The TSC-15 "Communications Central"...
View of a TSC-15 in an M-715 5/4 ton truck shown here in a 1967 M-715 1 1/4 ton truck.

  The TSC-15 was an outgrowth of Collins Radio Company's 'Project Birdcall'.  In the mid 1950's,  General Curtis LeMay felt a need for an automatically-tuned 1KW HF SSB radio to replace the 100-150 watt HF radios then current in his front-line SAC bombers.  No such radio existed at the time, and Collins Radio was selected by the Air Force to develop both air-ground and point-to-point high-power HF SSB radio systems.   'Old Iron Ass' LeMay had his way with Collins Radio, and after a few years of grief and pain and wailing and gnashing of teeth in the Collins engineering departments, the ARC-58 was born.

  The ARC-58 found it's way into most all B-52s & B-58s, and soon ARC-58 technology was incorporated into several other radio systems, like:
 -  The 18Z-3 & 18Z-4, pressurized & unpressurized commercial versions of the ARC-58.
 -  The TRC-75, a repackaged & watertight ARC-58, intended for the back of an M38A1 or M170 Jeep.
 -  The URC-32, a manually tuned, 500W rackmount version, for shipboard & fixed installations. (Was the URC-32 the first radio to have a synthesizer controlled by a single 'analog-style' tuning knob?)
 -  The TSC-15, a full-duplex version of the TRC-75, installed in a 6' x 6' x 6' radio shelter.
 -  A mystery radio, used by Australian Forces in Vietnam, which appears to be a full-duplex TRC-75 in an lwb Land Rover.
 -  At least one other mystery radio, RCA-built, which seems to be an automatically-tuned URC-32.

  The unpressurized 18Z-3, the unpressurized ARC-58, the TRC-75, and the TSC-15 all used the same receiver, the R-761/ARC-58.  The unpressurized airborne radios used the T-605/ARC-58, but the ground radios used a T-730/TRC-75.  The only electrical difference between the two transmitters seems to be a shorting jumper on the barometric pressure sensor switch in the ground models, presumably added to prevent the radio from reacting to concussion from nearby artillery.

  The TSC-15 "Communications Central" provides radiotelephone and radioteletype communications over a full-duplex HF radio link.  It can operate in USB, LSB, TWIN, ISB, AME, CW, FSK, and FSK-MUX modes.  Automatic switching allows the RF linear amplifier to operate Class C if transmitting a single tone signal.  A CV-976/U voice mux/demux unit allows four (bandwidth-limited) full-duplex voice channels to operate over both sidebands in ISB mode.  A TGA-1 radioteletype mux/demux unit allows four full-duplex teletype channels to communicate over 1 sideband, and also provides a single full-duplex 850 Hz. shift teletype channel using a full sideband.  An F-508/U preselector/filter provides isolation between the transmitter and the full-duplex receiver.  Five 2-wire or 4-wire field telephones and five 2-wire or 4-wire teleprinter machines can be connected to the TSC-15 through the signal entry panel.  An SB-1248/TSC-15 cordless switchboard allows any of the five field 'phones to be connected to any other field 'phone, or to either sideband, or to any one of the four voice mux/demux channels, or in a 'conference call' combination.  The operator can use a local TT-299B/UG (MITE) teleprinter to communicate with any one of the five local teleprinters, or any one of the five available radioteletype channels.  A TS-1635/TSC-15 test box allows the operator to perform a full-duplex voice or radioteletype loopback test of the entire Communications Central, with the exception of the RF power amplifier.  The TS-1635/TSC-15 is wired in parallel with the signal entry panel, and can generate test tones for the field 'phones, monitor audio levels from the field 'phones, and connect the local MITE teleprinter in place of one of the five local teleprinters.

View inside the TSC-15
   A view looking through the door of the TSC-15 shelter shows a 6 HP., 3 KW., 110 VAC., 3 phase, 400 Hz. generator inside the shelter.  A trolley/hoist arrangement (not seen in this photograph) attaches to the red bracket on the ceiling of the shelter, and is used to lift the generator, roll it out of the shelter, and lower it to the ground.  The operator's table is behind the generator, folded up and obscuring the main console.  Stored on the door is the full-duplex receive antenna mount and a reel of coaxial cable.  (When operating full-duplex, the receive antenna must be a minimum of 150 feet from the shelter's transmit antenna.)

View of TSC-15 operator's console
   The TSC-15 operator's console.  The black vertical panel to the left is the  C-3533/TSC-15 teletype control panel.  The black vertical panel to the right carries a C-3595/U preselector control box, the C-3141/TRC-75 duplex receiver control box, the C-3141/TRC-75 simplex transceiver control box, the C-3532/TSC-15 radio mode selector/wattmeter box, and the C-3534/TSC-15 power supply monitor box.  At the top of the main console is the C-3535/TSC-15 telephone terminal control unit, which contains the five telephone line interfaces, the five telephone line hybrid units, and four VOX amplifiers.  The TA-408/TSC-15 patch cord panel lets the operator set up the Communications Central in non-standard ways, in order to accomodate equipment failures or to facilitate troubleshooting.  The SB-1248/TSC-15 switchboard can create communications circuits between the five field 'phone lines, the four radio voice channels, and the operator's console.  The switchboard has no cords as such-  switching is accomplished by eight rotary stepping relays controlled by the rows and columns of lighted pushbutton switches.  Below the switchboard is the console accessory panel, which contains four self-leveling transmitter audio amplifiers, and circuitry having to do with the Automatic Telephone Ringing feature.  (Once set up, Automatic Telephone Ringing allows unattended operation.  Ringing voltage from a local field 'phone will cause a two-tone signal to be transmitted by the local TSC-15.  Upon receiving this two-tone radio signal, the remote TSC-15 will send ringing voltage to one of it's field 'phones, thereby establishing a connection with no operator intervention.)  Unseen below the folded-down desktop are the MT-2445/TSC-15 and the MT-2446/TSC-15 amplifier assemblies, the RE-502/TSC-15 Director Relay Assembly, and the PP-2868/U (regulated) and PP-2869/TSC-15 (unregulated) +28 Volt power supplies.

view of TSC-15 right-hand equipment bay
   The TSC-15 right-hand equipment bay.  At the top is the duplex receiver unit, an R-761/TRC-75.  The simplex receiver/exciter unit (also an R-761/TRC-75) is at the center, and at the bottom is the TGA-1 radioteletype mux/demux unit.  Mounted above the bay is a squirrelcage blower supplying cooling air to all three devices in the bay.  Also seen is the TS-1635/TSC-15 test set.

view of TSC-15 left-hand equipment bay
   The TSC-15 left-hand equipment bay.  On the bottom shelf is the T-730/TRC-75 1 KW. RF power amplifier & power supply.  Above it is a shelf carrying the CV-976/U voice mux & demux unit and the F-508/U preselector.  On the top shelf is the CU-749/TRC-75 antenna coupler and the C-2848/TRC-75 antenna coupler controller.  Mounted above the bay is a multistage vaneaxial fan supplying cooling air to the T-730/TRC-75.  The operator's TT-299B/UG (MITE) teleprinter on it's shelf is visible at the lower left.

close-up view of TSC-15 radio control heads
   A close-up of the C-3595/U preselector control box, the C-3141/TRC-75 duplex receiver control box, and the C-3141/TRC-75 simplex transceiver control box.  The label reads  "WARNING  DO NOT TUNE BAND PASS FILTER OR DUPLEX RECEIVER TO A FREQUENCY WITHIN +/- 10% OF TRANSMITTER FREQUENCY".

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