|zur vorigen Seite to previous page|
The AN/PRC-90 Legacy
D. Tasker, WA1NYR
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.
RT-10 (RT-10, RT-20,
RT-60B, AN/PRC-93) rescue radios
103, 106, 125, 96) rescue radios
AN/PRC-74 (74, 74A,
, 74B, 74C) HF Special Forces
68A, 68B, 126, 128, 136) Squad radios
The following discussion will explain to some
degree the story of the AN/PRC-90, the first tri-service rescue radio, and
additional rescue related products, production of which began at GTE-Sylvania in
Needham, MA starting sometime in the mid-nineteen sixties. According to Jane’s,
the approximate number of PRC-90 units of all types ordered is well over 50,000
units. Many are still in use today, sometimes even being preferred over the
supposed replacement (AN/PRC-112). This attests to the excellence of these older
designs. Ref #4, 5
In the 1965 period, GTE-Sylvania had produced the
smallest single frequency military rescue radio ever, the AN/PRC-63 (Navy
contract). They used modular techniques along with a space saving super
regenerative receiver, a new small 14-Volt mercury battery (BA-1568) that could
be enclosed in a waterproof compartment with a screw on cover, and possibly the
first military use of what we now call the “rubber duckie” antenna, although
thinner than most.
However, the experiences in the Vietnam conflict
were proving the need for at least a two-frequency unit. Adapting the circuitry of the PRC-63 for this task would not
work for the following reasons.
Super regenerative receivers are not particularly
selective, and thus not adaptable to two frequencies, especially if they are
close together. This type of receiver also radiates RF energy, not exactly a
Transmitter circuitry tends to be very selective in
order to keep down spurious emissions, and thus not too adaptable if the two
frequencies are far enough apart.
The Basic AN/PRC-90 Design
This led to the basic design principals of the
PRC-90, which were……
To employ a two receiver super heterodyne design.
To employ a two-transmitter design.
To use the same antenna and battery/battery
compartment type as in the PRC-63.
Because the case had to be longer than that of the
PRC-63, a separate microphone and earpiece was used, greatly simplifying T/R
switching. In addition, for the first time, an external earphone connection and
earphone were provided (a magnet in the ear piece plug activated a switch inside
the unit which disconnected the internal earpiece).
Modular construction was again used. The internal
module placement is as follows. All descriptions below assume that the unit is
sitting with its front side down (i.e. as in on top of a table), that the
antenna is pointing away from you, and that the back cover has been removed.
Once this is done, it is easy to see that the main interconnect within the unit
is a long, thin, vertically oriented printed circuit board running from the top
to the bottom, and centered half way between the right and left sides. The two
receiver modules are mounted horizontally, one above the other, and plugged into
this central board toward the bottom left, opposite the battery compartment (which
would be on the bottom right). There is a smaller module underneath them that
amplifies the receiver audio, and the Tx audio/modulator module is on top of
them, making it a four-module stack. The two transmitter modules are also
mounted one on top of the other, and plugged into the central board on the top
right, opposite the three controls (channel switch, PTT switch, and MCW switch).
Underneath them is the Tx/Rx switching board. One neat concept introduced here
was the frequency selector switch. It can not leak because no moving parts
penetrate the case. The switch uses an external permanent magnet that moves with
the switch rotation, and this causes internal reed switches to close at the
proper position (similar to how an RT-60 frequency selector switch works). Ref
The basic specifications are as follows. Ref #4
400-mW output power in voice mode, 243 MHz or 282.8
500 mW output power in MCW or Beacon mode, 243 MHz
Rx sensitivity, 2.5 uV.
There is some discussion about a patented circuit
that GTE-Sylvania employed in this unit, perhaps acquiring it originally from
NCR. By examining the circuit diagrams, it would appear this patent, if related
to a single circuit, was used to effect the receiver design. This single
conversion superhet employs only five transistors in the main receiver module.
The function of these is as follows.
AM Detector (30+ MHz super-regen I think)
Audio Pre-Amp (the post audio amp is external to the main receiver module)
This is an extremely compact design, even for
Ref #1, 4, 7
Known manufacturers include.
GTE- Sylvania (starting in 1967 or so, originally
for the Navy).
OAI (this symbol refers to both Oklahoma
Aerotronics and OAI)
Cost Saving Efforts
Although a very effective design, the PRC-90 was
expensive to produce. It was clear that in order to save money, major internal
changes would have to be made. Chief among these were the following.
Conversion to a more conventional two frequency,
single receiver, single transmitter design.
Elimination of modular construction as much as
Elimination of patented circuitry on which
royalties would have to be paid.
It is not clear to me who accomplished the design
changes. It is known that both ACR and OAI manufactured the first cost reduced
version, the AN/PRC-90-1. Ref #1
This unit was produced using cases originally made
for the PRC-90 production, and containing letter engraving for that unit.
However, in the 90-1, the MCW function had been eliminated and replaced with a
high power beacon function, so paste strip overlays were used as appropriate to
change external labeling.
