ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Received Freds woofers today, very well packed Fred...well done. Opened the packing case and carried out a initial visual inspection.....cones are frozen solid but look good with no signs of tears or damage
and the other one
However things go a little downhill from there, one magnet assembly looks in reasonable shape, should be an easy fix.
same woofer another angle
Its counterpart has not been so fortunate sadly....
It's skewed to one side at an alarming angle....I'm guessing this was done in transit, Fred could not have missed damage on this scale
Put the most badly damaged woofer in the vice and lifted the magnet clear.....sure enough the core had shifted as well as the magnet
surprisingly the voice coil is nice and free within the frame
though close inspection reveals some very odd rippling in the metalic coil tube
the coil measures up ok (I think) at 3.5 ohms.....so its not broken, I'm hoping the ripple are outside of the coils travel up and down the core as it plays, if so this one can be saved.
Removed the magnet from the better one and was not surprised to see the core nicely in place.....
However I was shocked by the condition of the voice coil, which was still held fast within the frame even with the magnet removed
another angle
It seems to be all concertinaed on itself, still measured up at 3.5 ohms the same as the other one.......will try to re align the coil but it doesnt look good
Taking what originally looked like the worst one that turned out to be the best one....I popped the magnet base plate off the magnet, easy enough in the vice, and wire brushed the magnet, plate and the core to clean every bit of old glue and debris away before epoxying the magnet back onto the frame, the core back onto the magnet plate.....will give it a good 24 hrs to set solid before moving on....the cone is still nice and free.
With the aid of the girlfriends hairdryer carefully removed the dustcap to have a really good look at the damaged voice coil, looks for all the world that the damage was occasioned by a turning motion...as if someone had tried to screw the magnet base off.....non the less it looks like the damage is above the area where the actual coils are wound, this may explain why the coil resistance is still intact.
As the epoxy adhesive is now hardened I put the magnet base plate complete with soft iron core in the vice just below the level of the jaws and carefully lowered the frame complete with magnet over the core, the vice jaws prevent the plate from snapping into place too quickly, then release the vice and the plate snaps safely into the voice coil with no further damage.
As can be seen the ripples in the voice coil sit above the height of the soft iron core...fitted the "alignment tool" to the magnet and made the required adjustments to the screws to bring the bore into the centre of the voice coil
The cone now moves freely back and forth with around 4mm movement before it hits the ripples in the coil.....I think this will probably be ok for reasonable sound levels but will wire it into one of my speaker outputs and see how it performs.....
Admire your work there. Are the ripples in the actual voice coil former material, or are they only on a surface coating? Could they be gently cut off, if it's a coating issue?
Jeff
I'm afraid I'm recovering from the BeoVirus.
Thanks Jeff
The more I look at the damage the stranger it looks...almost like blistering one one of them, its the metallic inner sleeve that's pushed out, there must be a paper or plastic inner tube around which the wire is coiled or the whole thing would simply short out, attempting to cut this away may damage the inner layers or possibly open circuit the coil.....I will wait until the epoxy is completely hard and remove the alignment tool before hooking up to a cabinet and hearing what it sounds like, if we get lucky and it does in fact perform outside of the damaged area I will leave well alone, if it doesn't work well I will look to trying to flatten it out, maybe rolling something around inside carefully.
Well....epoxy now had the required 24hrs and appears to be quite hard so removed the tool and, checking everything is still free, soldered the woofer into one of my cabinets and gave it a whirl.....sounds great on the end of a BM 4400 at volume level 6.....didn't go higher as there seemed little point, the woofer only actually moves a very small amount even with the bass wound up high......
The badly deformed one next in the vice to fit the magnet base and soft iron core onto the magnet....same procedure as before provided the same result, fitted the "alignment tool" and centred the core by eye....this is very quick....
Surprisingly the damage to this one sits higher in the voice coil than its predecessor, there is around 5mm of good coil for the core to travel into before it would foul against the damaged area.
Ensuring it still moves freely got the epoxy on it and will give it the 24hrs, I'm quite optimistic that this is going to have a happy ending too, Fred.....go buy yourself a Euro Millions ticket, it seems your on a roll ;~)
I sent this thead to a friend of mine who's a career audio engineer/speaker type. This is what he said when I asked if he'd seen this kind of thing before:
"Yes, a pretty common failure when the coil is pounded against the back plate repeatedly. I can't think of another root cause. It often occurs with some other thermal failure like spider and/or cone neck joint (which can then cool after failure and re-set -- this makes it hard to diagnose)."
Sounds like a symptom of beginning magnet slippage to me.
Or seriously overdriving the speakers sometime in their past.
When I asked Fred, if the cone had seized and told him to carefully try and see, if the cone would move by hand, Fred misunderstood andactually poked the dustcap inwards quite a bit repeatedly.The poking could easily have caused this, particularly since the cone couldn't move away - it had clearly seized.
He even sent me a video of it, referred to in this thread: https://archivedforum2.beoworld.org/forums/t/29296.aspx
Damage from the coil former hitting on the metal backplate usually causes damage to the coil former behind the voicecoil, not in front of it.
Martin
I think you usually see this kind of thing on this side of the gap in automotive or other harsh environment speakers. Being driven hard in a high temp environment can cause failures in the softening adhesives for things like the spider or cone/former attachment, often asymmetrical which can let the former scrape on one side like this, and it's not uncommon for the adhesives to rebond when the stress of temp and being driven hard is removed. Pressing on the caps as you describe is likely the root cause here, didn't know he'd done that. Without that as a cause it's hard to imagine a home speaker being in that high a temp environment, though for a vertically mounted speaker I could see sag occurring if some of the adhesives let go.
If high stress and high temps had happened I would have expected more damage than this. Then again I've seen some odd failures, one guy had a pair of old Cerwin Vega rock speakers, set a high power amp to almost full volume and dropped the tonearm on the record. Cantilever compressed, giving a nice big 1 hz or so push, DC coupled amp. blew the dust cap off and the entire voice coil out of the driver, hanging there like a Slinky. Exploded caps in the crossover, cracked a phenolic dome tweeter in half down the middle. Guy wanted us to cover it under warranty!
The worst one has also turned out good........Fred, be careful when you put them back....be sure to solder the red wire to the terminal with the red mark! and dont push on the dust cap ;~)
They will be dispatched tomorrow evening.....enjoy
Note that this problem seems to affect more or less all Peerless units from around this time;Beovox S45 and S45-2 woofers,Beovox P45 and P50 woofers,Beovox M70 and M75 midranges etc.all have this problem.
Though of different size and function, they are in many ways of similar construction and all the above mentioned Beovox modelsshould only be shipped with the above job done and only be moved with the greatest care if not - or you will risk damage.
A few others are in the same league, like f.e. the speaker used in the 1970s Beolit portable models.
Very nice job, Craig - as always!I enjoy watching your work.
Thank you for your encouraging comments, I appreciate the time you take to provide the guidance and advice that makes these threads possible, having now run out of projects I will sit back for a while and enjoy the activities of the other members until something else comes along.
Craig
Beautiful work Craig, excellent craftsmanship. Always enjoy seeing threads like these with repairs made by people who obviously love the old B&O gear.