The AN/PRC-68 Legacy

Alan D. Tasker, WA1NYR

(mailto:atasker@ix.netcom.com)

With additional information supplied by Dennis Starks

A. Introduction

In the annals of US Military Portables, only four types have produced more than one “offspring,” at least by my definition (A production unit that offered some significant difference from its production unit parent that was not just an MWO). These four are as follows.

¨ The RT-10 (RT-10, RT-20, URC-10, RT-60, RT-60B, and PRC-93) rescue radios.

¨ The AN/PRC-90 (PRC-90, 103, 106, 125, and 96) rescue radios.

¨ The AN/PRC-74 (74, 74A, 74B, and 74C) HF Special Forces radios.

¨ The AN/PRC-68 (68, 68A, 68B, 126,128, and 136) squad radios.

The following discussion will explain to some degree the story of the AN/PRC-68, the first synthesized U.S. military handheld radio, and the similar but improved products that followed, all of which came out of Magnavox (now Raytheon) in Fort Wayne, Indiana starting sometime in 1976. Ref #1, 6

B. Background

A “Squad Radio” is a small tactical radio, preferably operable with just one hand, which is intended for short-range communications (around one mile). The World War II SCR-536/BC-611 was the first real squad radio. Operating in the HF/AM mode with just 50-mW output power maximum, it met with limited success. The early-to-mid 1950’s saw the introduction of the AN/PRC-6, a much better radio. Operating in VHF/FM mode with better than ¼ watt output power, it proved very successful.

The “History of the Squad Radio” was written in 1976, and chronicled the Army’s attempts over the years at developing a replacement for the AN/PRC-6. Most attempts were either too heavy, lacked performance, or were too expensive. In its latest attempt, the Army had been concentrating on separate units; namely, the two-channel hand held AN/PRT-4 transmitter and the single channel helmet mounted AN/PRR-9 receiver. These met with some success initially, but interest died quickly after that. Ref #11

Meanwhile, the Marine Corps grew tired of the Army’s lack of response to its requests for the joint development of a multi-channel squad transceiver (i.e. both transmitter and receiver in one case). As an expedient, they combined the PRR-9 and PRT-4 in a single case and called it the AN/PRC-88. Ref #1, 12

At about this same timeframe (1964), the Marine Corps also initiated development on their own of an entirely new type of squad radio. Information is sketchy as to what firms might have been involved in initial trials, but single channel crystal/jumper synthesized prototype PRC-68 units made by Motorola do exist in the collecting community. An internal jumper selected one of twelve possible channels. A change of crystals would apparently change the location of these channels in the band of coverage. Smaller than the eventual winner, an interesting feature of the Motorola unit is that the antenna was built into the bottom of the case. There is also a provision for an external antenna. It used two mercury batteries in the center of the unit. The bottom is made of plastic and holds the antenna and crystals that are not in use. The nameplate carries a 1968 contract date.

Eventually, a ten-channel synthesized version made by Magnavox won out, and a production order was placed with them in 1976. Thus, the legacy began. Ref #1, 5, 6, 7, 10

C. The Basic AN/PRC-68 Design

The RT-1113/PRC-68 is the first of the synthesized hand held portables designed and built by what was then Magnavox/Ft. Wayne, Indiana (since swallowed by Hughes Electronics, which has since been swallowed by Raytheon). As such, it has some advantages and disadvantages over the units that followed. The good news...

· 10 channels, synthesized, covering 30-79.95 MHz

· Small size (about 8.5” high by 3.75” wide by 1.5” thick)

· Simple modular construction (8 modules)

· All solid state

· Optional SVM (secure voice module) can be added in the battery box

· Antenna Options, short rubber, long rubber, or tape (same as the PRC-25 short antenna)

· Battery options, BA-1588 Mercury, BB-388 NMH, BB-? NiCad with it’s own case, BA-3588 Lithium, BB-588 NiCad

· 150 Hz sub-audible tone on Tx., carrier squelch on Rx

· Repair is by module replacement. There were eight different modules, each used once. The functions were

· 1A2, IF/AF

· 1A3, FILTER/IF

· 1A4, Tx PA

· 1A5, ANTENNA COUPLER

· 1A6, CONVERTER

· 1A7, VCO

· 1A8, MODULATOR/MIXER

· 1A9, SYNTHESIZER

This is the unit most available today on the surplus market, selling for $200 and up, and is the easiest to fix. It also covers 51 MHz, which seems to be the right frequency for military communications equipment/vehicle owners to have. Ref #2, 4

D. Limitations

The limitations of the basic PRC-68 design were as follows.

