That is interesting! I have recapped many radios both AA5 (series heaters) and transformer sets and have never seen a common positive. I am not saying that is impossible, however! Black color wire on the old multi-section electrolytics almost always indicates negative or common. On the chassis mounted metal cans the negative is the case of the cap. Reversing the polarity causes a "Big Bang" that is not a theory . Can you post the schematic section from the Emerson? Thanks!