Second Monitor on iMac Not Sleeping Properly

[fcamilleri]

I have a second monitor (Dell 20 inch widescreen) hooked up to my 24 inch iMac, and while I'm running windows, when the iMac monitor sleeps, it sleeps.

But, in Leopard (v10.5.7) whenever the iMac monitor sleeps, the secondary monitor does what a monitor usually does when the video cable is unplugged (does the whole factory test thing with the box of red/green/blue randomly moving around.)

Is there any way to make it sleep?

 

[BlueRevolution]

My 17" Dell display sleeps automatically. Are you connected with DVI or VGA?

 

[fcamilleri]

It's a VGA monitor.

 

[BlueRevolution]

Does it have a DVI connector? It should work fine if you use that instead.