A question that still comes in email at least once a week without fail is, "How can I tell whether my missing color is a monitor problem or a game board problem?" I covered one way to do this back in 2001 by using RGB extension cables & another working vid game's output or monitor to get the answer, but it seems that the droves of newbies have only their first game & no way to do that. There is another way to bias the monitor's RGB guns to obtain the answer. I'm going to super size a model to work from & then you can use your imagination to come up with something that works for you. I'll expound on some other possibilities after the basics. A couple pieces of scrap plywood will allow me to make this large & open for viewing. I've drilled it for a standard size on/off switch & three microswitch pushbuttons. One advantage to using microswitch pushbuttons is that I can color code them.... R-G-B. This is the basic needs.. three pushbuttons, an on/off switch, a battery holder & a video cable. Wiring is simple. Each video wire from the harness goes to it's corresponding button color.. red to red, green to green & blue to blue. The black ground wire goes to the negative battery terminal. The positive battery terminal connects to the on/off switch. From the on/off switch each button is looped in via the common terminals. Ready to use in no time at all & can be made from old scrap materials... an affordable tester. With your game off you simply unplug the video connector & plug your tester to the header. Turn the power back on & power on the tester. Pushing the red button will produce a red raster. Pushing the green button will produce a green raster. Pushing the blue button will produce a blue raster. The camera picks up the hum bar, but it is not seen by the naked eye. If you get each color to come on as you push the button then your problem is not the monitor, but rather the game board, wiring from it to the monitor or perhaps any connectors in the line. If one of the colors does not come on then you know you have a monitor problem & better yet, you know what circuit it is in. Since you have three identical circuits this is an easy fix for a hobbyist without really having a knowledge of electronics. The easiest way would be with the power off & using an ohmmeter & simply comparing measurements between the bad culprit & either one of the good circuits. As soon as you find the component that doesn't measure the same as the good one you've found your problem. Should you not find anything different in your missing color circuit you may have a bad gun in the CRT & it would be time to find someone with a CRT tester/rejuvenator. Okay... some other ideas on building this cheap tester. First off you can build it in any size project box. You can shrink the whole thing down to a pocket tester by using mini-switches & a connector for the video harness. You can add an LED power on indicator. You can substitute the momentary switches to toggle switches or even on-off-mom so that you can combine on's to help you learn what color is missing by eye. Happy Gaming...