Bang & Olufsen BeoSound Century music centre - Page 1
Forum section: electronics / repair

author: Specmaster
author_rank: Super Contributor
timestamp: March 13, 2021, 12:47:35 pm
content: I have acquired a B&O BeoSound Century music centre that was sounding more than a little sick as it was all treble from the tweeters and nothing else but all the functions, radio, CD, and tape seemed to be working perfectly fine along with automatically sliding door to the controls. Straight away the finger of suspicion was beginning to point towards a possibly unobtainable amplifier module or dedicated chip set.Fearing the worst, I downloaded a service manual for it and proceeded to strip it down for inspection and the first thing that greeted me was the woofer driver on the left speaker had the cone ripped to bits and when the grill on the right speaker was removed, that woofer was looking OK. Before stripping it down any further, I thought that maybe I should check the connections to the woofers, and they were OK.I hooked up my multimeters, set to AC + DC display to the woofers, thinking that maybe DC was leaking through the output stage and had ripped the left cone to shreds. Switched it all and played a CD and the meters displayed that signals were reaching the woofers and that tone controls etc were working and zero DC but nothing was being emitted from the woofers. Switched off and checked the woofers for voice coil connectivity and discovered that they were open circuit, so it looks like the only problem with the stereo was that someone had been playing it very high volume setting and blown the voice coils and torn one cone.I had a couple small cheap speakers in cabinets from a cheap mini stereo and connected them the woofer leads and the sound was there OK but lacked quality as the speaker driver were crap.Now I need to locate one pair of 4" woofers with centres of 82 mm on the fixing centres with a depth of 45 mm in order to fit into the shallow cabinet. No problem you would think, except for the fact that the speakers do not state their impedance. Turn to the service manual for confirmation, and nothing other to say that it is a woofer of 4" diameter. I think either 4 or 8 ohms will fit the bill, leaning towards 8 ohms as they will limit the power and perhaps reduce the chances of blowing the voice coils in the future.Any advice or thoughts on this would be welcome and also thoughts where I can source these from at a sensible price, there is currently on eBay a single driver for the in my view, a stupid price of £50, second hand

author: Gyro
author_rank: Super Contributor
timestamp: March 13, 2021, 01:01:07 pm
content: Wilmslow Audio are probably the best place in the UK to find modern replacement drivers... and advice. They are still going strong after at least 50 years tomyknowledge [Edit, make that over 40 years but I definitely remember adverts much earlier than that]...https://www.wilmslowaudio.co.uk/CPC do a limited range, mainly Visaton and Monacor, probably cheaper but of course, no advice.

author: Specmaster
author_rank: Super Contributor
timestamp: March 13, 2021, 01:21:56 pm
content: Thanks, I did a search for speaker repairers last night and came across Wilmslowaudio and looked through their range of drives and the only ones I could see that where similar where 5" ones. I never thought about CPC though, I'll have a look now

author: themadhippy
author_rank: Super Contributor
timestamp: March 13, 2021, 02:09:13 pm
content: If you want them rebuilt wembley loudspeakershttp://www.wembleyloudspeaker.com,but they aint cheap.For replacement drives theres falcon acousticshttps://www.falconacoustics.co.uk/they also used to do rebuilds but cant find any mention of it on there site

author: SpecialK
author_rank: Regular Contributor
timestamp: March 13, 2021, 02:18:30 pm
content: I'd probably try these:https://www.ebay.co.uk/p/1337371619https://www.parts-express.com/GRS-4PF-8-4-Paper-Cone-Foam-Surround-Woofer-292-404

author: Specmaster
author_rank: Super Contributor
timestamp: March 13, 2021, 05:40:48 pm
content: Quote from: SpecialK on March 13, 2021, 02:18:30 pmWill not post to the UK. They look the same as another one from China

author: Audiorepair
author_rank: Frequent Contributor
timestamp: March 14, 2021, 01:30:41 pm
content: Better drivers here for more money:http://www.bluearan.co.uk/index.php?browsemode=category&category=Speaker_Components&masthead=Loudspeaker_Drivers&subheadnew=_4_Inch_DriversThis would be my choice from that lot:http://www.bluearan.co.uk/index.php?id=BMA4FR40&browsemode=category

