Honeywell display on a commercial aircraft, R/T type unknown.

Does anyone here know exactly what weather radar malfunction is indicated when the radar, in the test mode, displays only the red bands (no yellow or green, but all 3 colors are shown in the sample panel on the lower left of the screen)? The radar seems a bit weak, but will display ground returns in WX and MAP mode. The auto-gain position (detent) displays nothing, but manual gain will produce a display of all 3 colors. No weather was available to test the unit.

Have you tried a greater range? If a low range is selected a reduced test pattern is displayed.
When you select target alert mode does the entire red display flash?
Do you have bands where the other colours should be but are filled with red.
When you adjust the brightness how does the display change?
When in auto gain detent do you see small green specks on the screen? 10 to 20 specks are ok. Look real close and give it sufficient time to warm up.
Really need to know the model of the display and RT to go much further. Even the aircraft type may be helpful. Let us know how you get on

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Curiousity killed it

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.