· 50 kHz spacing instead of the then emerging new standard of 25 kHz

· Needs a field strength meter to tune the antenna-matching network (1A5) when changing channel sets

· The10 channels are in successive steps of 200 kHz spacing.

E. The AN/PRC-68A

The RT-1113A/PRC-68A was the first attempt to solve some of the shortcomings of the PRC-68. The designers lengthened the case by about an inch, changed to two modules (The one on the front side is called the RF/IF Module, while the one in the back is called the Synth/AF Module), and went to large chip Motorola microcontroller (all of which became the standard for all models to come). The set has 25 kHz channel spacing, does not need an FSM for tune up, and allows 10 channels to be randomly programmed as long as all ten are within one of the 4 slightly overlapping bands. These four bands are as follows.

· 30-39.975 MHz

· 40-53.975 MHz

· 50-63.975 MHz

· 60-79.975 MHz

Units carry a 1984 contract date. Ref #2, 4

F. THE AN/PRC-68B

The PRC-68B was ordered by both the Marines and Air Force on what I suspect was combined funding. Unexpectedly, this radio has the most functionality of any of the Magnavox designed radios, including all that came later. A display was added where the speaker/mic was, so the spkr/mic was moved to a projection of the top piece (they call it the "panel"), making the unit 0.6 inches taller than the PRC-68A (1.6 inches taller than the PRC-68). This size and the display became the standard for all units to come, the only changes being in the frequency setting controls. The antenna-tuning switch was moved to the front (the unit beeps at you when you do not have it set right). This allows all 10 channels to be randomly programmed anywhere in the band of coverage. This unit also handles Tx to Rx offsets (for repeaters). Two features unique to the "B" model only are that it allows the channels to be set in 2.5 kHz increments, making it compatible with almost any band plan anywhere, and each channel can be programmed as a narrow band channel (i.e. like the hams, police and fire, etc.) or as a wide band channel (military). The unit could be switched from low band (30-88 MHz) to High Band (130-174 MHz) by swapping out the RF/IF module. Production began in 1984. Fair Radio has these for $495 with antenna, low band only. From time to time, Murphy has these also, sometimes in both frequency bands. Ref #2, 4, 6

G. The AN/PRC-126

The RT-1547/PRC-126 came along as a simplified PRC-68B, low band only. It kept the display, but lost that complicated wide/narrow and 2.5 kHz stuff, and added a display light. Therefore, it is back to 25 kHz channels, and no possibility some grunt can set it wrong. This is the present Army/Marine combat hand held, and it entered service in 1986. Units sell for $500-750. Ref #2, 4, 6

H. The AN/PRC-128

The PRC-128 (Scope Shield I) is a PRC-126 but with 12.5 kHz channel spacing for AF guard duty. It has the ability to operate in “high band” by swapping out the RF/IF module. In low band, it has “wide band” FM deviation, while in high band, it has “narrow band” FM deviation. Market price seems to be similar to that of the 126. Ref #2, 4

I. The AN/PRC-136

The PRC-136 is, I suspect, an all narrow band version of the PRC-128. It is NBFM to be compatible with civilian services. A High Band only version is in service with the Marine Corps for their Crash Fire Rescue Service (CFRS). Ref #9

J. The competition for Scope Shield II

From what little evidence there is, it would seem there was stiff competition for the three band Scope Shield II radio. Ultimately won by Racal (with a product ultimately called the AN/PRC-139), this brought to a close the long production run of Magnavox (now Raytheon) handhelds. It is thought that the following models were submitted in this competition.

1. Racal, AN/PRC-134

· PRC-134(V)5(C), Low Band, NSN 5820-01-349-9173

· PRC-134(V)6(C), High Band, NSN 5820-01-351-4531

· PRC-134(V)7(C), UHF, NSN 5820-01-349-9171

2. Magnavox, AN/PRC-135

· RT-1675/PRC-135, Low Band, NSN 5820-01-351-6278

· PRC-135(V)2(C), High Band, NSN 5820-01-351-6279

· PRC-135(V)3(C), UHF, NSN 5820-01-351-6820

3. Motorola, ???

K. Mobile Power Amplifier Units

Some of these units had a vehicular RF/AF/Power Supply module to allow powering from the vehicle, amplification of RF and AF signals, and unit mounting while in the vehicle. These are as follows. Ref #8

PRC-68A, OG-174 for Army fire control

PRC-68B, AM-7302

PRC-128, OF-185

PRC-136, OG-196

PRC-139 (NSN 5820-01-369-6046)(Scope Shield II, Racal, not Magnavox), OF-228 (Note: this radio covers three bands by module replacement, not just the two bands of the PRC-68B, 128.)(Listed for reference only)

