[ecfisa]

Hola.

Código Delphi [-]
{ Form.OnCreate }
procedure TForm.FormCreate(Sender: TObject);
begin
  tbDetalle.MasterSource:= nil;
end;

{ DBLookupComboBox.OnCloseUP }
procedure TForm.DBLookupComboBoxCloseUp(Sender: TObject);
begin
  tbDetalle.MasterSource:= nil;
end;

{ DBLookupComboBox.OnDropDown }
procedure TForm.DBLookupComboBoxDropDown(Sender: TObject);
begin
  tbDetalle.MasterSource:= dsMaestro;
end;

Saludos.