Revenue warrants shall be secured by all or part of the revenues received by the district from charges fixed pursuant to Section 35470 or from any other source other than assessments, and any such revenues may be pledged to the payment of revenue warrants. In the resolution authorizing issuance of revenue warrants, the board may subject such revenues as it specifies to a lien or charge to secure the payment of the principal of, and the interest on, revenue warrants, but such warrants shall be not secured by any lien or charge upon any assessment.