AN~PRC-74

Overview

Radio Set AN/PRC-74 or the AN/PRC-74A is a lightweight, two-way communications set that is capable of receiving and transmitting voice and telegraph signals. Single-sideband (SSB) modulation is used for voice transmission, and continuous wave (CW) modulation is used for telegraph transmissions. The voice and telegraph receptions are demodulated and applied to a headset. The frequency range of the radio set for both receive and transmit modes of operation is between 2.0 mHz and 11.999 mHz for the unlettered and A models and 2.0 - 17.999 mHz for the "B" version.

The radio set may be portably operated or operated at a fixed station. When portably operated, the radio set is powered by either a rechargeable nickel cadmium storage battery pack, or a dry cell battery pack. An external battery charger and power supply assemblies are provided with the radio set when used at a fixed station. The power supply assembly allows the radio set to be powered by either the alternating current (AC) or direct current (DC) voltages available at the operating site. The external battery charger assembly is used to recharge the radio set batteries prior to using the radio set portably.

Three types of antennas are supplied with the radio set. Operating in the low end of the high frequency spectrum, the range of the radio set may be extended to hundreds of miles with the proper selection of frequency and antenna type.

The operational features of the AN/PRC- 74 and AN/PRC-74A are identical except that the A model has been modified by MWO 11- 5820-590-35/1 to permit operation with the AN/GRA-71 coder burst equipment. Refer to TM 11-5820-590-12 for operational procedures for coder burst operation. Refer to paragraph 1-59 of the technical manual for circuit differences between the two models.

Employment

Radio set AN/PRC-74b is organic to special forces units and to military assistance teams in southeast asia. It provides long-range patrols with a means of communicating with their base stations. The AN/PRC-74B is interoperable with other high-frequency single sideband radios such as the AN/GRC-106

Development Background

The AN/PRC-74 radio was developed by Hughes Aircraft Company primarily for the military market. It was first issues for user tests in 1964. in 1966 this radio was modified to provide a continuous wave (CW) transmission capability. The modified set, the AN/PRC-74A was initially delivered for Army use in June of 1966. The AN/PRC-74B, issued to Army units in early 1967, incorporated another modification, which extended the set's frequency range from 2 - 11.999 MHz to 2 - 17.999 MHz.

Specifications

Weight

28 pounds 2 Ounces

Size

18" x 12" x 3-3/8"

Power Source

BB-418 nickel cadmium battery

BA-386 dry cell battery

Frequency range

2.000 to 17.999 mc in 1.0 kc steps.

Frequency standard stability

± 1.5 parts/million from -30°C to +55°C (AN(PRC-74B).± 1.5 parts/million from -40°C to +65°C (AN/PRC-74C).

Intermediate frequency

1,750 kc.

Range

Whip Antenna - 15 miles

Doublet - 30 miles

Slant wire - 500 niles

Transmitter performance:

Modulation modes

Upper-sideband suppressed carrier, voice or cw.

Carrier suppression

40 db below level of a 1,000- cycle modulation usb signal of rated output.

Power output

15 watts peak envelope power nominal.

Monitor sidetone

Voice or tone as determined by modulation mode.

Tuning indicator

Meter (operated by vswr bridge).

Receiver performance:

Sensitivity

0.7 microvolt for 10 db, signal plus noise to noise.

Selectivity

.3-db points at +300 and + 2,700 cps, 45-db points at -350 and + 6,500 cps (referenced to carrier frequency).

Audio output

1 mw minimum into a 500-ohm headset.

Audio distortion

10 percent maximum at frequencies between 300 cps and 2,700 cps with an audio output level of 4 milliwatts.

Power source

Determined by fixed or portable requirements: An external power supply may be used as a power source during fixed operation. For portable use, wet or dry battery pack may be used.

Input power

+ 10.5 to + 17.0 volts

Images

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  Slant Wire Antenna

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  Horizontal Dipole

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  Sloping Dipole

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  Inverted "V" Dipole

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  Dipole Fixture

These Images are from a complete wiring harness swap. The front audio section of a PRC-74 was hacked to pieces in a botched HAM conversion. I replaced the entire wiring harness of the radio with a donor PRC-74B after confirming every connection in the harnesses were the same.

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  Front of a severely hacked PRC-74

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  Front of a factory PRC-74B

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  Topside of the donor 74B

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  Bottom side of the donor 74B

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  Front panel of the 74

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  topside of the 74 before the swap

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  Read of the hacked 74, note the power connector

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  backside of the radio cover

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  Inside the front panel of the donor 74B

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  Harness removed from the donor ready to be installed

I replaced the harness from left to right, only removing parts as i was ready to connect the new harness.

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Antenna

The radio includes a whip and an adjustable dipole on reels.

Required Wire Lengths

Dipole Wire Leg Lengths
Freq (MHz)	Length (Ft.)	Freq (MHz)	Length (Ft.)	Freq (MHz)	Length (Ft.)
2.00	117	5.0	47	13.00	18
2.20	106	5.50	43	14.00	17
2.40	98	6.00	39	15.00	16
2.60	90	6.50	36	16.00	15
2.80	84	7.00	33	17.00	14
3.00	78	7.50	31	18.00	13
3.25	72	8.00	29	MEASURE WIRE LENGTH ACCURATELY FOR OPERATING FREQUENCY USING THE WRONG LENGTH MAY DAMAGE THE RADIO SET.
3.50	67	8.50	28
3.75	62	9.00	26
4.00	58	9.50	25
4.25	55	10.00	23
4.50	52	11.00	21
4.75	49	12.00	20



Reference Files

PRC-74A

TM 11-5820-590-35

DS, GS, AND DEPOT MAINTENANCE MANUAL

RADIO SET

AN/PRC-74

JULY 1966

PRC-74B AND C

TM 11-5820-590-12-1

OPERATOR'S AND ORGANIZATIONAL MAINTENANCE MANUAL (INCLUDING REPAIR PARTS AND SPECIAL TOOLS LISTS)

RADIO SETS AN/PRC-74B (NSN 5820-00-935-0030)

AN/PRC-74C (NSN 5820-00-177-1641);

POWER SUPPLIES PP-4514/PRC-74 (NSN 5820-00-942-0821)

PP-4514A/PRC-74 (NSN 5820-00-177-4581)

BATTERY BOXES CY-6121/PRC-74 (NSN 5820-00-908-3127)

CY-6314/PRC-74 AND CY-6314A/PRC-74 (NSN 5820-00-156-3934)

March 1967

TM 11-5820-590-35-1

DIRECT SUPPORT, GENERAL SUPPORT, AND DEPOT MAINTENANCE MANUAL

INCLUDING REPAIR PARTS AND SPECIAL TOOLS LISTS)

RADIO SETS AN/PRC-74B (NSN 5820-00-935-0030)

AN/PRC-74C (NSN 5820-00-177-1641);

POWER SUPPLIES PP-4514/PRC-74 (NSN 5820-00-942-0821)

PP-4514A/PRC-74 (NSN 5820-00-177-4581)

BATTERY BOXES CY-6121/PRC-74 (NSN 5820-00-908-3127)

CY-6314/PRC-74 AND CY-6314A/PRC-74 (NSN 5820-00-156-3934)

JULY 1968

PRC-74 Schematics

These are the missing schematics from the commonly found PRC-74B manual found on Google books

PRC-74 Instructions for adding Lower Sideband Support

Defines the process required to add Lower SSB (LSB) support for the PRC-74

James P Karlow