video amp for use with arcade monitor

[phreak97]

hey all, long time no post by me..
im back for some info.
basically i have the task of putting a megadrive2/genesis2 into an arcade cabinet, and the video signal is too low, the picture is all dark and crappy looking. im assuming this is the same problem as you get when you plug in an unamped vga card hacked to output 15khz.

so basically, i need someone to refer me to a schematic or something so i can build up a video amp.

thanks people.

[viletim!]

I think this one is nice

[Segasonicfan]

The Genesis has given me a lot of hell with it's crazy sync output- that's the problem, not amplification of the RGB lines.  You can test this for sure by removing the sync and notice that the (now rolling) image is perfectly bright (!)  I wanted to do a similar hookup with my Gen3 to my RGB monitor (PVM-2530).  The only way I could get it to work was by using a sync splitter (the ISL59885- but others may work too) to rectify the crappy sync output.  Now the image is 100% perfect

-Segasonicfan

[NFG]

That's an interesting idea.  I know I have a ton of trouble with Sega's RGB output, my TV hates it, all my monitors hate it.

[viletim!]

I have a mega drive 2 pcb floating around at the moment (I've got a megadrive related project underway) so I just though I'd take a look at the video sync line and well... here's what I found...

The sync line comes from the big all-in-one sega chip and goes to the (fujitsu) video encoder, then straight out to the A/V socket. The line is connected to Vcc (+5v) by a 2k2 resistor which implies that it's an open collector line (cannot supply any current, only sink it). I pulled out the 2k2 resistor, and siwtched the board on and sure enough, the line sat at 0v. With no sync signal going to the video encoder its internal clamp goes wild and makes a mess of the incoming video signal.

So it's worth remembering that the sync line is unbuffered. Leading this signal into a low impedance input (anything less than 1k8 will reduce it's output below TTL levels) will loose you your video clamp and thus your video output. This is pretty horrible! A lot of monitors have pull down resistors on their sync input which is usualy in the range of 390-1000 ohms.

Also checking my megadrive 1 circuit diagram confirms that is also has an identical arrangement (despite it's video encoder, the CXA1145, actualy having a sync buffer output available). Master system too perhaps???