Allison tweeter and midrange decay: ferrofluid vaporized

[Guest abobotek]

In the last month I've had the misfortune of acquiring two fried

1988 tweeters from a pair of Allison 110 speakers and two shipping-damaged 1987 midranges from another pair of speakers.

I've dissected one midrange and one tweeter. the ferrofluid

has dried up in both, leaving only a greasy or waxy brown residue. I'm beginning to wonder:

* Is it typical for ferrofluid to dry up? Note that these drivers

came from speakers with fried tweeters.

* How does this affect performance (i.e., sound and power handling)

* How can dry ferrofluid be detected non-destructively?

* Can the ferrofluid be restored?

Has anyone run frequency response curves on their aging Allisons? One would expect the loss of damping to increase high-frequency output.

[Guest Baffle]

>I've dissected one midrange and one tweeter. the ferrofluid

>has dried up in both, leaving only a greasy or waxy brown

>residue. I'm beginning to wonder:

>

>* Is it typical for ferrofluid to dry up? Note that these

>drivers

> came from speakers with fried tweeters.

>* How does this affect performance (i.e., sound and power

>handling)

>* How can dry ferrofluid be detected non-destructively?

>* Can the ferrofluid be restored?

Hi Alex,

From what I have read (I am no expert) those drivers may have contained grease and not ferrofluid at all. I know the 3 way tweeters used ferrofluid but some of the 2 ways had grease and no clue about the mid range. Perhaps the banged condition of those mid range units led to the condition.

Did you get one of those mid range units to work? (Congratulations on out bidding me!) I figured for 10 bucks it was a good gamble to possibly save one of those puppies.

I hope so.......

I would not be too concerned about the longevity of your drivers based on the stuff picked up on Ebay. You never know what that stuff has been through.

Regards,

Bill

[Guest abobotek]

Allison's literature (and my dissections) confirm that the 2-way tweeters were damped with ferrofluid in the gaps, and the newer 3-way tweeters were damped with silicone oil. I don't know why one material is preferred over the other. In my dissections the ferrofluid has dried, while the silicone is present (but could have thickened with age).

Unfortunately, the midranges and tweeters I recently bought on eBay had their surrounds trashed by improper packing, aggravated by rough USPS handling. The seller tried, but goofed, in packing. He was quite honorable and gave me a refund, but obviously I would have preferred working drivers.

I'm considering having a batch of new tweeter domes made and conducting some lab tests to figure out which viscosity ferrofluid and/or silicone with which to replenish the fluid. Then damaged/dried Allison tweeters with intact voice coils and diaphragms could be repaired. Allison published frequency response curves for the tweeters, so (given the right lab equipment) a little trial and error should identify the correct fluid.

[Guest Baffle]

>Allison's literature (and my dissections) confirm that the

>2-way tweeters were damped with ferrofluid in the gaps, and

>the newer 3-way tweeters were damped with silicone oil. I

>don't know why one material is preferred over the other. In

>my dissections the ferrofluid has dried, while the silicone is

>present (but could have thickened with age).;

My literature copies confirm this 100%. It's been a while since I looked at this data.

Going back I could not find the reference to "grease" used in the damping/cooling process. They may have been referring to the silicone oil used in the newer designs.

>

>Unfortunately, the midranges and tweeters I recently bought on

>eBay had their surrounds trashed by improper packing,

>aggravated by rough USPS handling. The seller tried, but

>goofed, in packing. He was quite honorable and gave me a

>refund, but obviously I would have preferred working drivers.

It was a good gamble. I saw the seller had an excellent rating.

>

>

>I'm considering having a batch of new tweeter domes made and

>conducting some lab tests to figure out which viscosity

>ferrofluid and/or silicone with which to replenish the fluid.

>Then damaged/dried Allison tweeters with intact voice coils

>and diaphragms could be repaired. Allison published frequency

>response curves for the tweeters, so (given the right lab

>equipment) a little trial and error should identify the

>correct fluid.

Keep us posted. That is a very ambitious project. Perhaps it could lead to aftermarket repair?

>

[Guest jpk_1]

Hi,

The cooling fluid the 3-ways uses is more efficient, but will not stay in the gap of the two way tweets due to the longer excursion of reaching down to 2KHz.. The coolant can splatter! 3-ways only go down to 3750 Hz. The two way coolant is less efficient and will handle less power. Assuming the CAP in your crossover is O.K. the tweets are most likely a victim of severe clipping. Over the years I’ve seen extreme examples of this with Allison and other speakers.

I don’t think the oils/grease will dry out, maybe they boiled out? Howard F. has mentioned seeing slight leakage, but no apparent loss of performance. Members of the Yahoo discussion group have measured Allison drivers of various vintages, and they all hold their Freq response, dimpled tweets were said to be good to 12k or so? Shipping Allison speakers, whether complete enclosures or raw drivers is a challenge, I recently received a pair of mids and the way they were packed, one of them seems to have the surround pushed in slightly, visibly noticeable. It seems to sound OK, I don’t have the hardware to measure it. I’m hoping it will shrink back to normal, but it probably won’t! Do I worry about the surround slightly pushed in? It is frustrating some come well packed, others try but….

If you do come up with a repair/replacement for dented tweets, or even mids, let us know!

If Allison Ky. Ever sells it’s inventory or if the Chinese outfit ever starts making Allison’s, who knows maybe they’ll have quality, affordable replacement drivers? We can avoid the risk of the pre-owned market!

Joe