Turn the tender upside down. Note the position of the long spring beneath the screw that is between the front and centre axles. This is more tricky to replace if you are not aware of how it was fitted in the first place ! Remove the screw that holds the spring and thread the articulated draw bar from under the front axle. There is a lip at the end which needs to be lifted out of a slot. Keep the screw and the spring safe.
The screw is secured in a threaded hole in the tender weight, and the spring provides the earth connection to the loco chassis for normal DC operation. Except it isn't that reliable - I prefer to put in an extra earth wire for DC and DCC operation.