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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
r e g u l a t i o n i s v e r y p r e c i s e t o t h e t r a n s i s -
tor oBcillator,
frequency
stability
is
extiemely
good.
Automatic
Cain Control,
(AGC) is pro-
v i d e d b y t h e A G C A m p l r f i e r / D e t e c t o r ,
VlI,
which provides
an AGC signal for
c o n t r o l o f t h e g a i n o f V 6 , R e c e i v e r R F
A m p l i f i e r ,
V 7 , R e c e i v e r M i x e r ,
a n d V 9 ,
TRANSM]T
AND RECEIVE
SWITCH]NC
A I I t r a n s m i t
a n d r e c e i v e e w i t c h r n g i 6
p e r f o r m e d b y r e l a y s K l a n d K 2 .
I n
TRANSMIT
position,
only those tubes
that opelate
in the transmit
mode a!e
o p e r a t i v e , a U o t h e r s b e i n g b i a s e d t o c u t -
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
t h e o u t p u t p i - n e t w o r k t o t h e r e c e i v e r ,
and is used also to control
any external
switching.
In the TRANSMIT
po3ition,
the meter
indicates
the cornbined cathode
c u r r e n t o I t h e t w o P o w e r A m p l i f i e r s .
In the RECEM
po6ition,
it indicates
the
voltage acro6s R902 in the cathode of the
S e c o n d I F A m p l i f i e r ,
V 9 , w h i c h i s
iDversely
proportional
to the AGC volt-
a g e u s e d t o c o n t r o l t h e g a i n o f t h e t u b e .
Thus the S-Meter
reads le{t to right on
t r a n s m i t ,
a n d r i g h t t o l e f t o n r e c e i v e .
POWER RATING
T h e S w a n 4 0 0 i s c a p a b l e o f 4 0 0 w a t t s ,
PEP input under steady two-tone
te6t
. o n d i t i o n s ,
w h e n o p e r a t e d w i t h a n y o f
the recommended
power supplies.
The
peak envelope power,
when voice modu-
lated,
i5 considerably
more,
tyPically
500 \ratts,
or rnore,
R e c o m m e n d e d p o w e r s u p P l i e s p r o d u c e a
no-load plate voltage ol approximately
925 volts.
Under TUNE conditions,
or
CW operation,
this voltage rnay drop to
a s I o w a s ? 2 0 v o l t s .
U n d e r s t e a d y s t a t e
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
p o w e r i 6 c o n s i d e r a b l y I e s s ' t h e P o w € r
Amplifier
plate and screen voltages will
b e m a i n t a ) n e d h i S h e r , e v e n d u r i n g v o i c e
p e a k s , b y t h e p o w e r s u p p l y f i l t e r c a p a c i -
t o r s .
P e a k v o i c e p l a t e c u ! r e n t w i l l t h e l e -
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
o f 5 0 0 w a t t 8 .
Reading6 ol cathode current
would not
reflect
this 500 watt power inPut'
how-
ever,
because oI the dampiDg in the
c a t h o d e c u r r e n t m e t e r .
T h e m e t e r d a m p -
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
n o t e x c e € d a p p r o x i m a t e l y
I 5 0 t o 1 7 5 r n a .
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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