12cm Fostex F120A Surround Replacement
The surround material of Fostex F120A@driver is urethane made. This is good material in terms of its productivity but has shorter life term. Within 4 or 5 years, the surround is broken depending upon using condition. However F120A is very good full range driver which has large ALNICO magnet and robust construction. There is no way to through it away without replacing the surround.
To replace the surround itself is not a difficult work. But to re-generate the original TS parameters after the work is very important. Otherwise it looks the original driver but has deferent properties. This will create deferent low frequency characters when it is mounted in the original enclosure.

Refer to the following sites for shop and material.


http://www.hino-audio.com/SHOP/778310/807068/list.html

http://www.ecsaine.com/about/

Specifications and TS Parameters
----------------------------
Impedance: 8ohms
Frequency range: Fs-20KHz
SPL: 89dB/W(m)

Fostex data for the model_______My original data for this driver
Fs: 65Hz ___________________________Fs: 71.63Hz
Qts: 0.44___________________________Qts: 0.48
Qms: 3.1
Qes: 0.51
Vas: 9.876L________________________Vas:5.46L

May 2012, MJ Magazine
Replacement process

Broken or worn out
surround. Removal of the gasket.
Removal of the surround
by a cutter and cleaning process by the lacquer thinner or equivalent.
Selection of the required size of surround and making the inner and outer circle lines for cutting. Cutting out of the surround. Make cutting inner side of the surround with radial direction by 1cm interval.

Push the diaphragm gently to making sure that the voice coil does not touch the magnet gap several times.
Now, surround replacement work has finished.
Fitting up of the surround on the driver frame. And apply adhesive on the frame and put the surround on it.
Verify if the voice coil movement is free from the magnet gap. Then, attach gasket
Flip back the driver. Now you can approach back side of the driver. Then, apply adhesive for fixing inner side of the surround piece by piece. Next, we have to do is to measure the Fs at minimum.

Measurement of TS Parameters

Measurement
Equipment: CLIO
Temperature : Approx. 20 degrees C
Driven by constant current mode


Above is the impedance curve measured. Fs is approx. 110Hz which is much higher than the targeted 65-70Hz. To have lower Fs, we need to weaken the spring action (smaller compliance) of the surround. Tuning of Fs parameters
By using a sandpaper, scrub surface of the surround material gently. .
After some tune-up process, Fs reached 71Hz. This is acceptable value considering the variations of each unit and environment factor including temperature.

Notice:

The surround is one of the most delicate component in a speaker driver. Replacing this component means that we , most probably, change the compliance. This leads out the difference of Fs, Qts and Vas values at minimum. This potentially impacts the low frequency property of the speaker enclosure.
Therefore, we have to know about original TS parameters and also the TS parameters after the repair work is done. Then, we can recognize the magnitude of the differences. We can sometimes re-tune the enclosure resonance

One more important matter is the "Quality of work". The "Curve" of the surround relates to Xpeak and Xmax. Diaphragm motion exceeding Xmax generates distortion. Where Xpeak is physical maximum of the diaphragm motion and Xmax is maximum motion with linear domain.

Good luck!!

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