RGB amplification on the Sega Mark III

[ApolloBoy]

I got a Mark III in the mail today and upon hooking it up through RGB, I noticed that the RGB output looked like utter crap just as described on the wiki. Since I had some THS7314s laying around from my N64 RGB mods, I decided to build a THS-based amp just like I do with the N64, with 75 ohm resistors on each output and a 0.1 uF cap across 5V and ground. The end result looks far better than the stock RGB output and I think this is a preferable way to RGB mod a Mark III as opposed to using the transistor-based amp posted here. Any thoughts?

[NFG]

I've never actually tried the amp on the wiki.  If yours is awesome, do feel free to post schematics, photos and etc.  =)

[ApolloBoy]

Here's the connection points for the RGB signals on the mobo:


And here's where I tapped the 5V and ground for the THS7314 (yellow is 5V, brown is ground):


When you do this, you'll also need to cut the traces going to the RGB pins on the DIN jack so that you don't have both the unamplified signals from the VDP and the amplified signals going at once. I don't have a pic of the traces but if you follow the pinout of the DIN jack and compare it to the pin arrangement it should be fairly obvious.

[renesisx]

Hey man - you mod sounds a lot simpler than the one on the wiki. Can you go into more details on yours? Where did you solder in the caps and resistors exactly? Do you have some photos of that? I'm very handy with a soldering iron, I'm just not sure how you implemented the mod! Thank you!

[ApolloBoy]

Hey man - you mod sounds a lot simpler than the one on the wiki. Can you go into more details on yours? Where did you solder in the caps and resistors exactly? Do you have some photos of that? I'm very handy with a soldering iron, I'm just not sure how you implemented the mod! Thank you!

OK, first thing you want to do is build a THS7314-based amp as shown here: http://www.mmmonkey.co.uk/ntsc-nintendo-64-rgb/

The input pins of the THS are connected to the three points shown in the first pic I put up. For 5V and ground, there are plenty of places on the board that you can grab them from, but I chose to tap them from the work RAM chip as it was convenient. For your outputs, you need to cut the traces for the unamplified RGB lines as I stated before, then solder the output wires to the proper pins on the DIN jack.

[viletim]

ApolloBoy,

The video comes out of the Sega 315-5124 VDP at 1.4Vp-p, while video from the N64's display processor comes out at 0.7Vp-p. So if you use the THS7314 based circuit (which has a voltage gain of two) connected straight to the outputs of the Sega VDP, you will end up with a video signal which is twice as large as it should be. This will saturate any normal TV/Monitor input stange and look rather bad unless  the contrast is turned way down (and possibly not even still). Did I miss something?

[ApolloBoy]

The video comes out of the Sega 315-5124 VDP at 1.4Vp-p, while video from the N64's display processor comes out at 0.7Vp-p. So if you use the THS7314 based circuit (which has a voltage gain of two) connected straight to the outputs of the Sega VDP, you will end up with a video signal which is twice as large as it should be. This will saturate any normal TV/Monitor input stange and look rather bad unless  the contrast is turned way down (and possibly not even still). Did I miss something?

That would explain why it was so bright, didn't realize that. Should I try some resistors on the inputs of the THS to reduce the signal before it gets amplified or stronger resistors on the output?

[viletim]

That would explain why it was so bright, didn't realize that. Should I try some resistors on the inputs of the THS to reduce the signal before it gets amplified or stronger resistors on the output?

The resistors on the output should always be 75 ohm for best performance. Attenuating the input is probably the way to go but maybe this is already being done for the video encoder chip. Which video encoder does it use? (I think it's that 24 chip with the electros all around it)

[ApolloBoy]

Which video encoder does it use? (I think it's that 24 chip with the electros all around it)

It's a Rohm BA7230LS.

[viletim]

The Datasheet lists the recommended RGB input level as 0.7Vp-p - perfect. Somewhere between the Sega VDP and the video encoder the RGB signals will go through a resistive voltage divider. A resistor in series and a resistor to ground after it. Then the signal will go through a coupling capacitor (1uf according to the datasheet) before it goes into the video encoder.

You need to connect your THS7314 inputs to the RGB signals at the juntion of the two resistors, before the coupling cap.