compressor shut offf right after power is switch on

I'm not sure what you are asking. It is normal for the compressor to come on for 1 or 2 seconds after you turn the power on, no matter what the chamber temperature is. 

Are you saying the compressor never comes on after the initial power on?

Craig is absolutely correct, it is perfectly normal for the compressor to kick on and off at power up DEPENDING on what the chamber temperature is versus the ambient temperature.  If the set temperature is set below what the chamber ambient temperature at power up then the compressor should come on and stay on until the two temperatures equalize.  There is one very important thing to remember about J2-21 centrifuge temperature controls.  Always make sure that the MAX temperature control is set to maximum prior to turning on the main power.  The reason for this is that the temperature meter has break beam sensors in it.  The temperature meter needle frequently jumps up at power up and if the max temp is set too low, it will create and overtemp condition as soon as the needle passes theu the max temp set point and that condition can only be reset by cycling the main power.

Hope this helps.

Don

Thank you both those very helpful information, now that I got the switch on/off fixed by cleaning with alcohol the light is staying on but the start button doesn't lit and rotor not spin. The chamber latch seems to be engaged well. Do you know which board should I replace first.

this was used unit we bought from BTS at first time use we have to press & hold down the START button for 5 seconds to get the rotor speeding up then released the button AFTer 6 months now and it takes the operator 115 second to hold down then release. A former Beckman employee ( Larry Hurst) told me that there is a cap leak. So maybe now is good time to replace the board but I don't know which one.

the START,STOP buttons did energized and de-energized the relay but the green light never lit up

OK, let me clear up one thing right off of the top, a flaky circuit breaker is just that and it really should be replaced.  You may have developed a high resistance contact in it from all of the probable arcing that has taken place inside of it.  They do not last forever and was one of the more frequent replacement parts when I was working in the field.

Now, as to the capacitor issue you talked about.  I believe that maybe the capacitor being discussed is the one that sits on the high voltage board which is only visible when you open up the back of the centrifuge by first removing the four small hex head screws and then removing the four larger hex head screws which allows the aluminum back panel to drop down.  There will be a circuit board with a lot of wires soldered to it and this is what is referred to as the high voltage board.  The big capacitor on that board is the filter cap for the 24volt power supply.  If it dries out, which is the normal failure mode for it then you will probably lose reliable functionality of the electronics.  Please let me know if you are still having problems with the start button after you replace that capacitor.  If that does not fix all of your problems then you will definitely have issues with the low voltage board, the larger of the two boards that have a connectors on them.  That is the circuit board that generates a ramp voltage that actually gets the drive to start spinning.  You may also have a problem with the SCR pack if one of the diodes within it has started to crap out so there are other things to try if the cap doesn't do it.

Don