There are limits to the Atari 7800's video quality. This project tries to compromise between simple enough to build and best quality available.
Once I got started, I decided to be even less compromising. Sure, I could have pulled the composite video off of the input to the RF modulator and put that through an amplifier. In fact, I did that with a breadboarded arrangement. I was less than impressed with the colors I got.
Instead, I decided to go with an S-Video output. In S-Video, also known as S-VHS, the luma and chroma signals are run separately to the display device.
A breadboard arrangement that used a thick cluster of clip jumpers to connect to the board convinced me that this would in fact be the way to go. The video I got off the breadboard was so nice that I played with that hooked up to the system for two days before I buckled down to building the permanent version of the circuit.
Radio Shack Part Number:276-2016
Jameco Part Number:38359
JDR Part Number:2N3904
Radio Shack Part Number:276-1101
Jameco Part Number:35975
JDR Part Number:1N4001
The cap should be at least a 16V part, a 25, 35 or 50V part will work just as well, it will just be a bit bigger.
Radio Shack Part Numbers (pick any one):272-956, 272-1028,
Jameco Part Numbers: 10911, 93761
JDR Part Numbers: 100R16, 100A16
Radio Shack Part Numbers (pick any one):272-1013, 272-1025
Jameco Part Numbers: 93577, 94211
JDR Part Numbers: 10A100, 10R63
Radio Shack Part Number: 272-1434
Jameco Part Number: 33662
JDR Part Number: T1.0-16
I used everything from standard carbon comp resistors to high-precision military parts depending on what came to hand first, it doesn't matter. Where I didn't have a specific value, I used two resistors in series to get the same resistance value. For example, my 6.8K resistor is a 3.0K in series with a 3.8K.
Also, R9 and R10 would be 75 ohm resistors in a perfect world. These are available, but not common. Anything from 68 ohms to 82 ohms is OK here, just make sure R9 and R10 are the same.
In my perusal of the Radio Shack catalog, I didn't find a 10K trimpot that is stocked in stores (2XX-XXXX number), just ones that need to be ordered (9XX-XXXX number). If you need to order it, just get whatever 10K trimpot is cheapest.
Jameco Part Number: 43001
JDR Part Number: 72PR10K
Radio Shack Part Number:274-346 (four in a pack)
Jameco Part Number:159484
JDR Part Number: (Not Found)
I also considered using a DIN jack and making the output the same as a C-64's video port since that would not only allow me to use the C-64 monitor cables that I already have for the Commodore monitor, but it would also give me the incentive to make the C-64 S-Video cable that I've been thinking of making for some time now. But I decided to go with the 3 RCA jacks instead.
Radio Shack Part Number:276-150 (the board I used.)
Jameco Part Number: 105099 (a different board that would do as well as the one I used.)
JDR Part Number: JDR-BB65 (a piece of perfboard. Use a chunk of it.)
I laid my circuit out so that the signal inputs from the 7800 were all on one edge of the PC board and the outputs were all on the opposite edge. Each output was paired with a ground connection. Here is a picture of my board from just before I connected the output wires:
You'll notice a second pot on the board on the Chroma signal. This pot turned out not only to be unnecessary, but useless, so I deleted it from the design. Also, the pot in the above picture that I had on brightness did not have enough range, so in later pictures you'll see a square blue pot that I used to replace it.
And below is a shot of the completed board sitting in the 7800. The wires on the left are the outputs running up to the connectors I placed on the left hand side of the console. Placement of the connectors was tricky, I had to leave room on the outside for RCA plugs with a thick insulating boot around them, as well as room for fingers to grip the plugs while plugging them in and pulling them out.
I also needed to leave room inside for the 7800's PC board. This was a real balancing act. There are other places you can mount the connectors, or you can run them out of the box on cables, and use inline connectors. If you have an older 7800 you will have a connector port here so you'll have to figure out something else.
The connectors from inside the case.
The connectors as seen from outside the case.
Be careful as you assemble the PC board to make sure your circuit matches the schematic. Once you've finished the PC board, here is where to attach the leads to the 7800's internals:
The labels are placed immediately below the resistor lead where you will want to pick up the signal. Note that the Luma0 signal is out of order, over by the Chroma signals. Be careful to get the luma singals wired to the correct place, or your brightness levels will be really hosed.
The power is picked off of the connections for the RF modulator. The connection toward the front of the unit is a ground, the one at the back of the unit is +5V. The composite video/audio signal that the RF convertor uses is on the lead next to the +5V connection.
I picked off the signals using some Bell wire, a solid copper wire that's about 24 guage in size. I prefer to use solid wire for things like this, since I don't have to worry about stray strands of wire shorting things out. If you use stranded wire, twist it and tin it, then form it into hooks before you connect it inside the 7800. This will allow you to see any stray strands and nip them off before they cause any trouble.