author: Specmaster
author_rank: Super Contributor
timestamp: March 14, 2021, 04:48:50 pm
content: I might email them for some critical dimensions, in particular the fixing centres and depth of the drivers, thanks for the link

author: Audiorepair
author_rank: Frequent Contributor
timestamp: March 14, 2021, 07:22:54 pm
content: The photo on the Blue Aran site doesn't appear to match the model of driver, which uses a Neodymian magnet, so much smaller.https://www.beyma.de/fileadmin/seiten/download/pdf/Datenblaetter/4FR40Nd.pdfPhone them, they've been very helpful with me in the past, this is a proper Pro shop I trust.Beyma are a reputable make, and I would far rather buy something that has a full datasheet available, rather than some random cheapo on Ebay.

author: Specmaster
author_rank: Super Contributor
timestamp: March 14, 2021, 09:17:07 pm
content: Quote from: Audiorepair on March 14, 2021, 07:22:54 pmThanks for the info, I have also downloaded the other speaker pdf that looks like it might fit the bill, bad sadly, neither of them do, both are too deep for the slimline enclosure and the fixing centres are 108 mm and I need 117 mm. The baffle board is plastic and is moulded around that shape of speaker, so none of the others would fit either. So its back to the chase I'm afraid

author: Audiorepair
author_rank: Frequent Contributor
timestamp: March 14, 2021, 09:47:09 pm
content: Doh!

author: Gyro
author_rank: Super Contributor
timestamp: March 14, 2021, 09:55:21 pm
content: B&O were always rather 'form over function', making compromises to make things slim and stylish.For the saved time and additional cost, you might be as well just buying a pair of respectable speakers, eg:https://www.richersounds.com/hi-fi/standmount-speakers/wharfedale-9-0-black.html

author: Specmaster
author_rank: Super Contributor
timestamp: March 14, 2021, 10:31:37 pm
content: To add on a pair of standalone speakers I think is not a feasible option on many levels, it is a self-contained music system all housed within the same cabinet that can be either table mounted or hung on the wall. The other problem with separate speakers is the B&O system has dedicated power amps for the tweeters and woofers, 2 per channel and the separate speakers are driven by a wideband power amp and the crossover unit in the speaker cabinets. The music system has the crossover built into the preamp. Also, the main function of the repair is to restore it as near original as possible in order to sell it on as they are still worth a reasonable sum it seems, and the proceeds raised would fund further acquisitions of test gear.

author: james_s
author_rank: Super Contributor
timestamp: March 14, 2021, 10:44:07 pm
content: It looks like the cone got damp and the paper deteriorated, quite possibly the voice coil corroded from moisture. There are a number of places out there that rebuild drivers, most of them specialize in things like guitar amplifier speakers and antique radios but there's no reason they can't work on any arbitrary speaker. It's probably not going to be cheap, but B&O stuff never was. Sometimes if you wanna play, you gotta pay. The alternative is to modify the cabinet to fit some other drivers, or find some smaller drivers and fabricate an adapter. Check the value of a nice working example before you modify yours though, I don't know whether a system like that has collector value.

author: Gyro
author_rank: Super Contributor
timestamp: March 14, 2021, 11:03:01 pm
content: Quote from: Specmaster on March 14, 2021, 10:31:37 pmDamn, I forget it was biamped, shame.

author: Specmaster
author_rank: Super Contributor
timestamp: March 14, 2021, 11:24:08 pm
content: Quote from: james_s on March 14, 2021, 10:44:07 pmAccording to the original owner, it was working OK, but they went out one day and left the teenagers at home and when they came home the speaker was like that and there was nothing other than treble being played. They decided to buy another system and gave me this one to play around with. I believe that the kids were playing the music so loud and for so long that the voice coils got burnt out and checking the speakers confirms that both are open circuit. I connected a pair of speakers to the leads and the bass was back but sounded dreadful on inadequate speakers.Normally if a speaker cone has become damp in my experience, the cone gets pulled to one side of the magnet assembly and then rubbing can be heard if the cone is moved by hand, but in both of these speakers, the cone has remained perfectly centred and is free moving.

