GAREX TWOMOBILE MK II User manual

D R A F T D R A F T D R A F T
Garex Twomobile 1 Handbook
GAREX TWOMOBILE MK II
TWO-METRE FM/AM TRANSCEIVER
OPERATING MANUAL

D R A F T D R A F T D R A F T
Garex Twomobile 2 Handbook
SECTION I
INTRODUCTION
The Garex Twomobile is a transmitter–receiver for operation in the 144–
146MHz amate r band.
The receiver is f lly t nable over the entire band and is capable of
receiving both amplit de and freq ency (or phase) mod lated signals. A
f lly transistorised do ble s perheterodyne design is sed, incorporating a
sq elch circ it to eliminate backgro nd noise in the absence of a signal.
The appropriate detector for FM and AM is selected by a switch mo nted on
the front panel. FM reception employs an integrated circ it q adrat re
detector.
The transmitter has a nominal inp t power to the final power amplifier of
15 watts. The a dio and oscillator/mod lator nits are f l1y
transistorised; the m ltiplier/driver and. power amplifier stages se
q ick–heat twin tetrodes. The mod lation system (phase or amplit de) is
selected by a front-panel switch, which is independent of the receiver mode
switch, th s readily allowing cross–mode working.
The eq ipment is designed to operate from a nominal 12V DC s pply,
conveniently s pplied from a vehicle battery. For fixed-station se, a
mains–driven power s pply delivering 12V DC (stabilised) at 5 amps maxim m
is req ired. (Under standby conditions the c rrent cons mption is abo t
0.25A.)
All of the operator’s controls are mo nted on the front panel. An external
lo dspeaker is sed, any 3 to 5 ohm type being s itable.
THIS DOCUMENT
This handbook was prod ced by M1FDE sing notes and man script s pplied by
GAREX which I asked if I co ld p blish. This is almost the doc ment they
wo ld have p blished, b t p blishing this doc ment is my responsibility and
all comm nications regarding errors or f rther information sho ld be
addressed to M1FDE.
A.N. Martin M1FDE <at> G0MWT <dot> ORG <dot> UK
This doc ment is DRAFT and preliminary and details are tentative. It will
be pdated WHEN I’ve MADE ONE WORK.

D R A F T D R A F T D R A F T
Garex Twomobile 3 Handbook
SUMMARY OF DATA
Service A3 or F3 telephony
Freq ency Range 144–146MHz
Bandwidth 10 kHz (±5kHz) at 6 dB down
Power S pply 12 volts DC (nominal)
Positive or negative gro nd, or floating,
Power Cons mption RX: 200mA (no signal)
TX: (AM) 5A. (FM) 4A
Controls Power off – Standby – Spot check switch
VOLUME control
SQUELCH control
CHANNEL selector switch
RECEIVER TUNING
Tx mode (FM or AM)
Rx mode (FM or AM)
Transmit b tton (on microphone)
Toneb rst selector switch (if fitted)
Dimensions 12½” x 5½” x 9” deep
Weight l2½ lbs. (5.7kg)
Optional feat res Tone b rst generator (1700 or 1750Hz)
Up to 6 transmit channels
RECEIVER
Sensitivity 1µV p.d. inp t for 500mW a dio o tp t with
signal-to-noise ratio of 12dB.
A dio o tp t 1 watt with less than l0% distortion
Intermediate freq encies 1st IF 10.7MHz.
2nd IF 455kHz, with band pass block filter.
Sq elch An electronic sq elch circ it is fitted; it is
Carrier operated, and can be adj sted to c t
backgro nd noise in the absence of a signal.
TRANSMITTER
Power 15 watts inp t (nominal) to the final power
amplifier stage.
Mod lation a) High level amplit de mod lation.
b) Phase mod lation, adj stable p to 15kHz
deviation, normally set to ±3kHz.

