Date   : Mon, 19 Oct 2009 15:36:27 +0100
From   : jim@... (Jim Hearne)
Subject: Dual drive power supply

Rick Murray wrote:
> Alex Taylor wrote:
>
>   
>> It appears to make a really irritating and quite loud high-pitched
>>     
>  > whine when switched off, a noise which seems to stop when the PC's
>  > switched on.
>
> Eh? It makes the noise when it is OFF!?!?!
>
>
>   
I expect it was on standby not really off, the power switch on the front 
of pc's hasn't switched the power off for years.
Only the one round the back does that, if there is a switch on the back.

The 5v standby section of the power supply is very lightly loaded on 
standby and is probably slightly unstable on that pc hence the audible 
noise.
Almost certainly coming from the transformer and inductor windings/cores.

Jim