author: themadhippy
author_rank: Super Contributor
timestamp: March 15, 2021, 12:28:51 am
content: If you fancy the challenge you could give reconing it yourself a go

author: Specmaster
author_rank: Super Contributor
timestamp: March 15, 2021, 02:28:33 am
content: Ok, I've decided to go with 2 drivers from CRC following the suggestion from Gyro, the speakers are from Visaton and the data sheet is attached, they have the right dimensions as far as I can see in every direction apart from the originals had 2 small additional holes used to orientate them on the baffle. The pegs that locate in these holes are plastic, and so I cut them off flush and then the drivers should be a good match for the system. The speakers are available in both 4 and 8 Ohm versions, and I've gone with 8 Ohm for added safety, as already mentioned, the service kindly omits this dataThe price was a reasonable £26.39 including delivery, and I'll give a further update once these have arrived.Thanks to everyone for their help in resolving this problem.

author: Gyro
author_rank: Super Contributor
timestamp: March 15, 2021, 11:52:34 am
content: Glad you found something that looks as if it fits.I notice that they are full range drivers and have a small 'wizzer' cone attached. That won't cause any issues for bass use though. If you do get too much midrange, you can always take a scalpel to them!I suspect that one of the reasons the old drivers failed (tore) is than B&O may have used a shallower than normal cone profile to meet the depth requirement. If so it would be more prone to breakup (literally) if severely over-driven.Now you have replacements on order, you might be able to confirm the impedance of at least one of them by dismantling it. Assuming that the voice coils aren't too fried (maybe an end termination blew) you might still be able to get in and take a DC resistance measurement or the remains. An 8 ohm driver normally has a DC resistance of around 6R, and a 4 ohm, probably around 3R - enough to be able to distinguish anyway.It would be good to see some pictures.Don't worry, there are plenty on the web.

author: Specmaster
author_rank: Super Contributor
timestamp: March 15, 2021, 01:32:52 pm
content: Quote from: Gyro on March 15, 2021, 11:52:34 amI did manage to get the voice coil out from the ripped driver and it measured 6.9 ohms so my gamble on ordering the 8 ohm version drivers paid off. I have already snipped off the 2 plastic pins on the enclosure they were used to ensure that the drivers were installed with the connections were in the vertical plain.The reason why I had no pictures of the complete item posted was that in my small lab/workshop I just could not get far enough away from it to get decent photos.

author: Gyro
author_rank: Super Contributor
timestamp: March 15, 2021, 09:51:26 pm
content: It sounds like you're good to go.

author: Specmaster
author_rank: Super Contributor
timestamp: March 16, 2021, 12:00:53 am
content: Yeah, it looks that way for sure. Tracking on the drives indicates that they should be with me around midday Wednesday, so fingers crossed it will be up and running again in short order. I took an educated guess on them being 8 ohms as most decent Hi-Fi and reasonable sound system are generally 8 ohms, 4 ohms tends to be the lower end of the market. Now I might look for a remote for it as this is missing.

author: Specmaster
author_rank: Super Contributor
timestamp: March 16, 2021, 11:14:13 pm
content: Well the speakers have arrived at my local post office, can't wait to get them in my hand tomorrow.

author: Gyro
author_rank: Super Contributor
timestamp: March 17, 2021, 11:17:29 am
content: The suspense is getting palpable!One thing I forgot to mention, probably obvious, but make sure you get a decent seal between the rear of the speaker frame and the front baffle. I don't know if the original drivers had a suitable gasket, but air leaks around the driver will make a difference. Worst case, a very thin rolled length of warm Bluetack (and I mean thin) on the lip of the hole will do the job, as will thin felt, cut to size. Draught excluder strip is sometimes suggested, but that tends be too thick and distorts the edges of the chassis when you tighten the screws.P.S. I see gaskets on the back of the old drivers in your photo (or do I?). You might need to unstick them.

