1200xl black screen

[1200xl black screen]

Thank you for the suggestions. Unfortunately at this point I do not have a spare machine to swap parts from. They seem to be hard to come by, Best is out of stock on replacement boards. All chips have been removed and reseated with no change in behavior. I do not know if the keyboard is functional or not, the only thing I know for sure is that the reset key does work. I tried holding START during power-up but there was no additional sound. U14 does seem to be the warmest chip on the board, everything else is rather cool to the touch so perhaps that is the culprit. Keep the suggestions coming

[1200xl black screen]

We have an Atari 1200xl computer which is in perfect physical condition. History of this machine is not known. When powered on, the power LED lights, the screen flashes red for a brief moment, then goes to pure black. There are two soft "pop" sounds from the tv speaker during the start up process as well. Pressing the reset button causes the two pops to be heard, but the screen remains black. We have tried both the RF and composite outputs with the same result. We have also tried 2 different power supplies, and swapped the ANTIC chip but the problem persists. I have removed and reseated all socketed chips, and moved the RAM chips around but there is still no change. All RAM has also been swapped with known good, yet the symptoms remain the same. When any RAM chip is bad or removed, the system presents a GREEN screen. Any assistance is appreciated. Thanks!