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cope
09-30-2003, 03:08 PM
:banghead:

One of my 288-8K drivers is putting out less HF, dynamics and detail. The two channels used to be the same.
I suspect dirt (dust?) in the voicecoil or on the membrane is the cause. Any other suggestions?

Am I convicted to Altec service professionals?

thanks,
Coen

Earl K
09-30-2003, 06:45 PM
Hi Cope

Yes you are convicted ( of what I don't know ) - just kidding ! :duck: :spin:

Why do you think the gaps or diaphragms have muck in/on them ? Did you allow yourself to follow the internet band-wagon and remove those mesh bug-screens ? :scold:

Seriously now , I have a pair of 288-8K drivers. They are one of my favourite larger format drivers with their (almost) 3" diaphragm . One of my diaphragms measures quite different than the other. I figure its' from aluminum fatigue in the diaphragm ( old-age ) . When I get serious about listening to them - I'll have to buy at least one if not two new diaphragms from Bill at Great Plains Audio (http://www.GreatPlainsAudio.com/) . Do you have an RTA ( analyzer ) to measure the difference in the two drivers response curves ?

regards <> Earl K:)

cope
10-03-2003, 06:41 AM
Hi,

Yes, I boldly removed the bugscreens as was advised in an article in Soundpractices. I decided that I wasn't afraid of bugs and went for maximum resolution! :D

Besides: Altec advises to clean the airgap (with a piece of 1" maskingtape) before installing a new diaphragm. I suppose that it gets dirty in there even with the bugscreens in.

If dirt does not seem to be the problem (after inspection, still to be performed), I will try swapping the diaphragms. I may assume that a ferrite magnet stays magnetized over time and that the FR inconsistency results from worn diaphragms. Thanks for the link!

I have no way of measuring the FR. Is there extra info in a measurement?

kr, Coen

Earl K
10-03-2003, 12:23 PM
Hi Cope


I may assume that a ferrite magnet stays magnetized over time and that the FR inconsistency results from worn diaphragms.

-That's a reasonable assumption.

- Some of the guys here have recently posted about problems they had with drivers showing somewhat similar sonic symptoms. Some problems it seems, were the result of Lpads needing cleaning. Thats another thought to look into - do you use any in your crossover circuitry?

- We ( here at the forum ) are still trying to determine a sonic signature of a fatiguing diaphragm ( a visual picture from a RT-analyzer ). This would help in determining when an aluminum diaphragms has age-hardened and therefore needs replacing .
But we are not there yet and need RTA pics from drivers that have the symptoms ( diminished this & that spec. ) like yours displays .

regards ,> Earl K :)

GordonW
10-03-2003, 12:40 PM
I dunno about a frequency response plot from an RTA or whatnot, in diagnosing a fatigued driver diaphragm. I would be much more confident of seeing a pattern develop, in say an FFT or waterfall plot.

I'm guessing that there's a time-dependent change in the transient response, possibly more than a variation in the frequency response... possibly something like a part of the diaphragm going "inside out" instantaneously on transients (ie, "oil canning")...

Regards,
Gordon.

Earl K
10-05-2003, 12:30 PM
Hi Gordon


I would be much more confident of seeing a pattern develop, in say an FFT or waterfall plot.

- I too figure that would be the best analysis tool , but isn't that beyond most peoples budgets at this time? Or has FFT software & the attendant hardware bundle , come down in price to match the low cost RTA programs I see out there ? Since I'm on a MacIntosh platform - I haven't checked ( something I can't use ) the going prices for this stuff for quite a while now .

regards <. Earl K