From: Humphreys, David (david.humphreys_at_honeywell.com)
Date: Mon Jun 18 2001 - 12:56:56 PDT
Hello All,
OK, before any of you start unsoldering transformers
and potting them in resin, let me tell you why it
won't work.
First some facts.
The transformer has multiple windings are provides a
number of voltages for various parts of the system.
That is why the whine is constant.
It provides the high voltage for the EL backlight.
The whine occurs all the time but is most noticeable
when the BL is on.
It is caused by a phenomenon known as 'magnetostriction'.
When a Ferro-magnetic material (transformer core) is
placed in a changing magnetic field, the material will
change length slightly. It is this change in length
that accounts for the noise as it travels through the
surrounding parts.
> Okay, I thought about attacking the whining noise too:
> 1. Unsolder the transformer
> 2. surround its pads with wax or grease
> 3. put it under vacuum and seal it with any liquid laquer or
> epoxy resin
> 4. vent with air and every cavity should be filled with
> whatever was used
> (its a shame that Apple didn't do that!!!)
> 5. resolder the transformer and cross your fingers...
>
> I was so close to try this but I broke the glass dome of my
> home vacuum
> equipment ;-(
> --
> With best regards / Viele Gruesse
>
> Marco Mailand
Marcos' idea seems, at first, to be the solution. Just pot
the transformer and, he presto!, quiet.
Unfortunately, the movement we are talking about is very small.
Any encapsulent will, not only move with the core, but will act
as a conduit for the sound, coupling it to the case and making
more noise!
The shape of the pulse being fed to the transformer plays a big
part too. If the signal is a square wave or similar i.e.
has fast leading and trailing edges, the energy goes quickly
from one state to another. The corresponding change in size
is relatively large. If a sine wave was used, the core would
change size more smoothly and be more quiet.
I don't want to get into the electronics design too much as
I have probably bored the pants off a lot of you.
Bottom line, in order to 'quiet' the beast, you could try
rounding the edges of the TX drive with some strategically
valued and placed capacitors.
To be honest, I don't know what the drive waveform looks
like, (I've never looked at it) but I am willing to bet
it's not a sine.
But, even if you managed to do that, the circuit could become
so heavily damped that it quits oscillating.
It would be quieter though...
Regards,
David Humphreys,
Brains...food for thought!
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