The Newport Savings Bank of Newport, N.H., has become the first mutual savings bank in the country to receive a federal charter. It was granted yesterday by the Federal Home Loan Bank Board.

This is the first of what is expected to be a stream of conversions from state of federally chartered institutions. Under the Depository Institutions Deregulation and Monetary Control Act of 1980, federals enjoy much broader asset, liability and branching powers than do state-chartered savings banks, particularly in New York.

Thirteen banks from that state already have applied, and the American Banker reports that virtually all the large mutuals in the city and upstate have expressed interest in federal chartering.

Such flexibility is considered an important tool in maintaining the viability of mutual savings banks at a period when a considerable number of them are operating in the red.

Another tool is merger. This is also a method of pooling talent in preparation for the fast-approaching day when thrifts will have increased consumer lending powers and the authority to seek new sources of assets in such sophisticated areas as pass-through securities and the Eurodollar market.

Three New York City savings banks announced yesterday they were holding merger talks. They are the Empire, American and Franklin Savings banks.

Mergers also are increasing among mutual savings and loan associations.

In another action, the bank board approved the sale of rehabilitation loans by S&Ls to the Federal National Mortgage Association (Fannie Mae). The loan amount is based on the after-rehab value of the house. However, if the loan is in default before completion of the work or the rehabilitation is not completed within one year, Fannie Mae will require the S&L to repurchase the loan.