A question about RGBHV.

I was hoping someone here might be able to better help me understand an issue I get in my classrooms from time to time with analog video signals.

We have panasonic projectors of varying types deployed across my campus in rooms. I know from experience that when I get a blue image from the projector that it isn't receiving a video input signal, but sometime I get a black image. I know that the black image means the projector is getting a signal, but there is something wrong with it.

I'm trying to understand what exactly might be causing the black image. My best guess is that it relates to the sync signals, but I hate not knowing more.

Tonight I was troubleshooting a system, where 1 of 2 projectors were displaying a black image regardless of the input source. The important part, for this issue, of the video signal path starts with a Extron Crosspoint Plus 88 with DSVP. The output video signals for the 2 projectors are then ran through 2 Extron DA2 RGBHV units. (From a design standpoint, I don't know why we have the DA's anyway. They only ever use 1 input and 1 output, but that's for another day.)

Getting on with the point of this, I checked and verified that all the cabling and the DA's were functioning properly by swapping the #4 (proj 1) and #5 (proj 2) outputs on the switcher. The #4 output is throwing the black image btw. I also bypassed the DA on the #4 path, as that is a common point of failure that we often experience and it was still the same black image. I swapped the output #4 to the (unused) #6 output and manually tied the I/O with a successfull image. I obvious can't leave it that way due to the AMX programming.

I'm now working on the assumption that the #4 output on the switcher is bad, but why the black image? Could it be that the RGB signals aren't getting out, but the HV are?

Sorry for the super long post, the few AV guys that are above me aren't exactly the mentoring type and so I typically learn everything on my own.

Thanks in advance.