check the cpu to see if the red led is coming on, it should only come on once , then go out quickly. if it comes on again after going out the first time, you can unplug all the input and output boards from the rack, you dont need to remove the connectors, jus make sure the boards are not plugged into the rack. then turn on the power and look at the cpu, the little red led should come on once and go out indicating the program is running. if it still continues to go on and off, then it is most likely the power supply board. occasionally it is the cpu board or timer board or prom board. and far less likely is the rack itself. if the cpu led comes on and goes out correctly with the i/o boards removed, then it is one of them, just plug one in at a time until the problem occurs, and that will be the bad board. It is probably the powersupply board, this is the one i have found to cause this problem 90 percent of the time