KEF NT25 Tweeter Refurbishing (with Pictures) - and a question

[cr4zyg047]

THE MISSION:
Having purchased a pair of KEF 103/4s about a month ago - I have set off to tear the things to pieces and get it back to peak running condition. The previous owner had done a fine job refoaming the internal bass woofers - but there is work yet to do. One tweeter sounded 'OK' at best - the other was so quiet, it was practically a paperweight.

These tweeters are notorious for having issues caused by dried ferrofluid. This magnetic fluid was used to cool the voice coil and improve power handling - but over time, turns to a sludgy paste that impedes the performance of the coil. My goal was to replace this fluid and restore the tweeters.

The double-edged sword (that I didn't find out until I got the damn things open) - is that the dried ferrofluid grips onto the voice coil - making it harder to extract. While opening one, the coil was no longer attached to the diaphragm itself - which I believe accounts for the lack of output I had seen. The second unit's voice coil was stuck in the sludge - which I believe stressed the adhesives connecting it to the diaphragm. Lastly - the glue that is inserted into the little notches held the diaphragm firmly to the cosmetic dome shield on the outside - so separating the dome took the diaphragm with it. Had the ferrofluid not separated them - that would have.

NT25 tweeters are no longer in production - spare parts just don't exist. So repair is my primary option.

THE QUESTION:
Would one use speaker-grade black rubber cement to adhere the voice coil to the diaphragm? I ask this because every diaphragm I see online has the coil directly attached - and when I press down (gently) on the diaphragm while suspended over the coil (while running), the sound is full and bright. I believe the answer is yes - but it never hurts to ask before doing something potentially stupid.

Were I to do this, I would line up the coil and diaphragm - determine the points where they meet - and put four dots of the rubber cement on the coil and diaphragm respectively - and place them together with the cement points alternating. I bought the white speaker repair glue from parts-express, but I don't know if this has the tack-factor to last in this kind of application.

THE PICTURES:

KEF 103/4s after purchase

• cr4zyg047
• Mar 29, 2016

B160 woofer and NT25 Tweeter assembly from rear

• cr4zyg047
• Mar 29, 2016

Popping the tweeter out of the B160 magnet

• cr4zyg047
• Mar 29, 2016

KEF NT25 - Voice coil with diaphragm removed

• cr4zyg047
• Mar 29, 2016

KEF NT25 Ferrofluid Cleanup

• cr4zyg047
• Mar 29, 2016

KEF NT25 Tweeter Cross-section

• cr4zyg047
• Mar 29, 2016

 

[A400]

Good write-up and photos. I have used an epoxy to make a similar repair in the past, however this was on inexpensive speakers so I your case I'd do plenty of research. My understanding is that you need a rigid connection that doesn't damp any of the rapid movements of a high frequency driver. So I think you need to consider material type, rigidity and temperature resistance for your glue.

Also I'm not sure why you only want to use a few points of contact, unless that replicates the original bond.

 

[cr4zyg047]

Good write-up and photos. I have used an epoxy to make a similar repair in the past, however this was on inexpensive speakers so I your case I'd do plenty of research. My understanding is that you need a rigid connection that doesn't damp any of the rapid movements of a high frequency driver. So I think you need to consider material type, rigidity and temperature resistance for your glue.

Also I'm not sure why you only want to use a few points of contact, unless that replicates the original bond.

All good points - I knew posting here was the right idea.

You make a valid point - I was under the impression that greater elasticity (ie - rubber cement) would be more resilient to the vibrations from the voice coil. But that would obviously impede and insulate the vibrations of the coil from the diaphragm, thus causing a frequence response dampening effect?

I'm no audio engineer - just trying to common-sense my way through this. Learning a lot, though!

 

[cr4zyg047]

Replaced the ferrofluid a few days ago - and posted asking for advice regarding the diaphragms.

I ended up using rubber cement - both for its tackiness, cure time, and flexible (but stiff) hold. Time will tell if this was the right choice, but I was unsure about epoxy.

I used a toothpick to apply a thin ring of glue around the upper edges (inside and out) of the voice coil - and lined it up with the diaphragm taped to the ring with masking tape. Then just sandwiched them together with my fingers.

Now both tweeters sound incredible. Took one from 'OK' to marvelous, and brought the other back from the brink of death (detached voice coil caused it to output a fraction of what it should).

