Saturday, January 29, 2011

KEF 105.2 - Bass Unit (woofer) Replacement - The Full Story

UPDATE: You can purchase the recommended replacement woofer for the B300b discussed below HERE.

HiFiCollector Community member, Tony sent me some detailed, very well-written information on how he is replacing his KEF 105.2 bass units. I know this info will be very helpful to all of us who own these fine speakers. Enjoy!

The KEF 105.2 Bass Unit Replacement.
My Kef 105.2's suffered a damaged bass unit, which was also showing signs of a buzzing in the bass, which happens when the support of the heavy Bextrene cone sags over time, and is a known problem. Also the concave shape of the outer rubber ring was flattening and stretching. These 12" B300b KEF-built units are rare, but occasionally turn up on ebay, where they are either overpriced, or are in poor condition, judging by the accompanying pictures.

After a web search, I found Audio Components here in the UK, and asked if a repair was possible. They promptly replied, and confirmed the source of the problem;

"What you describe – the flattening of the rubber surround is caused  by the collapse of the inner suspension (the spider) which supports the voice-coil end of the cone. Once this weakens it sinks into the frame and pulls the cone downwards, stretching (flattening) the rubber surround – this makes the driver unusable. Even if you find secondhand replacements on eBay, it will only be a matter of time before these go the same way.These drivers can be re-coned but the cost of doing thisproperly (and with the correct cone and voice-coil assembly) can be as costly as replacing the driver – also the re-coned driver will behave differently to the driver in the other channel (which has an old cone and suspension). The most economic solution (and the one which will perform best) is to replace both the bass drivers with new drivers that match the original specification and closely as possible."

"The key parameters of the B300 are: Fs=23Hz; Qts = 0.46; BL = 12; Znom = 8; Sens = 90dB; EBP =50. It is important to find a driver that is a close match to these – the majority of 12 inch woofers manufactured today do not. The closest driver we have found is the Peerless SLS315: Fs 28Hz; Qts = 0.47; BL = 12.2; Znom = 8: Sens = 91dB; EBP = 60. That’s as close as makes little difference. Maybe some tuning of the damping material will be required. This unit has a different physical frame size and fixing arrangement so some cabinet work is required."

These units were £70 each, a very reasonable price. They also offered to fit the units for me, and to measure the speakers under anechoic conditions to achieve proper tuning. However, this would have cost another £300, which is approaching what the speakers are worth anyway, so I decided to do the work myself and tune by ear. I measured up the cabinets first, of course, and found that the new drivers would fit into the existing cut-outs. The extra cabinet work referred to simply means drilling some holes, as the new units have an eight-hole fixing arrangement, compared to the three-point fixing of the originals. The cones are coated paper, the coating appears to be rubber, latex or similar. It looks and feels lighter than the B300b cone, so I would anticipate no sagging problems with these units.

I have had the Peerless units on stand-by for a while until I can find the time to do a proper job of it. Simply putting one of the speaker cabinets on its back and sitting the new units in place shows that the new screw positions will be close to the edge of the circular cut-out of the cabinet, but with a little care the actual placement should be fine. This could be done with conventional wood screws, but something to consider is that KEF used a special screw fitting for their original units, in conjunction with a foam gasket at the back of the driver. As this was an integral part of the design, I don't want to just put the new units in and assume it will be ok. I think I'll try cavity-wall screws; the expanding wings would mean that the drive unit will be bolted through rather than having screws biting into the wooden cabinet.

I want to double-check everything, reinforce the cabinet around the cut-outs if necessary, and have a supply of  gaskets of various materials (soft foam, hard foam, rubber) on hand so I can fit the units and audition as I go to get the best results. The cavity wall screws will also enable me to remove and replace the units several times without damaging the cabinet, to change gaskets. I think the first step will be to simply fit them with any set of gaskets, connect to the amp, and run the units in before testing alternative materials.

Of course, an interesting project would be to build new cabinets, as the originals are not particularly sturdy by today's standards; a tap with the knuckles here and there shows several resonant spots. I gather that they were built to a particular price, otherwise the 105.2's would have been so expensive that KEF wouldn't have sold any. For now, I'll be content to get these wonderful speakers up and running again, and just sit back to enjoy the music.

So when refurbishing any speaker, you'll need the appropriate parameters to source replacement units. I freely admit to not knowing what all the above symbols or abbreviations mean, although free resonance (Fs) and nominal impedance (Zs) are familiar. I suspect Qts is the natural damping factor of the driver.

Replacing some units might be tricky - for example to replace the mid or treble units in the 105.2's, they each have to fit into a moulded rebate. Fortunately, I think these units will last a lifetime, but it's something to bear in mind if revitalising a cherished speaker.

If there's anyone else out there in the UK who needs any speaker repairs, here's the web address;
I think there are similar suppliers in the USA who will doubtless be able to advise as well. I think Peerless are well-known across the pond.

I've attached a technical diagram and a picture, so you can see the more conventional appearance compared to the B300b.

So that's the KEF story so far - please feel free to edit the above if you think it necessary.

Of course, I now have one original B300b in good working order - so if one of your units shows signs of wear, then I have a spare!

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