Mac Pro no more showing video image
Hallo
I have a Mac Pro 4.1 flashed to 5.1 and with a Westmere 3.2Ghz chip that suddenly stopped outputting video. At startup it chimes correctly but the display stays with the yellow LED as if it gets no signal.
When it’s off I see one orange led on in the top left corner of the motherboard with 5V stby written on the right and a green led a bit lower with S3 on the right. Pushing the DIAG button the orange led becomes stronger and the other sims, non other less lights up. Both go off when I hit the power button.
I connected Mac to another display on another port, no joy, says explicitly no signal.
I tried the video card (just bought to install Mojave’s, an RX550) on another PC computer, it works. And all worked well until a few days ago.
Then I tried the old graphic card and, surprise, the screen lit up just after the chime, the boot started but hanged at the end of the progress bar (it should be starting from another HD where I have an older OS version, since the card is not compatible with Mojave).
Since the self test is ok, I would exclude the motherboard, the CPU and the RAM.
if it were the Mojave SSD I would expect to have the screen on but no boot, or am I wrong?
I changed the 3V battery and reset SMC, reseated ram and processor card, disconnected all PCI cards and HDs except boot ones.
What else can I try????
thanks in advance for any hints.
Mac Pro, macOS 10.14