the bias on CR6 is taken from a fixed center-tap on R48, so that the frequency is independent of the
setting of the RECEIVER OFFSE T tuning control. The r eceiver and transmitter will operate on the
same frequency when the control is set at mid-rotation (center line on panel).
VOX CIRC UIT
A six pole relay controls the voltage and biases to the various stages of the transceiver , and
deter mines whether the unit is in the receive or transmit condition. Tr iode V3B has, as its plate
load, the coil of the relay. When V3B is cut off , the relay is unenergized and the condition is "receive".
Cutoff is maintained by a negative bias which is applied through R26 to the gr id of V3B.
When an audio signal is applied to the VOX amplifier V3A, from the first microphone amplifier
V lA , V3A tends to draw less plate current. This causes the plate voltage at V3A to rise, thus firing
the neon bulb PL 3 and causing the voltage of the grid of V3B to r ise. This br ings V3B into conduction,
and the relay closes. The transceiver is now in the transmit condition. Af ter the voice peak has
passed, PL3 extinguishes and there is no longer any electr ical connection between V3B and any other
part of the VOX circuit. The grid voltage of V3B now tends to return to its original negative value,
and does so with a time constant which depends upon R26, C 19, and the setting of the VOX-DE LAY
control, R32. With the arm of R32 set to the plate end of the control, C19 will have a "virtual"
capacitance of approximately its own value times the gain of the stage V3B. As the arm of the control
is moved away from the plate end, the eff ective capacitance in the grid circuit is reduced; when it is
at the ground side of the control, the circuit capacitance is merely the value of C 19. This change of
eff ective grid circuit capacitance with stage gain is known as "Miller eff ect".
When the voltage at the grid of V3B decays to a suff iciently high negative value, the relay retur ns
to the "off" condition and the unit is in
"receive"
again. The voltage at the gr id of V3A is adjusted so
that PL3 is just on the verge of firing when there is no speech input. This assures that the sensitivity
of the VOX circuit is always at a maximum. This bias is adjusted with R24, the VOX- THRESHOLD
control.
If
a speaker is being used there is the possibility that the microphone will pick up the audio
from the speaker and cause the VOX to trip. The anti-vox detector , CR
l
and CR2, desensitizes the
VOX amplifier by converting a sample of the receiver output signal into a positive bias which br ings
V3A into a heavier state of conduction. The amount of anti-vox injection is controlled with the ANTI-
VOX sensitivity control,
A
J.).
In order to prevent transient voltages, which occur during relay switching, from accidentally
tr iggering the VOX circuit, the relay control tube is cut off for a short period of time immediately
af ter the unit switches to receive. A high negative pulse is f ed through PL4 to the grid of V3B, and
its duration is such that it keeps V3B cut off (and, thereby, the unit in the receive condition) until the
switching transients have decayed.
MAINTENANCE
GENE RAL
Your transceiver should require little service except for nor mal tube replacement. Do not sub-
stitute for tube types used in this transceiver. Factory-wired transceivers have been completely
aligned and adjusted at the factory before shipment. Transceivers built from the kit must be aligned
as described in the Alignment Procedure given. (Note that the crystal filter supplied in the kit has
been pre-adjusted and aligned at the factory) .
CABINET REMO VAL
To remove the instrument from the cabinet, first disconnect it from its power supply, and then
remove four #6 sheet metal screws at the rear and seven #6 sheet metal screws on the bottom. Unit
can be removed from the front of the cabinet.
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