inserting 0's into nulls

[mech]

I have a query where certain values are blanks because of a left join. How would I fill up those blanks with 0's? Here is my code.

SELECT DISTINCT MainInput.[5100/5300], MainInput.Type, MainInput.CountOfSerial_Number, RadioOutPut.CountOfSerial_Number
FROM MainInput LEFT JOIN RadioOutPut ON (MainInput.Type = RadioOutPut.Type) AND (MainInput.[5100/5300] = RadioOutPut.[5100/5300]);