Section 421. Security interest in other property

Except as provided in
  section four hundred ten of this chapter, no retail instalment  contract
  or  obligation, or any other agreement executed in connection therewith,
  may provide for the creation of a  security  interest  in  any  real  or
  personal  property, other than the goods which are the subject matter of
  the retail instalment sale, to secure the payment of the time sale price
  under such  contract  or  obligation  or  any  part  thereof;  provided,
  however, that the prohibition of this section shall not be applicable to
  the taking of a mortgage upon real property to secure the payment of the
  time  sale  price  of a retail instalment obligation, if the transaction
  which gave rise to such  obligation  was  the  furnishing  of  goods  or
  services for repairs, alterations or improvements upon, or in connection
  with  buildings  upon, such real property. Any such prohibited provision
  shall be void but  shall  not  otherwise  affect  the  validity  of  the
  contract or obligation.