1 MODEL 4OO
TRANSCEIVER
A. Circuit
Theory
(Cont)
Frequency Control Unit is deterrnined by
circuit elements which are far removed
frorn
any heat source,
and
the
voltage
regulation
is very
precise
to
the transis-
tor oBcillator,
frequency
stability is
extiemely
good.
Automatic Cain Control,
(AGC)
is
pro-
vided by the AGC Amplrfier/Detector,
VlI, which
provides an AGC signal for
control of
the
gain of
V6,
Receiver RF
Amplifier, V7, Receiver Mixer, and V9,
TRANSM]T AND RECEIVE SWITCH]NC
AII
transmit and receive ewitchrng
i6
performed by relays Kl and K2. In
TRANSMIT
position, only those tubes
that opelate
in the transmit mode a!e
operative, aU
others being biased to cut-
off through the relay contacts, In
the
RECEM
position, with the relay
de-
energlzed, the
tubes that are norhally
used
only in tran3mittiDg are
cut oft in
the sarne
manner. Relay K2, which
when de-energized
feeds signals
frorn
the output
pi-network to the receiver,
and
is used also
to
control
any external
switching.
In the TRANSMIT
po3ition,
the meter
indicates the
cornbined cathode
current
oI the two Power Amplifiers.
In
the RECEM
po6ition, it indicates the
voltage
acro6s R902 in the
cathode of the
Second
IF
Amplifier, V9,
which is
iDversely
proportional to the AGC volt-
age
used to control
the gain of the tube.
Thus the S-Meter
reads le{t to right on
transmit, and
right to left on receive.
POWER RATING
The Swan
400 is capable
of
400
watts,
PEP
input under steady
two-tone te6t
.onditions,
when operated with any of
the
recommended
power
supplies. The
peak envelope
power,
when voice modu-
lated,
i5 considerably
more, tyPically
500
\ratts,
or rnore,
Recommended
power
supPlies
produce
a
no-load
plate voltage
ol approximately
925
volts. Under
TUNE conditions,
or
CW
operation,
this voltage
rnay drop to
as Iow as
?20 volts.
Under steady state
two-tone
modulation,
the voltage will
drop
to apploxirnately
750 volt3, If
Power
Amplifier
idling current
is 50 rna, and
two tone
plate current,
ju8t
before
flat-
topping, is
3?5 ma, the
peak two
-tone
cullent will
be 560 rna.
The PEP input
wiu then be
?50 volts x 560
ma
=
420 watts,
Under voice modulatio!,
beca[rBe
average
power i6 considerably
Iess' the Pow€r
Amplifier
plate
and screen
voltages
will
be mainta)ned
hiSher, even during
voice
peaks, by the
power supply
filter capaci-
tors. Peak voice
plate
cu!rent
will thele-
fole
also be higher
than with two-tone
test
conditions. Under
typical
operatinS
conditions,
peak
plate current befole
flat-topping
will
be 625 lna at 800 volts,
to
result in a
peak envelope
power inPut
of 500
watt8.
Reading6
ol cathode current
would not
reflect this 500 watt
power inPut' how-
ever, because
oI the dampiDg
in the
cathode current
meter. The
meter damp-
ing
i6 such that
the rneter i3
unable to
respond to variations
oJ
cathode
current
in the audible
range.
Cathode current
readings under
normal voice
input, should
not
exce€d approximately
I50 to 175
rna.
POWER
AMPLIF]ER
PLATE DISSIPATION
There
is olten a mi
s und€ r standing about
the
plate dissipation
of tubes
operated as
AB
ampliliers
under voice modulation,
In
the Swan 400,
while in the
transmit
position, and with
no
modulation'
the plate
voltage will
be 890 vo1te,
the
plate current
50 rna, and
the
power
input
will be 50 watts.
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