make or insure an equity loan in the form of a supplemental loan for the purpose of equity takeout to the owner of housing financed with a loan made or insured under this section pursuant to a contract entered into before December 15, 1989 , for the purpose of extending the affordability of the housing for low income families or persons and very low-income families or persons for not less than 20 years, except that such loan may not exceed 90 percent of the value of the equity in the project as determined by the Secretary;