Section 33B. During the period the polls are open for voting, the exterior of the voting machine shall remain in plain view of the election officers. If so requested, an election officer shall instruct voters outside the guard rail in the manner of operating the machine. When the name of a voter has been checked on the voting list, as required by sections sixty-seven and seventy-six, and he is found to be qualified to vote, he shall be given a voter's authority certificate and admitted within the guard rail. Before entering a machine, the voter shall hand his voter's authority certificate to the officer in charge of the machine, who shall deposit it in the receptacle provided therefor and shall then release the machine to permit the voter to close the curtain and cast his ballot. Except as otherwise provided in section seventy-nine, during the voting no officer shall be, nor permit any other person to be, in any position to see or learn how any voter is voting or has voted. The election officer attending the machine shall from time to time inspect the face of the machine to see that neither the ballot labels nor the voting indicators have been tampered with or injured. During the election the doors of the counter compartment shall not be unlocked or opened. If through mechanical failure a machine ceases to function, it shall be inspected and put in working order, if possible, by the custodian of voting machines, designated under section thirty A, in the presence of two election officers of different political parties, who shall make and sign a statement setting forth the cause of the failure of the machine to properly operate and such statement shall be filed with the election returns. If the machine cannot be put in working order, it shall be placed out of service for that election and a statement that the machine is out of order shall be signed by the election officers and filed with the election returns.