My 103/4's went from mud to stud! Sounds great now!

 

[TR6man98]

Great writ-up and pictures. I really appreciate this as I'd like to do this on my speakers as well.
Cheers

 

[cr4zyg047]

Great writ-up and pictures. I really appreciate this as I'd like to do this on my speakers as well.
Cheers

Literally refurbing the NT19 my KEF Model 90C center channel right now!

After getting the hang of this, I should be done in about 20 mins. Should have enough time tonight to refurbish the NT19s in my C55s downstairs too!

I'll post a detailed write-up for both

 

[TR6man98]

Btw,
How do you remove the driver (mid with NT25) from the cabinet?
Are these attached from the inside or screws are hidden behind decorative plastic?

I refomed and re-coned bass drivers on my 103/4's some 15 years ago but never touched mid/hi driver.

Thank you

 

[cr4zyg047]

At home depot waiting for my wife. But here's the basics.

Remove the crossover board by removing the 8-or-so screws on the fear.

After removing the screws, push the crossover up, and pull it out bottom-first.

Remove the HF and MF terminal connectors, and rest the crossover on the top of the unit, circuit board facing down. (My LF wires were long enough to allow for that).

Remove the batting and remove the three screws on the metal bowl on the inside of the unit connecting to the mid driver. Keep your hand on the front of the woofer frame to keep it from falling out, though very unlikely.

Once removed - pull the woofer out and remove the MF and HF terminals. And then set the woofer down on a flat surface. Because the foam surrounds do not protrude past the plastic ring - I set them face down.

De-solder the two wires running to the HF terminals running from the center of the driver - these are the tweeter leads.

Then remove the three large silver screws surrounding the driver - turning each one a full turn in sequence. Do not remove them one by one - give each a turn. Otherwise I've seen the magnet go all cockeyed as it gets caught on a screw.

Once those are removed - lift the maget straight up off the woofer cone - the tweeter will need to clear the voice coil - be gentle.

Then remove the screw holding the tweeter to the magnet (on the black plastic)

Then gently remove the tweeter from the magnet - pulling the leads back through

 

[cr4zyg047]

Wait for my full write up of the tweeter refurb before doing it - there's lots of little tips I've learned from refurbing these NT25 and NT19s (mostly how not to break them)

 

[Jan62]

I'm about to replace the ferro fluid of my NT25 tweeters of the Kef 103/4 speaker. I removed the protective cap and the diaphragm. The voice coil is still attached to the diafragm. The fluid seems not to be sludgy, since the voice coil came out easily. I still have to order the ferrofluid. How can you determine if the existing ferro fluid is not working properly?

Another question is about the type of glue to use to glue the diaphrgm back to th emagbet and the protective cover back on the diaphragm. I added a few picures below, were you can see some residue of the glue. What do you recommend to use?

 

[cr4zyg047]

I'm about to replace the ferro fluid of my NT25 tweeters of the Kef 103/4 speaker. I removed the protective cap and the diaphragm. The voice coil is still attached to the diafragm. The fluid seems not to be sludgy, since the voice coil came out easily. I still have to order the ferrofluid. How can you determine if the existing ferro fluid is not working properly?

Another question is about the type of glue to use to glue the diaphrgm back to th emagbet and the protective cover back on the diaphragm. I added a few picures below, were you can see some residue of the glue. What do you recommend to use?

View attachment 757324 View attachment 757325

The speakers are 20+ years old - you don't have much to lose by replacing it. You may have gotten lucky and had a unit that preserved the FF better - but in my case, the gap was relatively empty and tacky - whereas now the gap is now filled with fluid and is very smooth and lubricated.

Clean the voice coil with a q-tip and some rubbing alcohol to clean off any extra ferrofluid residue.

I used a tiny bit of rubber cement to adhere my VC/grill assembly to the magnet. Holds nice and firm - but also is elastic in case I need to open them back up at some point.

 

[Jan62]

There was not a lot of ferrofluid left in the magnet of the tweeter. You can see a small ring of contamination on the inner side of the voice coil. I will remove this with alcohol. and clean the gap in the magnet as well. How much ferrofluid do I need to apply?

 

[cr4zyg047]

Once you've got it bone-dry/clean of any and all of the old fluid - you can start work on adding the fresh stuff. (I used the stuff from Parts Express by Ferrotec)

I am not an expert - but I have done this on 6 KEF uni-q tweeters with great success...

