The NEC PC-9821 computers output a very unusual resolution that most western monitors struggle with: 640 x 400 @ 24 kHz. I tried at least half a dozen monitors of all types and ages and none of them was able to display a picture at all (except an “out of range” message). Video scalers like the DVDO iScan VP50 Pro don’t recognize the signal either. Some sources claim that the cheap GBS-8220 converter is able to convert the signal – that is only partially true. You can see a stuttering picture that eventually becomes clear when you start the Windows 98 Desktop, but that doesn’t work in DOS.
Of course you can buy a special converter like the Micomsoft XPC-4, that will cost you 300-400€ though. There are other solutions:
1. If you already have a Framemeister (another 300-400€ converter by Micomsoft) then it is fairly easy. The RGB-in port accepts 24 kHz too, all you need is a sync combiner. I modified this sync combiner that I had built for my Atari Lynx and Sega Dreamcast. The sync combiner needs 5V to work and usually draws it from VGA pin #9. The RGB Output of the PC-9821 doesn’t carry this voltage, so I added an external 5V power connector and a switch.
This is the title screen of the 5th Touhou game, Touhou Kaikidan ~ Mystic Square, taken with a 1080p video grabber. With the standard settings the aspect ratio is incorrect.
To have a better aspect ratio I recommend to turn on the ZOOM function of the Framemeister and modify these settings:
ZOOM_SIZE = 100
ZOOM_OVERSCAN = 100
ZOOM_WIDTH = 10 to 20
ZOOM_WIDTH = 20 looks like this:
And this is how the Framemeister recognizes the input signal: 720 x 403 @ 56.42 kHz
2. The other solution is much cheaper and only works on the later PC-9821 models:
Press [GRPH] +  when powering up the machine and it will output 31 kHz.