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SERV
I
CE
Tbe channel
common
circuit
is for
factory test
PurPoses
onlY.
The
circui t permits both
frequencies
to be checked regardless of
the
posi
tion of S151.
The circuit
consists of
Q154, Rt72 and
C168.
Groundirg channel
common lead
J911-6 turns off
Q154
and Permits
each transmit oscillator
to
be
turnecl on
by grounding
P911-7(F1)
and then
P911-8(F2
).
CIRCUIT
ANALYSIS
L8r31072
EXCITER
The exciter
consists of a 1.50
llHz
amplifier a (150 \llIz to
45O MHz)
tripler
and two 450
I{Hz
amplifiers.
The exciter
takes the output from the
osci
I
1a
tor
and
buffer circuits
(2 milliwatts
minimum)
and
multiplies and amplifies 1t to provide
140
milliwatts
minimum (45O-47O
I{Hz) to
the
power
amplifier circuitry.
In addition to
these functions, the exciter contains aIl
of the filters
whicb determine the
band-
width and spurious characteristics
(exclu-
sive of
harmonics)
of the transmitter'
The output of the osci
I
1a
tor
and
buffer stages is coupled to the input of
the exciter via J151, which is
connected to
a tap on L2O1.
This tap also supplies
voltage to the buffer
transistor
Q153.
L201 along with L2O2, selects the third
harmonic (150 \!lHz) rvhich is present
aL
J151.
C2O6
and
C2O7
natch the output of thls
two pole filter
to the
base
of
Q201'
which
provides approximately 10
dB
of gain.
C273
and C2l4 match
the collector of this tran-
sistor
to the input of another two
pole
filter,
consisting of
L2O4 and
L205.
The
emitter voltage on LzOl
(AMPL-1)
can
be
monitored at Tp201, and is typically
+0.3
vcl
ts.
C216
and C2I7 match
the output of
L205
to the base of the tripler
transistor'
Q202.
The base
voltage on
Q2O2
ean
be
read
at'lP2O2,
and
is t1'pically -1.2 vo1ts.
The
tripler
has approximately
3
dB
of gain.
A matching network consisting of
L206
and
C222
matches
the 450
MHz
output of
the
tripler
to the first
two-po1e
helical fi1-
ter,
L2O7
and L2O8. The output of this
filter
is tben
matched
by
C223
and L209
to
a two stage,
broadbanded
450
[4Hz
amplifier
consisting of
Q203
and
Q204
.
C227
,
C228
'
L210 and L211
provide proper matching
from
the collector of
Q203
to the
base
of
Q2O4,
providing the
necessary bandwidth
as
we11.
Although this amplifier is capable of pro-
viding 12-I5 dB of gain, both stages
are
hiased to provide adequate saturation to
ensure no
AM
components
are present at
the
exciter output. Typical actual gain is
7-9
dB,
and
typical
power
output is
150
mW.
The collector
of
Q204
is matched
to
the input of the second two pole helical
filter
(L214 and L215) by C233-C235,
L272
and L213.
An RF "sniffing"
circuiL
con-
sisting of
C236, C237, D2OI, and
R221
allow
relative
power available at the input of
this filter
to be monitored at TP203.
+0'6
volts is typically
present at thj"s point
after tuning the exciter.
There is no test point that indicates
rela
tive power coming out of this fil ter
TP251
in the
PA
sect.ion indicates relative
power coming out of Q251
(Amp1 4)
,
and
final
tuning for L214 and L215 should
be
done
while monitoring this test point.
No
test points require the use of special
RF
detector
probes.
POWER
AMPLIFIER
The three stage power amplifier
con-
sists of Amplifier Q251, driver q252
and
power
ampl
ifier
Q253
and associated cir-
cuitry.
Collector voltage for driver
Q252
is applied from A+ through power control
transistor
q257 and L256, L257 and
R256.
The
colleetor voltage for
Q252
is a result
of the output power setLing and voltage
variations at any given time.
The output
of driver q252 is coupled to the base of
Power
Amplifier
Q253
through an
impedance
matching network consisting of
W254,
C292,
C267
,
C268
and
C269.
Collector voltage for
Q253
is
provided
frorn A+ through L267, L262 and
R258.
The 20 watt output of the PA is
con-
nected to the
1ow-pass
filter
by
W256 and
then to
antenna
relay
K601.
RF
POWER
ADJUST CIRCUIT
The
output
power
adjust circuit
al1ows
the transmitter to be set to rated
output
power.
The power
adjustment is attained
by
control ling
the DC
col
lector
voltage to
driver
Q252
through pass transistor
q257.
The pass
tra
nsistor
i
s control
I
ed by
a
feedback loop consisting of
Q254 through
Q256. The power is set by
potenLiometer
R260.
SERVICE
There
are 4 points in the exciter
which are approximatelY 50
ohrns.
W201, W2O4,
W205
and
W251
(which
interface the 2 helica I fi1 ters
to the other exciter circuitrY).
These
points can conveniently
be
used
to monitor or inject signals
for troubleshooting and testing
using 50
ohm
sources and termi-
nations.
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