How can you tell woofers need remagnetization?

[hifi1]

Just curious on what the tell tale signs are of woofers needing to be remagnetized, I have a pair of 1976 Model 19's that still sound wonderful and are all original except for 1 horn diaphram. I purchased them from my father who purchased them new from Federated in Los Angeles. They are being amplified by an Adcom 5800.

[bfish]

Howdy, hifi1,
Sensitivity is the main parameter that can be tested to determine the extend of Alnico losses, if you've got access to an RTA and calibrated mic. Since the strength of the field is focused on the voice coil in the gap, it would take very specialized instruments to measure it directly.

However, if you've got a pair of like drivers with matching diaphragms, and one of them sounds noticably bad, just carefully switch the diaphrams, and test again. If the problem stays with the same driver, remove the diaphragm and inspect the gap closely for rust, crud, and centering of pole-piece in plate. If all appears ok, you've eliminated everything but the magnet strength!

Keep in mind that should isolate your problem, but it doesn't necessarily address the Alnico issue. Alnico loses a portion of its strength over time. Period. The greatest loss normally comes immediately after being magnetized in production, followed by slower, but steady, rate of deterioration. However, the rate goes out the window if the magnet structure has ever experienced thermal shock (coil burnout), or mechanical shock (dropping on something other than a soft foot). Alnicos should always be recharged when replacing a burnt coil, or you'll likely fry the new one soon if you don't (weak magnet=low output=increased volume=coil burnout).
If you're fortuneate enough to live near a reconer that offers this service (most don't have the equipment), I've found it a worthwhile investment to remagnetize all my 20+year-old Alnico 12s, 15s, and CDs. For the 15-20 bucks each, why wonder? I can only recommend Great Plains Audio for this service,(and others). Keep in mind that those rates likely cover little more than the power surge from the machine, so be nice. Make an appointment instead of just showing up,and get that pair of replacement 'phrams you've been thinking about, while you're there.

A word of caution for those leaving OKC with a pickup bed full of freshly-nuked Alnico; STEER CLEAR OF ALL BYCYCLES! Toll booths can be interesting also.......

BFISH

[RonSSS]

Bill at GPA told me that unless the drivers have been abused or dropped, they more than likely do not need remagnatizing.
I was going to send him my 416 woofs, but he did not think it was necessary.
So I get to save the shipping and have my local re-coner put in Bills cones......if they ever get here.
Ron