[gentoo-user] strange problem (no video output) on new PC

[Jack]

I'm hoping the cumulative wisdom of the assembled masses might be able  
to figure out what I'm clearly missing, assuming there IS something I'm  
missing.

I've recently assembled a new PC, with an MSI B350-Tomahawk motherboard  
and a Ryzen 5 2600 CPU.  (We'll skip that I ended up actually buying an  
older Ryzen just to upgrade the BIOS.)  The problem is that I have now  
tried three different PCI-E graphics cards, and have gotten no video  
signal from any of them.  I do get a video signal from an ancient PCI  
graphics card.  One of the cards is a very old Radeon, one is a  
slightly less old nVidia, and the newest is (from lspci) "Advanced  
Micro Devices, Inc. [AMD/ATI] Cape Verde XT [Radeon HD 7770/8760 / R7  
250X]."

What I find particularly odd is that if I log in blind, and then issue  
startx, X (startkde) seems to be running fine.  I ssh in from another  
PC, and the X log shows the Radeon driver loading, the monitor being  
recognized on the appropriate connector, the EDID received, and the  
right resolution being chosen.  (Even without all the AMD drivers and  
firmware loaded, at least it also seemed to start with the VESA driver,  
but still no output signal.)

I can easily believe the old radeon card is dead, and possibly even the  
nVidia card.  However, given what I see in the logs with the new radeon  
card, I find it hard to believe the card is actually defective.  (Just  
purchased used on eBay, so I do have to admit the possibilty.)   
However, I have trouble imagining what else could be the problem.  I've  
tried two different cables (both of which work fine for the PCI card)  
but both use DVI to VGA adaptors, although I can't imagine why that  
would matter now, if they worked for a different card.  I have ordered  
a new DVI cable to go directly from the card to the monitor, so  
hopefully I'll get that and be able to test it within a few days.

Can anyone else thing of what the problem might be, and if there is any  
troubleshooting I could try?

Jack