5. OPERATION OF INTERPHONE AND RADIO. a. The crew must practice continually with the interphone to obtain its maximum value during combat. It will be used for tank control during operation of the vehicle, radio operation being interrupted during that time.

(1) Helmets and microphones should be worn at all times during crew drill. As standard operating procedure, after mounting, headsets and microphones, are tested according to the following procedure:

(a) Cannoneer. 1. Turns OFF-ON switch of radio receiver to ON. (See TM 11-600 for operation of radio and interphone.)

2. Turns OFF-ON switch of transmitter (SCR-508, SCR-528) to ON. (Allow 30 seconds for tubes to warm.)

3. Pushes button of the assigned channel number until it locks.

(b) Crew members. Each crew member inserts the plug of the short cord, extending from his earphones, into the breakaway plug of the headset extension cord of his interphone control box. The microphone is fastened securely in its proper position on the throat or lip to produce maximum clarity of transmission. The microphone is connected to the breakaway plug on the microphone cord of the control box.

Figure 1. Medium tank, M4 (105-mm howitzer)—front view.

Figure 2. Medium tank, M4 (105-mm howitzer)—side view.

(c) Commander. 1. The tank commander depresses the switch on his microphone cord, and orders, CHECK INTERPHONE. (NOTE: This command is used when the crew mounts by any other method than the drills given in paragraph 8 or 24. In those drills the "Ready" report constitutes the interphone check.) Each member of the crew in the following order: gunner, bow gunner, driver, cannoneer, throws his radio-interphone switch to INT, depresses his microphone switch and reports: BOG CHECK, LOADER CHECK, etc. Upon completion of his report, he immediately returns his switch to RADIO. During this procedure, each crew member adjusts the volume control on his interphone control box to the desired level. Care must be taken that the microphone switch does not remain in the locked position. Likewise, the electric cords and the suspension strap must not be wrapped around the hand switch lest they press down on the switch button and cause the dynamotor to burn out.

2. Upon completion of the interphone check at the end of the Before Operation Inspection, or during combat at the last opportunity before the imposition of radio silence, the tank commander tests the operation of the tank radio within the net. To do this he turns his radio-interphone switch to RADIO and either waits for the platoon net to be opened by the NCS or, if the net is open, reports that the Before Operation Inspection is complete.

(2) Control box positions. Interphone control box positions are as follows:

(a) Driver. On blower bracket above transmission.

(b) Bow gunner. On blower bracket above transmission.

(c) Gunner. On right wall of turret to his right.

(d) Tank commander. On right wall of turret next to gunner's control box. He controls his transmission by manipulating the switch on his control box, marked RADIO-INT, to the type of transmission desired.

(e) Cannoneer. On left wall of turret to his rear beside the radio.

(3) Switches. The RADIO-INT switches on all control boxes, except the tank commander's, must be set on RADIO. This is the normal position for interphone operation. The tank commander's switch will be set at INT most of the time; he will change it to RADIO only as he desires radio communication. Except in an emergency, no one but the tank commander may operate the RADIO-INT switch on his control box. In an emergency, a member of the tank crew may communicate with the tank commander or another crew member by throwing his control box switch to INT; but this action will interrupt the tank commander's radio reception. It is the duty of the tank commander to monitor his radio receiver at all times except when speaking over the interphone or transmitting over the radio.

b. First echelon radio check. As a part of the daily Before Operation Inspection the tank commander will make the following first echelon radio check:

(1) Cords. (a) See that insulation and plugs are dry, unbroken, clean, and making good contact.

(b) Arrange loose cordage to prevent its entangling personnel or equipment.

(2) Antenna. See that—

(a) Mast is complete, held securely by lock screw on mast base, and sections are tight and taped.

(b) Leads at transmitter, receiver, and mast base are intact, properly insulated, and tightly connected.

(c) Mast base is clean, tight, and not cracked.

(d) Insulators passing through armor plate and bulkheads are whole and in place.

(3) Set mountings, snaps, snubbers, etc. Check for security and condition.

(4) Microphones, headsets, and controls. Check for condition and proper position. Replace from spares if necessary and turn in defective items for repair or replacement.

(5) Spare antenna sections. See that they are correctly placed in the roll and stowed to avoid being damaged or interfering with personnel.

(6) Ground lead. Check connection at both ends.

(7) Tubes. See that spare tubes are sealed in containers bearing date of last test. Turn in defective tubes at the earliest opportunity.

(8) Fuses. Check condition, and spare supply for numbers and proper rating.

(9) Cleanliness. See that both radio and equipment are clean.

(10) Battery voltage. Have driver check battery voltage. If it is low, warn cannoneer to start auxiliary generator (have this started whenever radio is operated continuously and tank engine is not running).

(11) Crystals. Check for number, position and frequency. Be sure required crystals are present.

c. It is the duty of each man invariably to check his personal interphone equipment upon mounting the tank; he should see that it is properly maintained, and report any difficulties to the tank commander.

d. Definite tank control, commands, and terminology are set forth in paragraph 6. The desirability and necessity of adhering to this specific language cannot be overemphasized. General conversation on the interphone causes misunderstanding and disorder and is harmful to discipline.

6. INTERPHONE LANGUAGE. a. Terms.

b. Commands for movement of tank.

c. Commands for control of turret.

d. Fire orders. See FM 17-12.