D R A F T D R A F T D R A F T
Garex Twomobile 4 Handbook
SECTION I
CIRCUIT DESCRIPTION
RECEIVER
An all-transistor do ble s perheterodyne circ it is sed. Two RF amplifiers
are followed by a 1ow-noise FET mixer; the first local oscillator is
variable over a 2MHz range to allow t ning over the two metre band. The
first IF is 10.7MHz which is amplified in three stages and passed to the
second mixer also an FET. The second local oscillator is crystal controlled
at 10.245MHz, prod cing the second IF of 455kHz. The second IF is amplified
and passed thro gh a bandpass block filter, after which there are a f rther
five stages of amplification. Amplit de mod lated signals pass via the
detector and noise limiter to the mode selection switch.Freq ency mod lated
signals are taken from the fo rth amplifier stage and processed by an
integrated circ it q adrat re detector. The res ltant a dio is fed to the
mode switch. From this switch the a dio passes via the sq elch gate to the
a dio amplifier and o tp t stages.

D R A F T D R A F T D R A F T
Garex Twomobile 5 Handbook
RF Unit
The antenna changeover relay contacts RLA1 connect the aerial inp t to the
RF amplifiers VT1, VT2 which are co pled to the FET mixer VT3 by the t ned
circ it L4. The local oscillator freq ency is generated by a temperat re
compensated FET variable freq ency oscillator having a 2MHz t ning range.
B ffer stages, incl ding VT4, isolate the wanted freq ency. The o tp t from
VT3 is the first IF of 10.7MHz.
1st IF Unit
VTI01, VT102, VT103 are transformer co pled cascade amplifiers at 10.7MHz.
The o tp t is co pled by transformer T104 to the second mixer VT203.
2nd Mixer Unit
The mixer FET VT203 combines the 10.7MHz first IF with the l0.245 MHz
o tp t from the local oscillator VT202, prod cing the 2nd IF of 455kHz,
which passes thro gh a single stage amplifier VT2O1 to the 455kHz bandpass
filter.
455kHz Bandpass Filter
This filter is a sealed nit which determines the selectivity of the
receiver. The bandwidth is 10kHz (±5kHz) which allows a good compromise
between selectivity and satisfactory resol tion of freq ency mod lated
signals with a peak deviation of p to ±3kHz.
2nd IF Unit
The o tp t from the bandpass filter is co pled to the two amplifying stages
VT302, VT303 and emitter followers VT301, VT304. When receiving amplit de
mod lated signals a third amplifying stage VT305 is sed. Transformer T302
in its collector circ it has two secondary windings, which are connected
respectively to the signal detector MR302 and the AGC detector MR303. The
rectified o tp t from MR303 is fed back via emitter follower VT306 as AGC
bias to the RF stages VT1, VT2, and the IF amplifiers VT302, VT303.
The rectified o tp t from MR302 passes thro gh the noise limiter MR304 the
receiver mode selection switch (S2).
Freq ency mod lated signals are taken from the emitter of VT304 to the
inp t of the q adrat re detector IC402. The a dio o tp t from IC402 is
taken to S2. A dio then passes to the emitter follower VT307 and the
sq elch diode MR305.
Sq elch Unit
VT40l is a switching stage controlled by the AGC bias level. Under no
signal conditions VT401 maintains the cathode of MR305 at a level less
negative than its anode, which is controlled by VT307, thereby MR305 is
prevented from cond cting. When a signal is received the AGC voltage
becomes less negative allowing MR305 to cond ct and permit the a dio to
pass to the vol me control and a dio amplifying stages. The setting of VR1
(located on the front panel) determines the emitter voltage of VT401, and
th s the level at which the sq elch operates.
AF Section
The AF from the vol me control VR2 (located on the front panel) is
amplified by VT503 and co pled via the emitter—fo1lower VT501, to the
driver VT505. Phase splitting transformer T502 in the collector of VT505
drives the p sh—p ll o tp t stage VT505, VT507. T503 is the lo dspeaker
o tp t transformer.

D R A F T D R A F T D R A F T
Garex Twomobile 6 Handbook
TRANSMITTER
The RF section of the transmitter comprises a transistorised crystal
oscillator and m ltipliers. The m ltiplier/driver and power amplifier se
q ick-heat tetrodes.
Phase Mod lation
The o tp t from the crystal oscillator is phase mod lated by the variable
capacitance diodes and m ltiplied by the following stages.
Amplit de Mod lation
The AF section (part of which is sed in the receiver) provides anode and
screen mod lation of the transmitter power amplifier.

