Date   : Fri, 16 Oct 2009 17:33:59 +0100
From   : rs423@... (Mick Champion)
Subject: Dual drive power supply

Mark Usher wrote:
> Hi all,
>
> I've noticed that the power supply on my dual drives is making a bit of a
> "noise" - audible, not electrical. Best described as an oscillating hum.
>
> With the previous discussions on the Beeb Power Supplies, I wondered if this
> was a sign the capacitors on this psu were also starting to show their age
> and should be replaced. Or was it just a bad design in the first place.
>
> -Mark
>   
In my opinion (and that's all  this is), this sounds more like the 
windings of a switch mode transformer rattling (at whatever frequency 
mains is being pulsed to it) rather than a capacitor problem. Capacitors 
tend to be silent, until they are ready to blow. On the other hand, 
switch mode power supplies are not unknown to make a whine / whistle 
sound.  Could this be what you are hearing? The audible pitch can vary 
as the switching transistors adjust the frequency of rectified mains 
input to the primary transformer windings, this keep up with the load 
(output) requirements.  Does the pitch change when you start the disc 
motors?


Cheers,

Mick.