CPRC-26 ARMY BACKPACK TRANSCEIVER
The CPRC-26 is a small battery operated transceiver, operating on
6 crystal controlled channels in the VHF frequency range of 47 to 55.4
mhz. It has a power output of 300mW with an FM (Frequency Modulation) deviation
of plus and minus 15khz. It has a range of about 1 mile (1.6km), is fully
sealed and weighs about 10 pounds (5kg). Power was provided by dry batteries
and had a useful life of 20 hours. It was designed in Canada and used by
many armed services, including the Canadian Army and Navy, the US Army,
NATO, and the Australian Army. There were about 4500 built by Philips and
Canadian Rogers.
The circuit is very interesting, as it uses only one crystal per channel,
used for both transmit and receive. The design is based on valve circuitry,
at the time when pencil valves were taking over from miniature valves. The
radio contains 11 pencil valves and 2 miniature valves. The receiver is
a single conversion superheterodyne, using a crystal controlled oscillator.
There is one RF (Radio Frequency) amplifier and the IF (Intermediate Frequency)
is at 4.3 mhz, with 4 stages plus one Limiter. There is an FM discriminator
and audio amplifier for driving headphones. The transmitter has a master
oscillator directly driving the power output valve. The modulator is driven
from the microphone and is also used to control the transmit
frequency with an AFC (Automatic Frequency Control) circuit.
SIMILAR RADIOS
The British built a near copy of the CPRC-26 and called it the Station
Radio A40. It looks almost identical, with the most obvious differences
being the different knobs, no cover for the second handset socket, a small
bar to hold the plug in place, a corner screwed case, and battery attachment
with a strap rather than snail cams. The external accessories are interchangeable.
Internally they are very similar, with exchangeable modules. Inside the
CPRC-26 the battery connector is wired to the radio, whereas the A40 has
an extra connector on the radio chassis. The previous PRC-10 radio has some
similar modules. The following design, the PRC-510 also has some common
modules. Philips built a transistorised version and called it the PRC-261.
It looks almost identical from the top view, except that it has an extra
switch to enable a second bank of 6 channels, bringing the total number to
12 channels. The radio is not as tall as the CPRC-26. This is because the
battery compartment is smaller, even though the radio is taller. The photo
shows CPRC-26 (left), A40 (centre), PRC-261 (right).
The following photograph shows a side view of the CPRC-26 (left), A40,
(centre), and PRC-261 (right).
OPERATION
The CPRC-26 was normally used by a radio operator, in a mobile role.
It was mounted on the operators back in its own canvas backpack, or placed
in any other larger pack. The A40 had a special belt arrangement. The 1/4
wave whip antenna would normally be used vertically, when the operator was
standing upright. The counterpoise wire can be attached to the radio earth
pin and hangs down, thus making a 1/2 wave vertical antenna and improving
the range. The counterpoise wire had a little attachment so that it could
be fixed to the operators ankle. The whip has a flexible base so that it can
be bent to the vertical when the operator is lying down, or the radio is
horizontal. If the operator wishes to be concealed, the whip can be removed,
and the counterpoise wire can be plugged into the antenna socket and used
as a trailing antenna, but giving reduced range. Alternatively, the homing
antenna from a PRC-10 can be used to determine the direction of another transmitter.
The CPRC-26 could also be used as a portable station with the larger 10
foot ground mounted whip, placed up to 44 feet away from the radio. There
are two sockets on the front panel for a handset. The handset was used for
normal operation, and when an officer used this, the radio operator could
plug in a single earphone for monitoring. There is a PRESSEL switch on the
handset. The A40 had a Larkspur type headphones and microphone. There is
a NORMAL and WHISPER mode, which changes the audio input and output levels.
There is an audio extension lead to allow the handset to be used up to 60
feet away from the radio. There is a battery extension lead for cold weather,
to enable the battery to be removed from the radio, and kept warm. There
were no facilities for rebroadcasting or relaying. There was no other power
than a dry battery.
MILITARY USE
In Australia the CPRC-26 replaced the post WW2 backpack PRC-10, and
was eventually replaced by the PRC-25 which was introduced during the
Vietnam war (about 1970). In Britain the A40 replaced the Wireless
Set Number 88 in the late 1950s, and in the mid 1970s was eventually replaced
by the Clansman PRC-350. The CPRC-26 was used by NATO until the 1980s. In
Canada it was declared obsolete in 1969.