L. Additional Information

Additional Nomenclatures

There are a number of additional nomenclatures floating around that would tend to indicate that additional different versions of these basic designs were produced. However, little physical evidence seems to exist that there was any significant production. There are many possible explanations of this. For instance, when the program began, the AN/PRC-68B with a low band module installed was called an AN/PRC-68B(V)1, and with a high band module installed was called an AN/PRC-68B(V)2. (Ref #3, Magnavox generated preliminary manual.) However, when the very same program was further along, most references to the “(V)1 and (V)2” had been dropped, although the actual units are labeled AN/PRC-68B(V). (Ref #3, Magnavox generated final manual.) This shows that nomenclature can sometimes change during the existence of a program.

In addition, Magnavox’s advertising literature for this product (assumed generated before a sale was made) shows a unit slightly different from what was eventually produced (having rear RF ports, presumably for the external power amplifier). They labeled the Low Band version PRC-68B and the High Band version PRC-68(X-4). Yet, in the 1989 Janes (Ref #5), the low band version is called the PRC-68(X-2) while the high band version is called the PRC-68(X-4). This shows that many times, a company will make a few prototypes with some kind of nomenclature, either government supplied or self generated, in order to show them off and perhaps entice an order.

Of course, there is always the possibility of an actual government contract for a few test units that never makes it into production, or has a very limited production run for a specialized purpose, or is bought by a foreign government.

In any event, the following is a list of orphan units, other than the ones listed above, about which very little is known. Ref #5, 6, 7

· AN/PRC-68X, a single channel unit whose frequency is selected by front panel switches. This appears twice in the 1989 Janes (Ref #5), once claiming to be in production, and once claiming “no longer in the manufacture’s inventory.” Others claim this designation refers to a high band PRC-68.

· AN/PRC-68L, a ten-channel unit that looks just like the original PRC-68 except it has only two modules inside. There is a switch to choose between 25 kHz channel steps and 50 kHz steps. This appears to be a developmental model, and most likely was not microprocessor controlled. It most likely offered the same channel arrangement as the PRC-68, i.e. ten at 200 kHz spacing. At least two examples are in the hands of the collecting community, but neither of them works. Ref #5 (1990-91 issue), 13

Manuals, NSN

PRC-68, NSN 5820-01-079-9260

TM 11 -5820-882-10/TM 07827A-10/1, Operator Manual

TM 11 -5820-882-23/TM 06827A-23/2, Org & DS Maint Manual

PRC-68A, NSN 5820-01-180-8943

TM 11-5820-882-23&P-1, Unit &int DS Maint Manual

TM 06827B-24/2, Org & Int Maint Inst. Manual

PRC-68B, NSN 5820-01-179-7027 (5820-01-248-2852 high band)

MX-63-107, Ops Inst.

MX-63-121, Ops and Maint Inst. with IPB

PRC-126, NSN 5820-01-215-126

TM 11 -5820-1025-10, Ops Manual

TM 11 -5820-1025-?? (MX-63-114B), Ops & Maint Inst. with IPB

PRC-128, NSN 5820-01-288-0626

TO 31R2-4-810-1, Ops & Maint Inst. with IPB, Org

TO 31R2-4-810-3, Maint Inst. with IPB, Depot

PRC-136, NSN 5820-01-390-9438

M. References

1. “History of the Squad Radio,” Marvin W. Curtis, ECOM Report # 4451

2. Observation

3. Manufacturer’s advertising literature and/or technical reports

4. The Technical Manuals for the individual units

5. Jane’s Military Communications, 1981 and/or 1989 and/or 1994-5

6. Military Collector Group Post, 7/20/99

7. Military Collector Group Post, Condensed PRC Data, 10/22/98-11/3/98

8. Information provided by Joseph Pinner

9. Military Internet Posting

10. Information and pictures provided by Tom Bryan

11. “Military Communications, Test for technology, the U.S. Army in Vietnam,” John D. Bergen

12. Military Collector Group Post, Mystery Radio, The AN/PRC-88, Backmail #41

13. Mike Frye

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The proceeding is an updated version of an article that originally appeared in the "Military Collector Group Post"; an international email magazine dedicated to the preservation of history and the equipment that made it. Unlimited circulation of this material is authorized as long as the proper credit to the original author(s) and publisher (or the group) is included. For more information concerning this group or membership, contact Dennis Starks at….

military-radio-guy@juno.com. A list of selected articles of interest to members can be seen at: http://www.softcom.net/users/buzz/backmail.html

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