Aged ferro fluid in HF drivers - Possibilities of ways to test?

[Bytemee6666]

I am the owner of a pair of AR90's (purchased new in early 80's) as well as a used pair of AR11's, and in both cases I was reading about the fact that the ferro fluid in the HF drivers ( tweeters) may be aged and affecting performance. I don't know about anybody elses thoughts of testing to determine if they are going bad, but, I did have the idea that a test for a "voice coil rub and buzz" on a HF driver could be done with a Dayton Audio DATS unit. This test first runs a frequency sweep from 20 to 20KHz to check impedance with a bode plot display, then, runs a lower voltage level sweep and compares them, checking for any abnormalities that some kind of mechanical interference might cause and would be more prominently affecting a low level signal than a higher level one that might overcome a mechanical issue, and designates a good/bad grade as a result (you can change the grading parameters prior to testing if desired) What are anybody's thoughts on this, or, has anybody else come up with a testing solution they would like to share?

[lsandev]

Hi @Bytemee6666, I'm in process of rebuilding my AR-90 tweeters. I will make a video and put it on youtube so you can check it out when I'm done if you are interested. 

I did the first tweeter yesterday, and yes the ferrofluid was so dry and so gummed up that I don't know how the tweeter worked at all. 

The voice coil was mostly stuck.

I tested the tweeter with the DATS unit.  I will post impedance charts before after here if you want. 

The tweeter with old ferrofluid had an impedance chart almost flat. There was no resonance rise. After the ferrofluid change and getting it back together there is a big change in the impedance. 

Both tweeters fail the rub and buzz test. The repaired one has better result.

I'm not sure why is that, maybe ferrofluid influences the test in DATS? I don't hear any rubbing.  

Now my question is if anyone with knowledge will read this:

Why are all resonances that I measure with DATS different from that written in AR manual?

- Woofers have lower resonance - ok that can be the spider loosing it elasticity or?

- Upper midrange has higher resonance. - don't know why.

- Tweeter after ferrofluid replacement has also higher resonance. - don't know why.

Best regards,

Lovro (SS)