Try this:

Qt Code: 
  1. void MainWindow::keyPressEvent(QKeyEvent *event)
  2. {
  3. 	switch (event->key())
  4. 	{
  5. 		case Qt::Key_Delete:
  6. 			qDebug() << "[Del]";
  7. 			break;
  8. 		case Qt::Key_Up:
  9. 			if (event->modifiers() & Qt::AltModifier)
  10. 				qDebug() << "[Alt + Up]";
  11. 			break;
  12. 		case Qt::Key_Down:
  13. 			if (event->modifiers() & Qt::AltModifier)
  14. 				qDebug() << "[Alt + Down]";
  15. 			break;
  16. 		default: QMainWindow::keyPressEvent(event);	//	<< important!
  17. 	}
  18.  
  19. }
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Works here; should work for you also...