weak magnet

[joejoe723]

How to know that an Altec driver have a weak magnet?

[scott fitlin]

The magnetic strenght of the magnet can be measured by Bill, at GPA, and regaussed if neccesary!

What type of driver do you have? If it,s an old Alnico magnet driver it might be in need of recharging, but if it,s a ceramic type magnet probably not as ceramic magnets dont lose their charge!

[joejoe723]

Hi Scotty j,

Thankyou for your reply, I know that it can be measured by Bill, at GPA and I think that GPA is very good for all Altec owner.
I have a pair Altec large format hollywood driver(may be 288) and the model no. are missing. Of course, they are old and have Alnico magnets. But, they are in working condition ^_^. I just wonder that any symptoms appeared if they had weak magnets.

[joyspring]

Larry,

Two certain signs of driver demagnetization:

1. Loss of sensitivity or change in frequency response in one or both drivers. This starts subtly and eventually manifests itself into...

2. `Monitor drift'. A sound that should be centered between the two loudspeakers either tends to favor one side or the image is spread across both sides at different frequencies.

This was VERY common with alnico Altec 604s in recording control rooms and became readily apparent with the introduction of stereo. This problem even more of a nuisance as high-power amplifiers became commonplace in the 1970s.

Alnico magnets demagnetise at a stable rate over time; however, thermal and physical shock in addition to high electrical power input (low frequency pulses, etc.) rapidly increase this rate.

BobR

[joejoe723]

Hi BobR,

Thanks ^_^

[Greg R.]

Cant the resistance of the magnet be measured with an ohmmeter? Greg