I tried all ranges from 50 to 300 miles. Same result - weak returns. MAP mode seemed to be weak as well.
I didn't try target alert mode, and no, in TEST mode, the area that should have painted yellow and green were blank, not red.
Yes, I saw small specks on the display in auto gain mode, but it wouldn't paint anything, even with 10 down tilt. Switching to manual gain would immediately bring up ground return. I checked it on and off during a 2 hour flight.
I know it's a Honeywell display, and selecting different ranges with a test display is not possible. The TEST mode is selected by rotating the range selector full clockwise, just beyond the 300 range detent.
It's a B727-100.
Thanks for any advice you might offer.