I had to do this very thing a couple weeks ago due to DP problems on my 508 which was hooked up to the same quad as my 721. You can't run a switch test when you disco the DP quad. What will happen is that when you try to click opt 2 for signal screen the unit will kick you out of the menu totally. Very frustrating. If I remember right, when I finally got adv tech on the line he had me power off the unit, wait 10 seconds and unplug the unit (no coax should be connected to the unit ) wait a couple minutes and plug back in the unit....it will go into a reboot mode and then you can reconnect the coax and run your switch test. Matter of fact, when it got through rebooting and I connected coax ch 101 came on. Hope this helps and I also hope I remember the right procedure.