CIRCUIT DESCRIPTION
There are several antenna arrangements for the CPRC-26. There is a
four foot whip, a four foot trailing wire, a four foot wire which acts
as a counterpoise, and a homing loop. The antenna socket is dual threaded,
so that any other antenna may be used. There is also a 10 foot long ground
mounted whip. There is a BNC socket used for the homing loop.
The antenna is connected to the main tuning coil, which serves as both receiver
tuned RF coil and the transmitter tank coil. Different trimmer capacitors
are switched in for channel selection. There is one RF amplifier (V7) which
is capacitively coupled into the grid of the mixer, the plate circuit has
a tuned circuit with a different trimmer for each channel. The mixer (V8)
has the oscillator signal injected via the filament. The 4.3 mhz difference
frequency is selected in the plate tuned circuit, and connected to the first
IF amplifier (V9). Each of the four IF amplifiers have two tuned circuits
at 4.3mhz. The output of the last IF amplifier is connected to a limiter.
The limiter is identical to the IF amplifier, except for the input coil,
which has greater coupling, which contributes to overdriving the grid of
the limiter (V10) resulting in amplitude limiting. The plate voltage is higher
at 90 volts, the IF only having 45 volts. The output from the limiter is
connected to the discriminator, which acts like a balanced bridge. Any deviation
from the IF frequency due to FM modulation, unbalances the bridge and causes
an output at the rate of deviation (which is the modulation component). This
is capacitively coupled to the audio output valve (V6), and then transformer
coupled to the headphones. The channel frequency determining component is
a crystal oscillator (V4) which has a separate crystal for each channel.
The valve oscillates on the third harmonic of the crystal, and is 4.3 mhz
below the channel frequency. The transmitter uses a free running Colpitts
master oscillator V2, with a different trimmer for each channel. The
output of the oscillator is capacitively coupled to the power output valve
V1 running in class B. The output tank circuit is the same coil and switched
capacitors, used by the receiver, and coupled to the antenna. The receiver
RF amplifier V7 is not powered, but there is sufficient capacitive coupling
within the valve to inject the transmit frequency into the mixer V8. The
output from the mixer is the difference between the transmit frequency and
the frequency of the crystal oscillator V4. This 4.3mhz signal is capacitively
coupled to the AFC valve V5. The output of this is connected to a discriminator
which produces a bias for the modulator valve V3. The modulator adjusts the
master oscillator frequency, until the transmit frequency is the same as
the receive frequency. It has a range of plus and minus 250khz. The handset
has a carbon microphone, which is transformer coupled to the modulator grid,
to provide FM modulation at voice frequencies. The changeover from
receive to transmit is done simply by turning off the filament supply to
some valves , and energising other valves. The receive valves are RF amplifier
V7, and the four IF amplifiers V9, V9, V9, V9, and the limiter V10. The
transmit valves are the AFC amplifier V5, modulator V3, master oscillator
V2 and power output valve V1. Valves common to both receive and transmit
are always powered up, and these are the oscillator V4, mixer V8, and audio
amplifier V6. The two miniature valves used are type 3B4 (CV2240), in the
transmitter power output V1 and master oscillator V2. The pencil valves
used, are four type 1AD4 (CV2237) for the RF amplifier V7, crystal oscillator
V4, AFC control V5, and modulator V3, six type 5678 (CV2254) for the
mixer V8, four IF amplifiers V9, and limiter V10, and one type 5672 (CV2238)
as the audio amplifier V6.
