How can I check if the D550 is delivering full heat. I sense that it is not operating to full potential. Thanks/Marv

When it comes to Aluminum and Copper conductors joining (disimular metals), the resistance should always be kept to a minimum.
The use of a special conductive paste where the "copper element" goes into the aluminum secondary winding holes or nuts (if such a winding is used instead of a copper secondary) is very important for maximum element heating, plus keeping the nuts tightened. When it comes to Aluminum and Copper conductors joining (disimular metals), the resistance should always be kept to a minimum. The use of a special conductive paste where the "copper element" goes into the aluminum secondary winding holes or nuts (if such a winding is used instead of a copper secondary) is very important for maximum element heating, plus keeping the nuts tightened.

Your comment has merit but not in this case; the secondary of the transformer is a formed, heavy copper bar and unless changed, the tubular extensions are plated brass.
The solder tip is (if original) iron plated copper.
The tightness of the mounting nuts is extremely important to insure low resistance but it must be in milliohms to transfer the high current and no conductive paste will aid at that level of resistance.
On my Wellers, I have found that drilling the holes present at the end of the tubes slightly larger, then making a near-90 degree bend in the straight ends of the tip so they will protrude through the enlarged holes (tilt to install), then finally tightening the nuts down on the modified tip leads gives better contact with less need for tightening as the tip decays. Your comment has merit but not in this case; the secondary of the transformer is a formed, heavy copper bar and unless changed, the tubular extensions are plated brass. The solder tip is (if original) iron plated copper. The tightness of the mounting nuts is extremely important to insure low resistance but it must be in milliohms to transfer the high current and no conductive paste will aid at that level of resistance. On my Wellers, I have found that drilling the holes present at the end of the tubes slightly larger, then making a near-90 degree bend in the straight ends of the tip so they will protrude through the enlarged holes (tilt to install), then finally tightening the nuts down on the modified tip leads gives better contact with less need for tightening as the tip decays.