If you have purchased a Tandy Radio Shack TRS-80 Color Computer 3 from an eBay seller, and all you get when you hook it up is a BLANK GREEN SCREEN, then there are three likely causes.

The most common cause is that the G.I.M.E. chip has unseated itself in the socket. This is the only SQUARE chip in the CoCo 3. All other chips are rectangular. So what you want to try first is to carefully remove the GIME from it's socket (make sure you note the orientation because it is not keyed and can be accidentally plugged in backwards and that would be the end of it - GIME chips are almost as rare as the DoDo Bird). This can be accomplished by prying at opposite corners with a small jewelers screwdriver. It is more easily accomplished with an IC puller designed for those type chips. Once removed, clean its pins with a simple pencil eraser and push it firmly back into the socket (making sure to put it in exactly the same orientation as it was when you removed it). Turn the computer on. Hopefully this has solved the problem.

If not, the second most common problem is that the 74LS245 buffer has gone bad. It is located near the cartridge slot and IS NOT socketted. It must be removed. A socket soldered in and a replacement 74LS245 placed into the socket. This should be done by someone with experience in such matters. If this does not fix the problem, then there is the third possibility:

The third possible cause: The CPU has gone bad. 68B09E cpu's are becoming rare. These days, you can make your own in an FPGA chip, but it is not cheap. You can still find the real deal for under $10. The 68B09E is also NOT socketted. It will have to be removed, a 40-pin socket put into place and a replacement 68B09E or even better a 63B09e or 63C09E, placed into the socket.

Note that I have not mentioned the possibility that the RAM might be bad. Bad RAM usually gives symptoms other than the green screen, and often gives garbage screens. I have repaired CoCo's since 1986. One of the above three is the most likely culprit if you have a green screen condition Colour Computer 3.

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