This guide will instruct you on how to figure out why your Axiom 20 bed is not heating.

1 - First, go into Apex and verify that your settings are instructing your bed to heat.

2 - Go into Control->Temperature->Bed and set your bed to 60.  Scroll back to the top of the menu and click to exit to the main screen.  See if manually instructing the bed to heat in this way brings it on.  If the bed goes on, this indicates that your hardware all checks out and that you inability to heat the bed is settings-related.

3 - Testing the fuses:

There are two fuses that you will have to check first.  To access them, turn the printer off and unplug it from the wall.  Then, remove the back panel.  There is a blue fuse in the top right corner on the rambo mainboard and a yellow fuse contained within the red rubber cover (red arrow).  Check and replace those if necessary.

4 - If those two fuses check out, you'll move on to the small black power relay board.  Because of the high amount of power required to heat the bed, the power for the board is not drawn through the mainboard as the power for the hotend is.  Instead, the bed power output from the mainboard controls a power relay.  

5 - Testing the mainboard:

This connection from the mainboard to the power relay is made by the white wires (orange arrows).  With the power still off, remove the white wire connector from the port on the power relay.  Then, turn the printer back on and using the Control->Temperature->Bed command, instruct the printer to heat it's bed to 60.  With a multi-meter, measure the voltage by touching the contacts within the white wire connector that you would normally have plugged into the power relay.  If the mainboard is correctly instructing the power relay to turn on, you should see 24v coming out of it measured by your multi-meter.  The absence of this voltage would indicate that your mainboard needs to be replaced.

6 - Testing the relay input:

Turn the printer off and unplug it from the wall.  Plug the white wire back into the relay that you removed in step 5.  Plug the printer back in and turn it on.  The relay is always getting power into it from the bottom two contacts where it connects to the power supply (green arrows).  This is true whether or not the bed is being instructed to heat at any particular moment.  Take your multi-meter and confirm that these contacts are indeed getting the power into the relay by measuring for the 24v you expect to see coming into there.  Failure to detect this voltage would indicate faulty wiring or insufficient power supply.

7 - Testing the relay output:

Using the Control->Temperature->Bed command, instruct the printer to heat it's bed to 60.  With your multi-meter, measure for the 24v we expect to see coming out of the relay by touching your multi-meter probes to the two upper screw down terminals of the relay.  Inability to detect this voltage while the printer has been instructed to heat it's bed would indicate that your power relay needs to be replaced.

8 - Testing the heated bed component:

Lastly, we're going to measure the resistance of the bed to see if it is in spec.  Turn your printer off and unplug it from the wall.  Set your multi-meter to the 20 Ohms range and touch the probes to the outputs (upper two screw down terminals) on the power relay.  The resistance you measure should be between 1.1 and 1.4 Ohms.  Again, this measurement is done with the machine off and unplugged from the wall.  If you find that this resistance is drastically higher, a new heated bed will need to be installed.