Both the receiver and transmitter were reduced in
size to fit as much as possible on a single conventional PC board, and each is
located in the same relative positions as discussed above. In the ACR design,
the receiver made use of the LM1868 AM receiver chip, certainly a great space
saver. There was, in addition, a small audio amplifier module located on the
central board. The transmit modulator used the LM386 amplifier chip (located on
the central board) to drive the modulation transformer. I assume the OAI design
is similar if not identical. There may have been other changes, but with the
exception of the PRC-90 modules, I do not have any schematic diagrams for any of
the radios discussed in this report. If some come my way, I will see if there is
anything else that pops up as significant. Ref #1
Both ACR and OAI take credit for changing the ¼
wave antenna into the much more effective ½ wave type. It should be pointed out
here, however, that ½ wave designs were nothing new to ACR. The manual for the
AN/URC-10/RT-10 (The URC-10 is a separate battery version of the ACR designed
RT-10) states that there are two types of units in the field, those with ¼ wave
antennas and those with ½ wave antennas.
The 90-1 ½ wave has a base matching section to
transform the high antenna impedance to the radio’s output impedance (50 Ohms
assumed). Above that is a springy section just long enough to allow the antenna
to fold around the top of the radio for storage. Lastly, above that is a
Specifications, AN/PRC-90-1 Ref #3, 4, 7
600 mW output power in voice mode, 243 MHz or 282.8
500 mW output power in beacon mode, 243 MHz only.
750 mW in high power beacon mode, 243 MHz only.
Rx sensitivity, 2.5 uV.
It would appear that the only changes contained in
this unit as compared to the 90-1 are as follows.
Use of cases with proper engraving for the high
power beacon function and elimination of the MCW function.
Incorporation of a ½ wave antenna without the
matching section (cost saver).
In order to accomplish the latter, the matching
section is incorporated inside the unit in discrete form. The PRC-90-2C is
apparently a non-rescue frequency version of the 90-2.
Known manufacturers include the following. Ref #1,
-2, NSN 5820-01-238-6603
-2C, NSN 5820-01-338-3036
Addresses Where the AN/PRC-90 is mentioned
T.O. 31R2-2PR-103 (formerly 12R2-2PRC90-? /Navy
16-30PRC90-?, 30 March 197?, incl C24, 1 June 1991 (some dates unclear due to a
T.O. 12R2-2PRC90-3/Navy 16-30PRC90-3, 30 Mar 70,
inc C3 15 Nov 72
T.O. 12R2-2PRC90-4/Navair 16-30PRC90-4, 30 Nov 72,
inc C2 1 July 73
TM 11-5820-800-12, Nov 73, NSN 5820-00-782-5308
T.O. 31R2-2PRC90-1, 31 Jan 86, inc C4, 15 Aug 88
(covers –1 and –2)
TM 11-5820-1049-30, 15 Aug 90 (covers –2), NSN
Other than being a haze gray in color as opposed to
the green of other units, and having an attachment that essentially enlarges the
target size of the PTT button, there appears to be no other differences between
this and other models. The Navy ordered a small quantity of these to provide
communications for wearers of HAZ-MAT suits. These suits had their own internal
headphone and microphone, so some sort of interface had to be designed and
built. This interface device is called the HMF-3 acoustical coupler. It clamped
to the front of the PRC-90A, and pressed its small speaker to the 90A’s
microphone. It also had a plug to connect to the 90A’s earphone jack. Powered
by small batteries, this coupler amplified and buffered both the Rx and Tx audio
signals between the 90A and the audio components of the suit. The only
manufacturer of these components seems to be OAI. Ref #1, 2
PRC-90 “T” Units
Both the PRC-90 and the PRC-106 (see below) were
bought in a training version, that is, crystaled for training frequencies
instead of rescue ones. Generally, this was 251.9 MHz and 236 MHz. The
designation was changed to AN/PRC-90T, for example, to indicate this change. In
addition, training units are generally painted a strikingly different color so
there would be little likelihood of grabbing the wrong unit in a pinch. The
PRC-90T is yellow, and the PRC-106T is red. Ref #7
90T, NSN 5820-00-469-5658
90-2T, NSN ?
106T. NSN 5820-
The AN/PRC-106 is similar to an AN/PRC-90 except
its two channels are 121.5 Mhz and 243 MHz. In order to accomplish this, the
original 90 design of two separate receivers and transmitters had to be used.
The beacon function has been eliminated from the rotary frequency selector
switch. However, that function is now available on either frequency by pushing
the top mounted switch (which was the MCW function switch in a PRC-90). There is
a new feature provided that was not used on the original 90. Each transmitter
contains a detector circuit that feeds actual transmitted audio to the earphone
for Tx sidetone.
Manufactured by ACR, OAI, and C-RAN since 1974 for
the AF, DEA, and probably others, it has the following specs.
Power output, 100-mW average (400-mW pep) voice,
125-mW average (500-mW pep) beacon.