D R A F T D R A F T D R A F T
Garex Twomobile 7 Handbook
AF section
The transistor VT651 is a crystal contro11ed oscillator. The req ired
crystal and trimming capacitor are switched into circ it by the channel
switch S1 (located on the front panel). The oscillator is capacity co pled
to the phase mod lator which is t ned to the crystal freq ency. When sing
phase mod lation the microphone o tp t is amplified by VT501, VT502 and the
res ltant o tp t, limited by MR501, and MR502, is connected to the variable
capacitance diodes MR651 and MR653. Capacitance changes at these diodes d e
to the a dio inp t res lts in phase mod lation of the carrier. The AF inp t
to these diodes is shorted o t when sing amplit de mod lation.
The emitter follower VT652 matches the impedance of the phase mod lator to
the freq ency m ltiplier and b ffer stages. VT654 is co pled by means of a
pi–network to the two final m ltiplier stages V1a and V1b. The amplifier V2
is a twin tetrode operating in p sh-p ll. The o tp t from its tank circ it
is fed to the antenna via the co pling coil L609 and the relay contact
RLA1.
AF Section (phase mod lation)
O tp t from the microphone is amplified by VT50I and VT502 co pled by T501
to the series diodes MR5OI and MR502 which limit the a dio, and hence the
peak deviation. After limiting, the a dio is taken thro gh a lowpass
filter, integrated by R511 and C508, and via the peak deviation control
VR501 to the variable capacitance diodes MR651 and MR653.
AF Section (amplit de mod lation)
The microphone o tp t is amplified as described above. After limiting, the
a dio passes to the potential divider R515 and R516. The req ired a dio
level from the divider is fed via the relay contacts RLB2 to the amplifier
and driver stages VT503, VT504, VT505. The phase–splitting transformer T502
co ples the driver to the p sh—p ll stage VT506, VT507. The relay contacts
RLB3 and RLB1 connect the o tp t from the p sh-p ll stage to the mod lation
transformer T501. A secondary winding on T502 provides anode and screen
mod lation of the RF power amplifier V2.
Mod lation Switching
Switch S3 (located n the front panel) selects the req ired mod lation mode.
In the phase mod lation position, the o tp t from the potential divider
R515 and R516 is shorted o t by S3a effectively blocking the passage of
a dio to the amplit de mod lation stages. In the amplit de mod lation
position, S3b shorts the peak deviation control VR5O1, preventing a dio
from reaching the variable capacitance diodes MR651 and MR653.

D R A F T D R A F T D R A F T
Garex Twomobile 8 Handbook
POWER SUPPLY SECTION
The 12V DC s pply is connected to the eq ipment via the 7A f ses FS1, FS2
and the power switch SI on the front panel. When the eq ipment is switched
on the positive and negative sides of the s pply are connected to the
receiver and the green lamp ill minates. The positive side of the s pply is
connected to the DC converter.
The DC converter consists of VT70I, VT702 in a p sh—p ll blocking
oscillator circ it. AC is developed in the secondary of T701, rectified by
a f ll—wave bridge rectifier MR70I(a-d) and smoothed by C7O1, R701, C702a
and C702b. Another secondary winding provides heater s pplies for v601,
V602 via a choke-capacitor filter. When the transmit b tton on the
microphone is pressed the transmit relay RLB is energised, RLB1 changes
over to break the negative s pply to the receiver and connects it to the
converter to provide the HT and heater s pplies to the transmitter valves.
The red TX lamp is connected in parallel with the energising coils of RLA
and RLB and ill minates when the microphone switch is operated.
Reversed Polarity Protection
A seleni m rectifier MR702 is connected across the DC s pply when the power
switch is on and is normally non-cond cting. If the s pply polarity is
reversed, MR702 will cond ct and blow one or both of the s pply f ses. It
is vital that the correct f ses are fitted (7A rating, 16mm belling-lee).
RELAYS
The coils of the relays RLA and RLB are each connected to the negative side
of the 12v s pply. The positive side is connected to the rel&ys via the
microphone press—to-talk switch.
RLA is the antenna changeover relay and transfers the antenna
connection from the receiver inp t to the transmitter o tp t.
RLB is the transmit relay.
RLB1 transfers the 12V negative s pply from the receiver to the DC
converter.
RLB2 transfers the inp t connection to the a dio amplifier (VT503) from the
receiver vol me control to the o tp t from microphone preamplifier (VT502).
RLB3/RLB4 transfers the AF o tp t from the lo dspeaker transformer T503 to
the mod lation transformer T504.

