The photo I found of the Lyman D5 showed it to be a standard Ohaus scale. Set all the weights on zero. With the empty pan in place, screw the thumb-wheel that's the 3rd foot (on the far left) either up or down until the scale zeros.
If the scale won't "zero" repeatably, then there may be minor rust on the knife edges. Remove the balance beam and look closely at the wedged-shaped pivot point. Don't sand it, simply rub some grease on it to dissolve the rust and keep it from re-forming.
If with all this you still can't get the scale to come to zero, remove the pan to discover a screw. Under the screw is a compartment you can add or subtract sand (or other small weights) to bring it to zero.