CONSTRUCTION
The radio is in a sealed cast aluminium box. The battery is in a removable
box with a seal around the edge. The radio contains a H style
aluminium chassis, with many plug in modules on one side. These are all
the same shape and height and contain all the active electronics. They
are packed in rows 3 across and 6 along. All are colour coded for their
special function. They plug in so that they can be changed easily, and
are held in place by a cover. Each module is a (usually) square tall metal
can, with a 7 pin valve type base. Each one may contains a pencil valve,
tuned circuits, capacitors and resistors. They are sealed at the base with
solder and contain dry air. There is one vacant position. This normally
contains a desiccant when the radio is sealed in the case. When removed
from the case, a cable can be plugged into the vacant socket, to
measure aspects of the radios performance. The other side of the chassis
has a cast aluminium cover, which holds the switch mechanism and all the
trimmer capacitors. This cover is made of magnesium for lightness. It is
removable so that the bases of the 7 pin sockets can be accessed There
is a bank of 6 crystals on the same side at the rear. On the rear of the
chassis is the battery connector. At the front of the chassis, is the wiring
to the front panel switches and plugs. The whole of the radio insides is
closely packed. The four IF amplifier modules are interchangeable. The two
discriminators are interchangeable. The two 3B4 valves are interchangeable.
The normal dry battery is a BA-289/U which provides 4 different voltages,
from one battery block. On receive the requirements are: 90v@3ma, 45v@12ma,
1.5v@550ma, 3v. On transmit the requirements are: 90v@30ma, 45v@8ma, 1.5v@850ma,
3v.
TEST EQUIPMENT
There are three main pieces of test equipment for the CPRC-26. There
is a small Condition Indicator which shows the transmitter output.
There is also a Battery Tester which checks all battery voltages on a
meter. Those are mainly field testing units. The main testing
device is intended for bench repairs and is called the CTS-3/PRC Test
Set. This a test meter in a bakelite case that has several functions.
It is connected between the battery and the radio. The switch on the front
then enables the battery voltages to be checked, and the radio current
consumption to be monitored. It is actually a VTVM (Vacuum Tube Volt Meter)
so it also has a self test position. When the radio is opened, there is
a flying lead with a probe, that can be plugged into the vacant position
on the radio chassis. This allows the monitoring of several conditions
within the radio to enable alignment. It has switch positions for, AFC
TEST, TUNE RF, TUNE MO, TUNE PA. There is also a 7 pin socket to enable
the condition of the valves in the radio to be checked for emission, which
include the 3B4, and the modules AF, MODulator, AFC, XLO, MIXer, RF AMPlifier,
IF amplifier and LIMiter. The modules would normally be checked before
tuning and alignment of the radio.
BENCH TESTING
Several items are needed to align the radio. A power supply that provides
1.5 volts DC at about 1 amp (for filaments), 90 volts DC at 50 milliamps
(for the transmitter), 45 volts DC at 25 milliamps (for the receiver),
and 3 volts DC at about 1 milliamp (for bias). A shorting link or handset
to enable the receiver. A shorting link or clamp to enable the transmitter.
A dummy load, or field strength meter for the transmitter. The CTS-3 provides
most tuning functions. If no CTS-3 is available, a VHF signal generator
with FM modulation, and an audio output meter is necessary. For tuning the
IF amplifiers, a soldering iron capable of opening the modules is required,
and a sweep generator is useful. There is little space behind the front panel,
and if the wiring loom is not placed correctly, it can foul the ON-WHISPER-
NORMAL rotary switch, and as the panel is tightened up, the switch will jam
or break. The tool used for tuning the trimmers needs to be non conductive,
as some trimmers have high tension on them.
