Design concept failure
As we all know, after about 15 or so years the foam used in B&O & many other manufacturers speakers turns to mush, cones fall away, speakers become seemingly useless.
A market has arisen providing a readily available supply of replacement foam rubber rings, and appropriate glue that breaths new life into these majestic wonders of 20th century design. This is great, but ABR's (Auxiliary Bass Radiators) are rectangular.
Mushy foam rot on a rectangular bass radiator found on Red Line Bang & Olufsen Speakers
Bang & Olufson speakers seem to suffer badly, and in the case of the Red Line series, the design concept fails completely after about 15-20 years.
A rectangular metal plate supports a foam/rubber material that is glued across a metal diaphragm, it radiates a resonance, as the air contained within is distorted from the traditional drivers to the right of the speaker cabinet. This concept provides a good bass output.
Background
The foam had perished in my newly acquired Red Line 45’s, and after visiting the wonderful B&O fan site, www.beoworld.org members advised that B&O aware of this failing, released a tuned port conversion that sadly is no longer available. So my challenge was to replace the sticky mushy foam with new, perhaps longer life foam. I scoured many DIY shops, building merchants, only managing to find carpet underlay, which is made of a very similar grade of rubber but quilted and attached to a canvas or fabric back.
Fishy Inspiration
Laminate floor underlay also seemed to have similar properties, but alas at only 1mm thick, and quite week, it was no good. Almost giving up I took my daughter to see rabbits at our local pet shop. Eureka!! The foam rubber used to lay at the bottom of fish tanks was perfect, although a little thick, approximately 4-5 mm; it was the best I had found for the purpose. It’s tough, resistant to heat, oil, water obviously, and light, it also happened to be black unlike underlay which is brown or white. So my mini project is off to a good start.
On removing the foam from the plastic I noticed it had properties very similar to memory foam, found in expensive mattress toppers. I am yet to find if this has any additional value but it would certainly enable me to man handle it. It also cuts very well, with a knife gently heating the blade tip, it go’s through like butter. So forming two gaskets was easy. There are several holes so I simply used the end of drill bit, warmed in a candle and then pressed to form holes where required.
Template drawn on the rubber/foam gaskets surface then cut using craft knife, drill bits, scissors & candle
To dismantle the speakers I removed the 10 retaining screws at the rear of the cabinets. Carefully prising the fronts away, and removing the green and black wires from the rear terminals. There are 6 cross head screws holding the ABR's metal diaphragms. If you look carefully you can see one end of the diaphragm is narrower, allowing easier removal.
Once removed I vacuumed the old residual foam and lifted the dislodged ABR's from the speakers. Using a credit card! and tooth brush most of the mushy foam was removed from all metal surfaces. Finally I pasted them all with Nitro Mors, left for an hour then rinsed in cold water, wire wool took the remaining stubborn foam/glue off. I placed each part in the oven on high to dry and all the stubborn glue burnt off. You don’t have to do this I am just finicky about preparation, plus its quite dangerous with toxic fumes.
Using a credit card! and tooth brush most of the mushy foam was removed
Now the ABR's diaphragms, plates and gaskets are ready for reassembly. I used Araldite Rapid, about £4.99 from Home base. Very strong and tough, I tested an off cut of foam on my steel ruler and left for an hour. It was fine just very slight deformation nothing to worry about, more then enough to support the ABR's.
After cutting the gaskets, I mixed the glue spreading evenly over the ABR's diaphragms, the glue gives you about 4-5 minutes to adjust.
I then placed the foam gaskets onto the ABR's diaphragms. I placed quite a bit of weight on top evenly spread, and left for an hour.
Tricky Rads
The ABR's centre plates (Radiators) proved quite tricky, when aligning to ensure equal space around the edges it was more guesswork than a precise science. The glue again was applied in a similar pattern to the original shadow left on the metal radiators surface. I carefully placed Weight equally across the surface by placing upturned cups and a flat surface where weight is then applied for an hour.
Reassembly is a reverse of above, although be careful when replacing all the screws, if you use an electric driver make certain its torque setting is at about medium, as the cabinets screws can be brittle.
Proof is in the pudding; my authentic repair is almost complete
Before & after repair measured frequency levels displayed using Adobe Audition
A fully repaired and beautiful sounding Red Line 45's
Fully reassembled I tested both speakers at various volume levels using a Beomaster 3000, 3500 and Beogram CDX/2. I am personally happy with a good solid bass reflex, perhaps too much deep tone at low volume, only a trained ear would pick up, but as the Redlines tweeters provide such clarity it is a happy balance. I am still on the look out for B&O replacement ABR's so if anyone out there has any or a pair of good RL45/60's drop us a line, thanks to all 
Beomatt
If you have any questions then you can contact me via Beoworld.