author: Specmaster
author_rank: Super Contributor
timestamp: March 17, 2021, 11:48:22 am
content: Yes the old drives did have a foam gasket behind the metal frame (speakers mounted from the front). Not sure if the new will have this already or not, assuming that they don't then I was going to use a thin line of white tack beneath to perform that seal for me., otherwise the air pressure from the inward cone movement will escape via the poor seal (reducing the bass) instead of from the reflex port to boost the bass response.Still awaiting delivery of them, should be anytime now, fingers crossed.

author: Specmaster
author_rank: Super Contributor
timestamp: March 17, 2021, 11:15:28 pm
content: Update. Speakers arrived safe and sound, no rear gasket supplied so carefully cutaway the old ones from the duff drives and glued to the new drives. After glue had set, drives fitted to the unit and powered up. All is well once again, the sound is full and rich, but the bass is somewhat lacking for my liking considering the B&O name is held in such esteem. Then again it is really asking a lot for a plastic speaker cabinet, especially when considering both its size and small drive size as well. I do have a Bluetooth speaker that has really rich and deep bass sound, but that does employ a 7" woofer so will move far more air and has a longer throw as well.I will post some pictures here shortly for anyone who is interested.

author: SpecialK
author_rank: Regular Contributor
timestamp: March 17, 2021, 11:55:26 pm
content: The unit probably uses some equalization to try to extend frequency response down below 80Hz. I'm not sure if these new drivers match the originals, but if they are lower sensitivity then you'll want to boost the bass obviously. I do believe the suspension on the drivers do become more compliant as they break-in and will become better sounding.Never having owned a B & O, I speculate it's more about image than sound. Attractive packaging and a convenient form-factor is a virtue that I can appreicate though. I think I want one.

author: Specmaster
author_rank: Super Contributor
timestamp: March 18, 2021, 12:54:50 am
content: Quote from: SpecialK on March 17, 2021, 11:55:26 pmThe specification lifted from the service manual states that the speaker system is capable of 75Hz - 20,000Hz and the new drives are from their specifications capable of 80Hz to 20,000 Hz, so they are a pretty good match. The crossover is set at 3,500Hz for woofer/tweeter and the system is an active acoustic system, class G amplifier and although being bi amped, meaning that each drive has its own dedicated amplifier, its distortion level is high at 1%.I think that you are correct in your assumption that B&O is more about image rather than all out sound quality, but they have certainly made some serious efforts to get as good a quality sound as they can while retaining that sleek appearance.

author: james_s
author_rank: Super Contributor
timestamp: March 18, 2021, 02:48:58 am
content: B&O has definitely always been very Apple-like, the stuff mostly performs pretty well but it isn't high end by any stretch of the imagination, their thing has always been in the presentation. They do pull that off well, this and their other systems are sleek and quite neat looking. I would never pay what they originally cost but I wouldn't turn one down if somebody gave it to me.

author: Gyro
author_rank: Super Contributor
timestamp: March 18, 2021, 11:09:12 am
content: Yay, good result. I'm glad you got what sounds like a pretty good match for the originals.If you want to lower the FS of the drivers a little, you could try adding a small amount of cone mass. Maybe a few tiny calibrated blobs of white tack at the base of the wizzer cones. It's easy to overdo it though and you don't want to risk misaligning and rubbing the voice coils. It might make an improvement, it might not. You don't exactly have the luxury of being able to match the T-S parameters to the cabinet.The finished unit looks nice and sleek.P.S. You might want to satisfy yourself that it is the bass that's a bit weak rather than the midrange being too forward due to the new drivers being full range. If so, the surgery that I mentioned previously might be the thing (if you can't correct it with component value adjustments).

author: Specmaster
author_rank: Super Contributor
timestamp: March 18, 2021, 01:42:26 pm
content: Although the new drivers are full range, I doubt that the wizzer cones are coming into play and colouring the sound seeing that the amplifier for the bass channel is being fed from the built-in cross-over which sends all signals over 3,500Hz to the tweeters, so the woofers should only be seeing bass and mid bass signals.

