Discussion Category: Centrifuge
Beckman J2-21 will not start
I have a Beckman J2-21 that will not start; the start light does not turn on and the lid doesn't latch. Refrigeration and vacuum work fine but nothing happens when the start button is pushed. I have another working J2-21 and I have tried replacing the 2 plug in boards, the timer, and the latch from the working machine with no change in results. This machine also had a blown large 20A fuse that I believe is for the brake resistor but replacing the fuse didn't help our starting problem at all. Any help will be appreciated.
Scottie, You are correct about the 20A fuse being part of the brake function. It sounds to me that you have a problem with the 24VDC power. If you lose this, you lose almost all voltages because all other voltages are generated off of this power. The first thing that you need to check in the condition of the capacitor on what is known as the high voltage board. It is the boards with all of the wires soldered to it that is located inside of the back of the console. To access it you must first take the black cover off of the back of the control console. You should then see four hex head screws that hold the back panel in place. You definitely want the power off while you're doing this until you see where all of the problem areas can be relative to getting shocked. When you release the back panel, you will see the board that has all of the wires that are soldered to it. There is a large blue capacitor at one end of the board. First, check out the physical conditon of the capacitor. Does the black end of the look dried and cracked? If so, then just replace it. If not, you need to check out the AC ripple across the capacitor. It should be very low, less than 0.5volts AC across the capacitor. That one component can make a J2-21 centrifuge totally inoperable. You should also be aware that there were several updates made to the high voltage board that also affected whether or not the centrifuge would start the drive motor spinning. If you see a small circuit board soldered on top of the high voltage board then the main update has already been performed. There were also modifications performed to the low voltage board, the larger of the two socketed boards that also impacted the start function. Unfortunately, I do not have access to all of the modifications and updates so I will not be able to help you with those items. Before you worry about any of that, please check out the 24VDC circuit and we'll go from there.
I've inspected the large blue capacitor on the high voltage board and it looks like new. The high voltage board has the small update board on it and so does the socketed low voltage board. I had already checked the start button for continuity and it has 24vdc on one terminal.Where should I look next?
Did you check the 24 volts for the AC ripple voltage in addition to the 24VDC itself?
Don, I just went back and checked the ripple at .15 AC volts. The DC measurement at the cap is 23.4 VDC. Scottie
Lets try another check. Turn the temperature SET knob and the MAX temp knob such that they are at a point that is below what the current temperature setting is. If the power is off, just set them well below room temp. If the power is on, turning the MAX temp below the current temperature should cause the OVERTEMP lamp to illuminate. There have been instances where that lamp has failed and it actually causes a break in the circuit such that it will not allow the drive to spin or the door to latch.
You said that you tried replacing the door latch assembly. The other possibility is that the door latch micro switch that comes in contact with the eyebolt when the door is closed is either defective or is slightly out of position such that the contacts in the switch are not engaging when the eyebolt hits the wiper arm. This too can and will cause the door to not latch and the drive to not start spinning. You should check for continuity between the switch contacts with your DVM to check for normal switch operation. If the contacts are functioning properly, then you may just need to adjust its position. There are two small screws and nuts that keep the switch in position but the switch can move slightly out of position over time. Make sure that the lower portion of the switch is situated such that it will be more likely to come in contact with the eyebolt. In other words, it should be moved as far towards the front of the centrifuge as is possible without preventing the eyebolt from actually engaging with the latch assembly. Get back to me after you check these items out.
Don, The overtemp lamp lights as it should. The latch microswitch tests good and we tried tripping it manually to see if the motor would start with the lid open. We also tested the replacement latch the same way. Scottie
If you close the door and hit the start button, do you get any noises at all from the latch assembly indicating that it is even trying to latch? If not, then that means that the latch solenoid is not working at all and the question then becomes why, based upon all of the items that you already have tried to replace. Something else to try, remove the cover plate over the latch. You should see a brass arm that is attached to the solenoid. Normally it is in a position such that the arm is pointing at around 11 o'clock. You can manually move it so that it is now pointing towards 12 o'clock. At the same time, engage the other microswitch and have someone hit the start button. If the drive engages then you know that part of the circuitry is functional and that gets us back to figuring out why the solenoid doesn't engage. There is a 10 second delay that provides the timing for the drive to start spinning so that it can start to generate tach pulses. Since you have another J2, have you also tried replacing the motor assembly? If none of this helps then I will have to try to check with some old colleagues to see what else they might suggest since they have access to electronic service manuals and I obviously don't.
When you turn on the timer the vacuum pump starts. When the start button is pressed nothing happens; no start light, no door latch and no motor spin.
I forgot to mention that this J2-21 is one of the models without the manual slide door latch; only an automatic latch. I know the solenoid works because it automatically locks the door when you turn off power. When power is applied it immediately snaps back to unlatch the door. From what I have read I believe the start button latches the door and my start button never lights up to show its ready. I've had the latch out and tested the microswitches and the adjustments and I can't find anything wrong.
I haven't tried swapping the motors but I did check the brushes and commutator and all looks good.
Anything else you can think of?
I can't say that I'm wild hearing that you still have the original style door latch mechanism. They were problematic to say the least. At least you are seeing the solenoid activate as it should when the power is cycled.
Anyway, have you checked out the start pushbutton as I had to replace a few of those during my career. There were times that if I pushed on the start button and kind of wiggled it under my finger that it would start the drive motor turning. If that's the case then you will need a new pushbutton switch. I believe that you previously said that you had the voltage at the pushbutton but do you also have it on the other side of the PB when you push it? That voltage goes right to the low voltage control board to trigger the start of the run. Get back to me and we'll go from there.