I have a matched set of Spectrumn F 7A-B units that I assume are older, but new in the box unopened. I want to convert them to DCC and I am using a Soundtraxx decoder function only, no sound. Can you tell me if the copper tab on the bottom of the motor needs to be removed? And is this where I should attach the decoder ground wire? When I took the engine apart, I found the red (+) wire and the negative (-) soldered to the top of the engine, seemingly on the same post. I have two pictures for you, but I can't send them. Seems the files are to big.
The motor in these have brass flywheels on them, so I guess they are a newer design? Thank for your help. And let me know if there is another way to send pictures to you. Thank You.

Rusty Rail

Good day to all. I have recently purchased non DCC Spectrum HO F7A and F7B units at a local show. They are new in unopened boxes and are DC only. I have purchased a sound decoder and a function only decoder to install in the locomotives. It seems like a straight forward install, but I have been given some mixed advice. On the internet I have seen where you are required to snip off the wheel wiper and then solder a wire on the truck frame. Nowhere on the Soundtraxx web site does it instruct you to do this step. So my question is: should I snip the wiper off, or not? 
Any help gentlemen would be fine. I am also installing a sound decoder in a Bachmann GP 7 as well. Thanks to all who respond in helping with my dilemma. 

Rusty Rail