NLA ferrofluid change?

[sparklow]

I own two pairs of NLAs, both from 1978, which with the help of the experts on this forum I refurbished and they sound great. I have two good extra tweeters which I will keep until (and if) I need them. My question is can the ferrofluid in the second generation NLA fried egg tweeter be changed? I don't think that I need to do this currently but I am curious as to the answer.

[Pete B]

I would guess that it can be changed but it would not be an easy job.

I have a blown NLA tweeter that I believe was driven so hard that the insulation

broke down and the voice coil shorted, then the current went way up and the

red donut part of the cone was burnt brown under the tinsel lead in wire.

I want to take a look inside and the red paper lip at the outer edge is already

lifted about 1/3 of the way around.  Not sure what to use to release the rest of

the glue.  Acetone or MEK perhaps.

[sparklow]

When I did the center cone repair on two NLA tweeters in the past I think that I could have changed the ferrofluid at that time. I was too busy trying to do the actual repair to add a ferrofluid change to the process, but it could have been done successfully I believe. Typically what are the sonic symptoms of a tweeter that require fresh ferrofluid?

[Alkaloid]

I had that problem on an old pair of ADS L1290/2s. I bought them off of Craigslist. When I picked them up and took them home and put them in the rear channel of a 5.1 surround system I ran some pink noise signals to set the volume with a SPL sound pressure level meter. The front pair were the same model so when the signal panned to the rear channel the 2 speakers were 3 db softer than the front. Also the treble was a tiny bit softer. Bad ferrofluid gets like gel. I sent them off to Rich So and he rebuilt the entire system, tweets, mids, and woofers. He also did the caps on the crossovers and a few cracked posts. There is many videos on YouTube showing tweeters being rebuilt, and the ferrofluid being replaced. Check them out before you do it. It makes a mess and is caustic to your skin so be careful.

[Pete B]

I tried to open them up here without much success.  Is there a materials person who might

know how the paper could be treated to make it less brittle?  Or was it designed to be stiff and brittle from the start?

https://audiokarma.org/forums/index.php?threads/disassembling-new-large-advent-nla-tweeters.953989/

[DavidR]

On 12/16/2020 at 7:55 PM, Pete B said:

I would guess that it can be changed but it would not be an easy job.

I have a blown NLA tweeter that I believe was driven so hard that the insulation

broke down and the voice coil shorted, then the current went way up and the

red donut part of the cone was burnt brown under the tinsel lead in wire.

I want to take a look inside and the red paper lip at the outer edge is already

lifted about 1/3 of the way around.  Not sure what to use to release the rest of

the glue.  Acetone or MEK perhaps.

Try Lacquer Thinner; it contains many solvents and one in the mix might do it.

[Pete B]

On 2/1/2021 at 3:34 PM, sparklow said:

When I did the center cone repair on two NLA tweeters in the past I think that I could have changed the ferrofluid at that time. I was too busy trying to do the actual repair to add a ferrofluid change to the process, but it could have been done successfully I believe. Typically what are the sonic symptoms of a tweeter that require fresh ferrofluid?

Burnt FF causes diminished output and/or a rise in the moving systems resonant frequency.

Where did you get that replacement center dome?

I have a working NLA tweeter with a smashed dome.

I agree that if you cut the dome off, you can certainly add the FF inside the VC, and if a tiny

hole was cut into the VC former a needle injector could be used to put some FF on the outside

where it is really needed.

The other issue is how to remove the old FF?

I wonder if those glued on center domes have the same response as the originals.

[sparklow]

Pete B. - I used the 5/8" paper dust caps from SpeakerWorks.com. If there is a loss of quality in the pair of NLA tweeters with the glued on center domes that I repaired it is not evident to me. I have my other pair of NLA's that have nice original tweeters and they do not sound "better" than my repaired set. Both of these sets of NLA's have their capacitors replaced with Dayton polypropylene caps of identical value to the original caps. As to the ferrofluid change, I would not hesitate to attempt it if I ever have a pair of these tweeters apart in the future.