Heathkit HA-14 - Page 46

(Gag earserr] HEATEHEIT Page 43
While receiving, cut-off bias voltage from the
‘power supply is applied through the coil ofrelay
RL1, and through resistors R2, R3, and R4 and
choke RFC3, to the grids of tubes V1 and V2; ‘The
Antenna Relay lug is“ungrounded and no cur-
rent will flow through the coil of relay RL1, which
allows the relay to remain open, Thus it connects
the RF Output jack (antenna) to the RF Input
‘jack of the Linear Amplifier,
When transmitting, the Antenna Relay lug (12)
is._-grounded by the VOX relay of the exciter,
This allows current to flow through the coil of
‘relay RLI and actuate the relay, When actuated,
relay RL1 connects the RF Output jack (antenna)
ito the output circuit of the Linear Amplifier,
_Grounding the Antenna Relay lug also grounds
‘the grids of tubes V1 and V2 through RFC3,
and resistors R3 and R4, This removes all
‘but -2 volts from the grids of the tubes, This
'-2 volts which is developed across resistor R9
is operating bias, which limits resting plate
Lcurrent to approximately 90 milliamperes,
When operating with the Linear Amplifier turned
_ Off, there is no bias voltage to operate relay
RL1 and the RF Input jack remains connected .
:to the antenna, through RF Output jack, This
permits low power operation directly from the
'Exciter into the antenna without changing any
-eables, Because instant-heating-filament tubes
‘are used in the Linear Amplifier, you can go
to high power operation as soon as the Linear
_ Amplifier is turned On,
During modulation the ALC control voltage is
| developed across a voltage divider that consists
' of capacitors C8 and C9, A sample of this volt-
age is dire tae by diode Di, Diode D1 is re-
verse biased with approximately +6 V DC to
form a threshold level, A negative DC control
voltage is pioducéd that is proportional to the
modulation level, This control voltage is then
applied back.,to the ALC input of the exciter,
Relative Power 4nd Standing Wave Ratio are
measured with a’ bridge circuit consisting of
coils L8, L9, and Li0, diodes D2 and D3,
resistors R6 and R7%, and capacitors C20 and
C21, Relative Power Sensitivity control R5 sets
the sensitivity of the meter, The FWD position
is used when adjusting the Tune control for max-
imum output of the Linear Amplifier; this meter
function is also used to establish a ''set'’ meter
level for making SWR measurements,
Because coil L9 of the bridge circuit is con-
nected in series between the RF Input and RF
Output jacks when the Linear Amplifier is
€urned off, the meter can be used to measure
relative power and SWR when operating the
Exciter alone, This method of checking SWR
is recommended for accurately determining
antenna performance,
‘In the bridge circuit, RF current is inductively
and capacitively coupled from L9, to L8and L10,
The RF currents in L8 and L10 are rectified
by diodes D2 and D3, and then filtered by
capacitors C20 and C21,
For relative power measurements, the rectified
RF voltage from L10is appliedtothe meter, For
SWR (reflected power) measurements, the recti-
fied RF voltage from L8 is applied to the
meter,
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