Here's a look at my connections.
In my case, I plan to use two different displays, a composite monitor and my TV. The TV display is not as bright as the monitor's with the monitor and TV at their nominal settings.
Also, between the breadboard circuit I built and the permanent one I found that there was a pretty significant variation in the transistors. I don't have a curve tracer on hand to compare the two transistors against each other, but while I was able to get all the adjustment in brightness I needed out of a 1K pot in the breadboard version, I needed at least a 5K pot in the permanent model to get the same range of adjustment. So I decided to put in a 10K pot to be safe.
So if you are planning to use more than one display for your system, I recommend doing the adjustment using both, going back and forth between them to get the best compromise on brightness adjustment.
Also, you will want to check the display with both 2600 and 7800 cartridges. And for each type of cartridge, use at least one game that has a black background, and another that has the display pretty well filled with color.
The cartridges I used are:
I also dropped in several other cartridges to check the adjustment including both versions of Ms. Pac-Man, 2600 Pitfall, Xevious, etc.
Now turn the system off and put in your black background 7800 game. Turn it on and make sure that the dark objects are clearly visible. Dark greens seem to be the worst. Adjust the brightness up until you can see them well.
Now put back the filled-screen 7800 cart and adjust back down again until you get the display stable. You'll probably want to do it with a finer hand than the first time through.
Go back and forth until you get an acceptable image. If you must err, err on the side of darker.
Now put in your 2600 filled-screen cart and make sure the image is still not too bright. It should not be, since the 7800 image has slightly greater range than the 2600 display (Luma0 is not used by the 2600 mode).
Check things out with your 2600 black background game cart as well. Make adjustments as necessary. Whenever you make adjustments in the 2600 mode, go back through both your 7800 carts and check things and make any tweaks necessary before going on.
While you are doing the adjustment, make sure that you are using decent cables. The first time I ran through the adjustment process, I was using clip jumper cables instead of proper video cables. I was having a hard time getting the dark greens to be visible without having the brightness too high for the filled-screen games. Once I made up my S-Video cable, I found that I had plenty of room for adjustment. I just turned the brightness down a bit from where it had been and everything was fine. After that it was just a matter of cycling through several different carts to make sure the display looked good with everything I like to play commonly.
And the display is spectacular. I can see the dark green ships in 2600 Galaxian that I couldn't see through the RF modulator. I feel like I'm in an arcade when I play Ms. Pac-Man and Centipede. The colors are all bright and the display is crisp. And the screen isn't covered with snot from some kid sneezing all over it, so it's even better than the arcade!
The S-Video plug is available at Radio Shack for about five
bucks, I haven't found it at any other suppliers that sell in
small quantities yet. The part number is 278-451. What I did was
also pick up a high quality audio patch cable that is six feet
long. I clipped off the RCA plugs at one end of the audio patch
cable (with enough cable left on them to have them still be
useful for breadboarding and other projects), and connected the
appropriate wires to the S-Video connector.
Remember to put the backshell on the cable before soldering!
Then I took the center conductor of the line that went to the
white RCA plug and soldered it to pin 3, the shielding went to
pin 1. The red plug's center conductor went to pin 4, and its
shielding to pin 2. The strain relief was then crimped around
the cable sheathing, and some extra shielding strands from both
sides were soldered to the strain relief.
The S-Video Cable
If you want to put the output of your newly-enhanced 7800 into
an ordinary composite video port of a display, use a Y-cord to
bring the Chroma and Luma outputs together then run these into
the Composite input of your display. If you want to get the full
value of your 7800-plus, you can either use RCA patch cables
with a composite monitor that accepts separate Chroma and Luma
inputs, like the Commodore composite monitors, or you can make a
cable to go from your 7800 to an S-Video plug.
Thanks to Jay Tilton for his original posting on which my design was based, and thanks to Lee Krueger for making sure it is still available on the web here.
Thanks to Matthias Hartl for catching an error in the type of tranny shown in the schematics, and to B. Cross for catching that I dropped R13 and R14 out of the B.O.M. at some point.
The S-Video plug is available at Radio Shack for about five bucks, I haven't found it at any other suppliers that sell in small quantities yet. The part number is 278-451. What I did was also pick up a high quality audio patch cable that is six feet long. I clipped off the RCA plugs at one end of the audio patch cable (with enough cable left on them to have them still be useful for breadboarding and other projects), and connected the appropriate wires to the S-Video connector.
Remember to put the backshell on the cable before soldering! Then I took the center conductor of the line that went to the white RCA plug and soldered it to pin 3, the shielding went to pin 1. The red plug's center conductor went to pin 4, and its shielding to pin 2. The strain relief was then crimped around the cable sheathing, and some extra shielding strands from both sides were soldered to the strain relief.