SCOPETREX vector gaming on your oscilloscope!
Have you ever wanted to buy a Vectrex, but you can't afford the high prices on auction sites? Do you already have a hoard of Vectrexes, but want another one? Well now you can build your own!
The SCOPETREX is basically a Vectrex on a single board without a monitor. To play games, you have to connect it to an oscilloscope or an XY monitor.
This board is not just an exact clone of a Vectrex; it makes a number of improvements over the original.
All of the components are readily available, either from Mouser or from various brokers in China. The bills of materials include Mouser part numbers.
There are two separate bills of materials. One is configured for people who wish to build their SCOPETREX with the 68A09/68B09 (no suffix option), and the other is configured for the 68A09E/68B09E (note the E suffix). In general, the E suffix is easier to find and it is less likely to be a counterfeit part. One caution: the 6809 and 6809E (no A or B) chips are only rated to 1MHz while the SCOPETREX requires 1.5MHz operation.
Please note that the 0.1" header pins are not listed on the BOM. They are just standard breakaway headers. Jumper shunts (you will need three) are also not listed on the BOM.
The game cartridge slot is designed to be used with the Sullins EBC18DRAS (listed in the BOM as S3311-ND) but you can also use the EDAC 395-036-520-202 since it is available from Mouser and it may be more convenient to order all the parts at once. If you do this, you will need to install the EDAC slot at a right angle and add 18 bodge wires (see the Assembly Notes section).
Board dimensions are 7 x 5 inches. Order the board in whatever cool colors you like!
The SCOPETREX also has a controller.
The board is 5.8 x 2 inches. It's very simple, so before fabbing it out, considering going into Kicad and customizing it to your tastes. Add some fancy silkscreen artwork or whatever you want.
The thumb joystick is listed with a Digi-Key part number since Mouser doesn't carry it, but you can also get it from Sparkfun. There are similar joysticks sold by a number of different vendors but this is the only one that's been tested.
There is a slide switch with 4 positions that selects which button (1 through 4) the top hat control triggers (when you push down on the joystick).
The SCOPETREX will run with the standard Vectrex firmware. Just program it into the flash memory chip (the AT28C64). Note that the "Vectrex BIOS" ROM that you may find does not include the MINE STORM game. Often, the "MINE STORM" ROM comes with the BIOS so you should use that instead. I will not tell you where to get ROMs, nor will I send you any. You could go buy a broken Vectrex and transplant the ROM from that.
In general it is a good idea to socket the chips that are hard to find and thus may be counterfeit. It makes troubleshooting much easier. Therefore, consider socketing the 6809, the 65C22, and the AY-3-8910/12.
The U101/MC34063 power supply chip should be soldered directly to the board for best performance since a socket adds resistance.
The game cartridge connector has two different part numbers on the BOM. If you ordered the EDAC part from Mouser which is designed to mount vertically on the board, you will not be able to plug the cart into it because the case will bump into components on the board. I recommend rotating the connector 90 degrees and soldering the bottom 18 pins directly to the footprint and connecting the top 18 pins using bodge wires.
Don't forget to install the jumpers J301, J302, and J303. If you aren't sure what polarity you need, just set them all to "NORM."
Set all the trimmer potentiometers to the halfway position. You'll tune them later on.
Don't forget to flash the firmware into the AT28C64. There are many devices that can do this. I like the MINIPRO programmers that are available cheaply from many different sources.
When you assemble the controller board, be sure to install the DE-9 connector on the bottom of the circuit board rather than the top.
You can connect your oscilloscope probes to the X, Y, and Z (aka blanking) directly or by making up adapter BNC cables that go directly into your oscilloscope or XY monitor inputs.
To start with, connect just the X and Y outputs to your oscilloscope's channel 1 and channel 2 inputs. Set both channels to 1V/div and put the oscilloscope into XY mode (on some oscilloscopes, you do this by rotating the horizontal sweep rate knob all the way counterclockwise).
Be very careful about setting the brightness/intensity control on your oscilloscope. A spot that is too bright can permanently damage the phosphor coating inside the CRT, leaving a dark spot!
Connect a 5V power source to the input jack on the SCOPETREX and move the power switch to the ON position. After a second you should see lines and dots appear in various places on the oscilloscope. You may have to adjust the intensity knob to be able to see them.
Next, connect the oscilloscope's Z axis input (sometimes called video or blanking) to the Z axis output of the SCOPETREX. Try adjusting the brightness trimmer on the SCOPETREX. If you still see a lot of lines and a bright spot in the middle of the screen, switch the polarity jumper on the Z axis (JP301). Adjust the brightness trimmer again until the spot in the center goes away.
At this point you can also plug in a controller and some speakers into the audio jack. Make sure you can get sound, and check if the controller works.
The SCOPETREX requires some calibration to be performed to get the best image quality. To do this, you will need a Vectrex test cart (often included in multicarts, which you probably want to have anyway.)
The first step of calibration involves correcting the DAC offset voltage. With the test cart installed (no hot swapping carts! label side up!) you will first see a grid displayed on the screen. Hit button 4, and the display should read "DAC OFFSET," slowly blinking on and off. Connect a voltmeter between ground (I like the convenient mounting holes in the corners of the board) and the left side of R303 (marked with an arrow on the board). This is the voltage output, pin 1, of the U304 op amp. With the text off, the voltmeter should read 0V. If not, adjust the DAC offset trimmer until you get the voltage to 0V.
The second step involves fixing the X and Y adjustments. You can do this with the test cart's third screen by pressing button 4 again. You'll see some diamond-shaped targets; adjust X and Y so that the vertical and horizontal lines are center inside the diamond shapes.
The third step is less specific, try hitting buttons 3 and 4 to switch to various screens. Adjust the X and Y as you like to get things to look good.
The final step is the joystick calibration. Go to the last screen--the one that you can't seem to exit by pushing a button--and adjust the two trimmers on the controller board until the line segment turns into a dot. Swirl the joystick around to make sure it hits all the square boxes. To exit this screen, you have to push buttons 1 and 3 or buttons 1 and 4 at the same time.
XY monitors such as the Tektronix 602 or 604 offer a nicer experience for playing Vectrex games. Typically they have a fixed analog deflection voltage of only a volt or two, so you will need to use the width and height trimmers to dial back the deflection so that all the objects are on the screen.
Pro tip--turn a landscape-mode XY monitor up on its side and swap the X and Y inputs to mimic the Vectrex's portrait orientation.
The SCOPETREX is a fairly complex mixed-signal board. I can't tell you all the possible ways it can go wrong, but here's a list of things to check... * Power supply voltages: Are +5V, -5V, and the -12V rails all present? * Reset signal: Does the reset input on the 6809 go high? * Does the 6809 have a valid set of clock signals? Check E and Q, and the crystal or oscillator * Is there activity on the bus? Check the address and data lines * Does the ROM chip ever get selected? Both the CS# and OE# lines must be pulsing for that to happen. How about the RAM chip? * Check for activity on the 65C22 outputs * Does sound come out of the AY-3-8910/2? * Look at the waveform on pin 1 of U304A. You should see a fairly complex waveform * The SCOPETREX works by using a constant voltage into an integrator to generate ramps that drive the X and Y deflection signals. Work your way back along the analog signal chain starting from the individual deflection outputs * Take a look at the blanking signal (Z axis) output magnitudes. You might want to check the manual for your oscilloscope to find out the acceptable range of voltages
After all that work, don't forget to enjoy playing some Vectrex games!
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