The fluid is magnetically suspended in the gap by the magnet - cut the vial close to the tip to make a relatively fine point - and go 50%-ish into the gap and squeeze. As you dispense the fluid, it will suspend in the gap and form a ring. If you don't dispense enough - you'll have gaps in the ring. Work the vial around the gap just until you've made one continuous ring of fluid around the gap. If you add more than that - you're on your own. I have added only the minimum necessary to complete that ring and have found it to work very well.

I have attached a picture of an NT19 tweeter I had refurbed. You can see one continuous ring of ferrofluid in the image.

 

[tpotter]

I'm gearing up to rebuild my 105/3s with a suspect tweeter. This thread you started is great! Thank you!

 

[Jan62]

Thanks for your quick responses and helpfull information.
I ordered the fluid at good hifi, a company in the Netherlands. On their website they advise to fill the gap up to 75% of the height with the ferro fluid. You have to take into account that the level will rise once you put the voice coil back into its position. By reading your application method it seems that you do not fill the gap all the way up to 75%. Am I correct? I also believe that too much fluid creates a problem.
Hoe did you clean the gap. Poor some alcohol in it and clean it with a piece of cloth?

 

[cr4zyg047]

Thanks for your quick responses and helpfull information.
I ordered the fluid at good hifi, a company in the Netherlands. On their website they advise to fill the gap up to 75% of the height with the ferro fluid. You have to take into account that the level will rise once you put the voice coil back into its position. By reading your application method it seems that you do not fill the gap all the way up to 75%. Am I correct? I also believe that too much fluid creates a problem.
Hoe did you clean the gap. Poor some alcohol in it and clean it with a piece of cloth?

With the 6 KEF tweeter's I've done, the fluid becomes suspended once it flows into the gap - towards the surface. there's no telling exactly how deep the fluid goes because it's mainly pulled towards the front of the air gap (so far as I've seen.)

My rule of thumb has been to sink the vial about half way in, and dispense enough to complete the ring. All my tweets have been bright and lively (just about everything is an improvement over the gunk that was left in that gap before). Remember - it's supposed to conduct heat from the coil - and help cool/lubricate it. You don't need a ton - but what you do have, needs to be complete around the entire gap.

Cleaning the gap involved using heavy paper stock (think post-card thickness paper)- and wetting it a little with rubbing alcohol and running it around the airgap. Taking ordinary printer paper folded in half (for thickness) worked too - but the card stock paper was easier to work with once it gets a little wet. Keep using clean pieces until you quit getting "skid marks" on the paper.

 

[Jan62]

Thanks, I'll give it a try this weekend.

 

[jpeepers16]

My 103/4s are 24 yrs old and I am going to replace the ferrofluid. After google-ing this topic, it seems that the ferrofluid viscosity may be important. Someone on one website said viscosity should be 75 for Kef tweeters. You said your tweeters sound great after a refurb, so I will defer to you. Where did you get your ferrofluid and what viscosity? Thanks

 

[cr4zyg047]

My 103/4s are 24 yrs old and I am going to replace the ferrofluid. After google-ing this topic, it seems that the ferrofluid viscosity may be important. Someone on one website said viscosity should be 75 for Kef tweeters. You said your tweeters sound great after a refurb, so I will defer to you. Where did you get your ferrofluid and what viscosity? Thanks

I used the ferrotec fluid from Partsexpress. I will say that I did some searching and merely opted for the easiest audio-quality ferrofluid to purchase. I don't think viscosity fluctuates wildly amongst ferrofluids for speakers - I'm sure it'll become less viscous as it dries up anyway

You will need about 100ml per tweeter - I bought the 900ML so I could do a bunch of tweeters

http://www.parts-express.com/ferrof...source=google&utm_medium=cpc&utm_campaign=pla

Feel free to ask any questions - I get instant updates to this thread and try to respond quickly. I've done refurbs on 3 NT-25s (like in the 103/4) and three NT-19s. They all used this Ferrotec fluid and sound great.

 

[Slingshot05]

Wow. Thanks so much. I just realized my reference 3s unis have stopped working. Kef says too bad. Debating wether to buy new. (Ouch) or search ebay. Your solution rocks! Thanks so much! Expect dumb questions.