author: mnementh
author_rank: Super Contributor
timestamp: March 18, 2021, 02:30:49 pm
content: Mmmmhmmm... Looks like a good restoration. Well done. IIRC, later editions of this unit had black grillcloth; if yours are discolored (as they were prone to be with the slightest provocation) you may wish to update the look with new black cloth.Carry on!mnem

author: Gyro
author_rank: Super Contributor
timestamp: March 18, 2021, 05:55:46 pm
content: Quote from: Specmaster on March 18, 2021, 01:42:26 pmHa, I know. 3.5kHz always seems high to me (my Tannoys cross over at 1kHz) but is quite normal for small 2-ways. Yes, it is probably unlikely that the wizzer cones are making any significant contribution below the crossover frequency - I'm just prone to 'optimising' (tinkering!),I'd have a couple of cotton wool balls in there by now!so best to ignore me!

author: Specmaster
author_rank: Super Contributor
timestamp: March 18, 2021, 10:25:10 pm
content: Quote from: mnementh on March 18, 2021, 02:30:49 pmI've never seen one with black cloth, I know they come with a choice of grey, blue, green or red. Mine is in grey and is in good condition albeit there are a couple small liquid stain marks on the very top of the grills that I think I'll try and remove using some car interior cleaner, unless you have other suggestions?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: March 18, 2021, 10:49:30 pm
content: Looks like the Whizzer cones have a tendancy to spice up the response above 8kHz or so with lots of harmonics.https://www.eevblog.com/forum/repair/bang-olufsen-beosound-century-music-centre/?action=dlattach;attach=1194156Seems the Beyma achieves a smoother response without one.https://www.beyma.de/fileadmin/seiten/download/pdf/Datenblaetter/4FR40Nd.pdfQuote from: Gyro on March 18, 2021, 05:55:46 pm

author: Specmaster
author_rank: Super Contributor
timestamp: March 18, 2021, 11:14:03 pm
content: Quote from: Audiorepair on March 18, 2021, 10:49:30 pmOnly marginal though, it is also 13mm deeper and as such I think it would bottom out on the back of the cabinet, it really is a very shallow one and the entire enclosure is 1.1 litres each side.

author: james_s
author_rank: Super Contributor
timestamp: March 18, 2021, 11:44:52 pm
content: With such a small cabinet volume it might be quite closely tailored to the specific drivers that came with it. It would be interesting to compare side by side with a working original unit.

author: Specmaster
author_rank: Super Contributor
timestamp: March 19, 2021, 12:55:32 am
content: Could be but I don't have access to another sadly, but however, it has now got speaker drives and it sounds pretty good anyway.

author: Gyro
author_rank: Super Contributor
timestamp: March 19, 2021, 01:25:27 pm
content: Yes, the Beyma is pretty much the same up to your crossover frequency, it's only higher that the Visaton gets ragged. I did at one stage consider suggesting taking a hole saw to the back of the cabinets so that you could fit deeper drivers and then sealing the magnet assemblies into the rear baffles with epoxy!As SpecialK mentioned, brand new drivers do tend to break in as their spiders become more compliant with use. Playing at high volume accelerates the process - turn it up until the conesjusttear and then back it of a couple of notches! (derived from the old mechanical 'tighten it up until the threadsjuststrip and then back off half a turn').The only other tool you have at your disposal is to add a little cone mass. As I mentioned, this would lower the FS but at the expense of some efficiency so it's a compromise. It all depends what frequency that the reflex port is tuned to - it's usual to set this a bit lower than the driver FS to give a bit more bass extension.I'm sure it won't take you long to become accustomed to the sound anyway.

author: Specmaster
author_rank: Super Contributor
timestamp: March 19, 2021, 01:50:04 pm
content: Its not for me to become accustomed to how it sounds, I intend to flip it real soon on the bay of evil. Reading the manual more, there is apparently a test and adjustment I could make pages 60 & 61 of the manual refer to it. Apparently if I was able to get new drivers from B&O for it, they come pretested and have a number printed on them that I just have to enter that number into the microcontroller to return the stereo back to its optimum settings with the amplifier / speaker tuned to the size of the enclosure.