hums after one hour or so.

That’s the C-Core transformer that gets loose.

I have it on a 1979 Quad 405 power amp I’m working on.

Fortunately and unlike B&O, Quad still has the part and it is not expensive.

 

Jacques

Krolroger
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Craig,

I'm having the same problem with my 4000 even after extensive component replacement.  It sounds fantastic but the buzzing transformer makes it borderline unusable.

Just wondering if winding down the idle current would make a difference.  Mine certainly seemed to be quieter round the 70mA mark.

Simon

Craig
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Don't know if reducing the idle current would help, as it is only the idle current Dillen will know. Removed the transformer from the doner machine and gave it a bit of a clean.....its inpretty good shape and the rectifier looks good too 


Craig
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Removed its noisy friend from the chassis...………….. 


Craig
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amazing how much lighter the whole thing is with the transformer removed...…..


Craig
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replacement fitted and ready to go..


Craig
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No humming noise whatsoever...…good result! will proceed with the restoration now I know its going to be worth the effort. Test the FM side, make sure the tuner is good, try the phono input and perform the recap and I think this will make a very desirable amplifier.


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