Rx sensitivity, 5 uV
Immersion, five minutes/15.2 m, 24 hours/0.6 m.
T.O. 31R2-2PRC106-1 (formerly 12R2-2PRC106-1), 1
Dec 1974, include C1 15 June 1979, NSN ?
Para-Rescue Units, the AN/PRC-103 and the AN/PRC-125
Produced since 1976 by C-RAN, the AN/PRC-103 is an
Air Force para-rescue radio. It is
a PRC-90 type with the following changes.
The controls are mirrored side-to-side with respect
to those of the PRC-90 series.
There is no microphone or speaker.
There is an added control, oriented horizontally
near the top of the unit, and labeled “VOX Sensitivity.” This controls the
VOX level so that the wearer can transmit while keeping both hands free.
There is a rubber boot attached to the bottom of
the unit that contains the connection jacks for the microphone and the earphone,
which are part of the headgear.
There is a belt loop device attached to the back.
When used to carry the unit, there is an antenna extension cable to allow the
antenna to be up on the shoulder or on the cap (Velcro). Either a stubby antenna
or a regular antenna can be used.
Internally, the construction is similar to the
original 90, i.e. two receiver and two transmitter modules, etc.
31R2-2PRC103-3, 1 MAY 1983
12R2-2PRC103-11, 1 August 1977
Produced by OAI, the AN/PRC-125 is the para-rescue
radio for the Navy. Although the unit is similar in size to those discussed
above, and although it uses the same battery, the only control on the main body
is very different (although it still uses the magnet/reed switch arrangement),
and the link back to GTE becomes a bit tenuous here. The frequency selector/off
switch is a side mounted slide type as opposed to the rotary type on the above
units. The antenna, volume control, PTT switch, and earphone jack are on a
remote speaker/mic that connects to the radio by means of a 36” cable. The
radio has a jack on the top for this purpose. This set is stored in, and meant
to be used with the LPU-28/P Life Vest. The speaker/mic mounts to the vest
(Velcro) in such a way that the wearer’s chin can be used to depress the PTT
switch. Internally, there are a series of plug-in printed circuit boards (right
to left orientation) that plug vertically into a motherboard located just behind
the front panel. The circuitry is primarily discrete transistor in nature.
NAVAIR 16-35PRC125-1, 1 October 1992, NSN ?
The AN/PRC-96 Lifeboat Radio
In 1972, GTE received an order from the Navy for
1200 Lifeboat radios, AN/PRC-96. Ref #4. This was a unit similar in some
respects to the PRC-106, i.e. it transmitted and received on either 121.5 or 243
MHz. The main difference was the use of “D” cell diameter Lithium batteries
(slightly longer than two “D” cells in series). At two per radio (3
Volts/cell), the terminal voltage was only about 6 Volts as opposed to the 14
Volts used in the other designs. This would have necessitated many circuit
design changes compared to previous designs. One wrinkle was the fact that
Lithium batteries are not allowed in a submarine environment. This was
accommodated by adding a sleeve (screws in to the existing battery compartment
and uses the existing battery cap) to allow four “D” cell alkaline batteries
to fit for these applications. “D” cells have a larger diameter than the
battery used in the other GTE designs, so the case dimension of the PRC-96 was
necessarily larger in the thickness direction, thus leading to many physical
changes. There is only a combination speaker/mic, and the other controls are
mounted on the top of the unit as opposed to the front. A battery test meter or
test pin was added to the backside. Internally, there are eight small circuit
boards that are soldered into a motherboard. Some ICs are employed, but the
soldered nature of the boards and lack of circuit diagrams makes investigation
GTE, tiring of market pressures at this point,
sub-contracted production to C-RAN Corporation.
Tx power output,
Rx sensitivity, 5 uV
Immersion, 1.8 m/2 hours
EE1 50-PA-OMI-010/5101 PRC-96 (formerly
0967-LP-588-4010), 1 March 1981. NSN 2Z5820-00-334-8407
All of the above products, with the possible
exception of the AN/PRC-90A, remain in use today, coming out of service as they
fail. The supposed replacement for the above for normal rescue duties, the
Motorola AN/PRC-112, is itself heading for replacement. A number of special
PRC-112 units with internal GPS, called the Hook-112, have been acquired for
interim duty in the hot spots of the world. Meanwhile, Boeing won the contract
award for CSEL (Combat Survivor Evader Locator), the next step. They awarded the
radio contract to Racal, and its nomenclature is AN/PRQ-7. It is, however, an
expensive set, as it operates on VHF and UHF guard channels as well as 600 MHz,
being capable of transmitting both voice and data, and having an internal GPS
capability. Rumor has it that a contract has been awarded to Tadiran for a less
expensive set. Information is
starting to show up on the web about something called the PRC-149. This could be
- Manufacturer’s Literature
- The Technical Manuals for the individual units
- Jane’s Military Communications, 1981 and/or 1994-5
-Military Collector Group Post, 20 January 1999
-Military Collector Group Post, Condensed PRC Data, 10/22/98-11/3/98
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)