ALIGNMENT
The alignment of the radio involves adjusting three trimmers
for each channel. The antenna circuit is tuned with the PA trimmer, and
the RF amplifier is tuned with the RF trimmer, for maximum output indicated
on the CTS-3 meter, with the switch set to TUNE RF. That is all that is
tunable for the receiver. For the transmitter, set the CTS-3 switch
to TUNE MO, and adjust the MO trimmer to set the meter to the red
line on the scale, after tuning through a peak. Switch the CTS-3 to TUNE
PA and the CPRC-26 to transmit. Readjust the PA trimmer for maximum meter
reading and dummy load lamp brightness. Leave the CPRC-26 on transmit, switch
the CTS-3 to TUNE MO, then readjust the MO trimmer for no deflection, as
you switch the CPRC-26 from receive to transmit. Repeat for each of the 6
channels. I didn' t like this procedure much. Tuning for a bright lamp
is reasonable, but it doesn' t take into account what antenna you are using,
so I prefer to tune the PA trimmer using a field strength meter. I looked
at the transmitter output with a spectrum analyser, and noticed that if the
AFC circuit was maladjusted, the transmit frequency would lock in after a
second or two. I could see the carrier move and lock in on the screen. With
the spectrum analyser it was easy to adjust the MO trimmer so that it was
correct immediately. Modern equipment can make things easier. I used
a VHF FM signal generator and output meter, to tune the RF and PA trimmers
for maximum, and seemed to get better sensitivity. However, there seemed
more than one peak (receive frequency) and they all seemed a little off
what they should be, according to the crystal. The receiver alignment assumes
that the IF is correct and this appeared not to be the case, so I used a
sweep generator to check them. The first graph shows that the IF was not
exactly tuned to 4.3 mhz and showed a non uniform passband. Tuning the CPRC-26
with this characteristic could produce any of three different receive frequencies,
and a poorer sensitivity than possible. I could hand select IF modules for
the correct frequency. That would be fine if I had spares. I
decided to open the IF and limiter modules. The coils had dobs of glue on
them which chipped off with a sharp point and gentle pressure. I freed up
the slugs which are a small ferrite cylinder that rotates around the outside
of the coil. Since each IF has two slugs, and they interact, I had no success
tuning the ten slugs, either by ear, by output meter, by oscilloscope or
by sweep generator. I made up a little jig that plugs into the CTS-3 module
test socket, which enabled me to tune each module to 4.3 mhz individually,
while powered up. I then plugged them all into the radio, and tuned them
for one single peak on 4.3 mhz. This was measured after the input coil of
the limiter. I then monitored the discriminator input and tuned the last
coil in the limiter module to give a double peak of the same height, centered
on 4.3 mhz. The result is shown as the middle graph. The lower curve shows
the discriminator characteristic, which is responsible for the double peak
in the output. The slugs are brittle and easily broken, and I have yet to
find a supplier of them. The receiver alignment was redone and better
output was obtained, and on the correct frequency. There is so much gain
in the IF and limiter strip and I was operating them without the shielding
covers, that I had a lot of instability. I had to reduce both the 90 volts
and 45 volts supply to about half, to do the alignment. The modules have
to be dried in a dessicator before reassembly.
FEATURES
The CPRC-26 is extremely small and light compared to its predecessors.
It has six crystal locked channels that ensure stability over normal temperature
ranges. Each channel can be set with only one crystal. Repairs can be
done extremely quickly by changing internal modules. The coloured disc
on the side, indicates which set of channels are fitted. It was common
practice to have several different groups of channels, for different battle
roles. All frequency groups had one common channel for liaison. The frequencies
commonly fitted are: 50.0, 50.2, 50.4, 50.6, 50.8, 51.0 mhz. There are few
controls so that operator error is reduced and unskilled operation is possible.
Only one of the handset sockets has transmit capability, so accidental transmission
is avoided, and battery life is extended. Battery supply is through the
handset, so if no handset is connected, the batteries cannot go flat. The
whip is short so obstacles can be avoided. The flexible base allows any
radio orientation. The radio is sealed with dry air, so a long life in
tropical conditions can be achieved.
SHORTCOMINGS
The battery is small, so operation time is limited. Transmit power
is only 300mw, so the range is limited. The transmit function is provided
by energising the output valve filaments, and even though they are quick
heating, the operators first few words may be lost. Operators needed to
be trained to be aware of this. On some channels, the strong FM band radio
stations can enter and beat with the ninth harmonic of the receive crystal,
to generate interference.
INFORMATION
CPRC-26 Circuit Diagram (92k)
CPRC-26 Parts list(65k)
CPRC-26 Battery Top view(33k)
CPRC-26 Battery Terminals (9k)
CPRC-26 repairs (3k)
REFERENCES
RADIO SET CPRC-26, EMEI (Aust) F542, March 1957
TEST SET, RADIO CTS-3/PRC, Canadian Army Operators Handbook, January
1955
WIRELESS SET TYPE A40, Working Instructions, EKCO
Wireless for the Warrior, (http://home.hccnet.nl/l.meulstee/larkspur/larkspur1.html)
Wireless Set A40, Murray McCabe, June 1999
Copyright: Ray Robinson
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