I'm trying to run the self-cal for a 54845A scope and it hangs up on channel 3. Common and channels 1 & 2 proceed normally, but when it gets to channel 3 in just sits. If I cancel it, the cancel window goes away, but nothing else happens. The only way to get out of it is to turn the scope off. I've upgraded the firmware, but that didn't help. If I attach a signal, it displays normally. Any idea what I can do to fix it?