D R A F T D R A F T D R A F T
Garex Twomobile 9 Handbook
SECTION III
OPERATION
Before operating the eq ipment, remove the o ter case and check that all
valves, crystals, lamp holders and hinged printed circ it boards are firmly
seated in position and that no damage has occ rred in transit.
The o ter case is sec red by the 6BA screw located at the top centre of the
front panel. The feet beneath the case sho ld also be removed by nscrewing
their 6BA screws, to avoid fo ling the lower edge of the rear panel. The
nit may now be withdrawn from the case in a forward direction, taking care
that the flange at the top front of the case does not fo l components
mo nted high in the chassis assembly, in partic lar the power switch, a dio
smoothing electrolytic and voltage reg lator panel (if this is fitted above
the mod lation transformer).
To reassemble, reverse the proced re, ens ring that the correct n mber of
packing washers are sed on the screws fixing the feet.
The red s pply lead m st be connected to the positive terminal of the. 12V
DC s pply and the black s pply lead to the negative. Reversal of these
connections will blow one or both of the 7A f ses. It is important to
ens re that the correct rating and type of f se is sed as a replacement.
The s pply voltage for alignment and test p rposes is 13.2V meas red at the
inp t to the eq ipment.
Receiver
Connect a 3—5 ohm lo dspeaker to the terminal block on the o tside of the
rear panel. With the 12V DC s pply connected as above, t rn the power
switch to ‘ON’. The green pilot light sho ld ill minate and pon t rning
the sq elch and vol me controls in a clockwise direction receiver noise
sho ld be heard. Connect a s itable antenna (50 ohm impedance) to the
antenna socket. The t ning dial is directly calibrated for the 2 metre
band. Set the Rx mode switch to ‘FM’ or ‘AM’ as req ired. For general band–
searching it is s ggested that this switch be kept in the ‘FM’ position;
signals are then easily located by the q ietening of the backgro nd noise
when t ned to the centre of the carrier. AM signals are resolved instantly
by switching modes. When receiving freq ency mod lated signals having a
deviation in excess of ±5kHz some distortion will res lt.
To silence the Rx noise in the absence of a signal t rn the sq elch control
caref lly anticlockwise ntil m ting j st occ rs.

D R A F T D R A F T D R A F T
Garex Twomobile 10 Handbook
TRANSMITTER
Do not operate the transmitter nless an antenna or d mmy load is connected
to the antenna socket.
Select the appropriate crystal by setting the channel switch on the front
panel. T rn the power switch to ‘SPOT’ and t ne the receiver to the nominal
transmitter freq ency. A strong carrier sho ld be heard. Reset the power
switch to ‘ON’, when the carrier sho ld disappear.
Select the req ired transmit mode, listen briefly to the receiver to verify
that the selected channel is clear, then press the transmit b tton on the
microphone. It sho ld be possible to hear a whine from the DC converter.
Wait for approximately one second for the q ick-heat valves to reach
operating temperat re, then speak at normal voice level across the face of
the microphone at a distance of 2—3 inches (do not speak directly at the
face of the microphone).
When first sing a partic lar sntenna system the transmitter sho ld be
correctly loaded to the antenna as follows:
sing field strength meter or reflectometer (VSWR meter) adj st the antenna
trimmer (accessible thro gh the side of the case) for maxim m radiation.
The Twomobile transmitter is designed for intermittent operation, and
especially when sing amplit de mod lation, long periods of transmission
sho ld be avoided. If sed as a fixed station, where long periods of
transmission are more prevalent than nder mobile conditions there is some
merit in red cing the power inp t to the final power amplifier by abo t 20%
(see Tx adj stments). Adeq ate ventilation, especially of the PA and driver
area is essential.