The city may incur indebtedness by issuing bonds with the approval of the electors of the city at an election in the manner prescribed by and to the extent and for any purpose or purposes authorized by the state constitution and law. Sinking funds shall be established, maintained and administered as required by the state constitution and law and this charter. The proceeds of all bonds voted for a special purpose or purposes may be used only for such purpose or purposes or to retire the indebtedness created thereby. Interest payments and retirement of bonds voted and issued for the purpose of the purchase, alteration, re-equipment or improvement of public utilities shall constitute an obligation against the receipts and income from public utilities operated by the city. The city may refund its indebtedness in accordance with the state constitution and state law.