What type of monitor can I use with the 200-VGA-HM90 ?

You can use any monitor that supports standard 525 line VGA.
Used VGA monitors are very common and inexpensive. You may also use a flat-panel display if you prefer.

Can I replace the Mark-90 display with an old EGA monitor ?

We have not tried this nor have we heard if it being done before.
The connections and output signals on the Mark-90 are not the standard EGA that was used by the IBM-PC. Since EGA monitors are becomming so rare and expensive it makes more sense to just convert to VGA instead.

Can't I just make an adapter with some wire and connectors ?

No. In order to run a VGA monitor you have to convert the scan rate and blanking times. This requires scan conversion techniques using memory and a sync generator.

Why do I have to put jumpers on the Mark-90 CG board and what do they do?

The jumpers connect to unused pins on connector PH0 and provide the 200-EGA-HM90 with +5V and pixel clock from the Graphics Controller board. This is the easiest way to obtain the power and clock and not be bothered with power adaptors and excessive wiring.

How does the 200-VGA-HM90 work ?

The 200-EGA-TK features a custom sync generator, frame memory and a crystal-controlled pixel clock. It converts the Mark-90 display signals to VGA using standard scan conversion techniques.