RTvax 1000 monitor connection help needed

I am new to old vax computer. Help is appreciated. DEC rtvax 1000 620QY-A2 (1989 or 90?) From hard drive LED and LED display on the back the booting is all fine. It has two DB9 connectors on the back. I could not get matching monitor. Tried CGA nothing come out. Use DB9 to DB15 connector cable nothing came out on modern VGA monitor. But the monitor senses something and LED in front of VGA became green for a second and became yellow again. Tried Sony triniton with analog, digital 1 and digital 2 selection on the back, something came out (at least bright) but no characters. Is this syn issue or analog/digital issue? Any way can I open the chasis and figure out what kind of monitor I need? How do I prepare one hardware backup plan like floppy and tape similar to floppy and CD on model PC? How do I restore? thanks Henry