ActiveX control checkbox to (un-)check all checkboxes

Hello,

I'm using MS Excel 2003 and I have a sheet with altogether 6 ActiveX checkboxes. I want to have a 7th that (un-)checks them all. I can do that for form controls in VBA but not for ActiveX. Here's my code for the form controls:

Sub CheckAll()
    Dim lngIndex As Long
    Dim lngState As Long
    
    lngState = ActiveSheet.Shapes("Check Box 7").ControlFormat.Value
    For lngIndex = 1 To 6
        With ActiveSheet
            With .Shapes("Check Box " & lngIndex)
                .ControlFormat.Value = lngState
            End With
        End With
    Next
    
End Sub

Unfortunately, I have to stick with ActiveX controls because I need them to trigger a worksheet change event.
Thanks.
Stefan