Heathkit SB-201 - Circuit Description

Page 50
Ge QuearHxit:
CIRCUIT DESCRIPTION
POWER SUPPLY
The power supply uses a power transformer which
has dual-primary and triple-secondary windings. The
primary winding may be connected for either 120 or
240 volt AC operation. The blower fan is connected
across one-half of the primary winding. The trans-
former is protected against overload by reset type
circuit breakers CB1 and CB2.
One secondary winding of the power transformer
supplies 6.3 volts AC for the tube filaments and the
pilot lamp in the meter. Another secondary winding
is used with silicon diodes D1 through D16 and elec-
trolytic capacitors C4 through C9 in a full-wave
voltage-doubler circuit to provide plate voltage for
the tubes. Resistors R5 through R10 insure that the
voltage across each of the electrolytic capacitors is
equalized; these resistors also serve as a bleeder net-
work.
Resistor R11 at the bottom of the bleeder network
provides an ALC threshold voltage of approximately
10 volts DC. Resistor R12 provides a means of measur-
ing plate current independently, without also
measuring bleeder current.
The third secondary winding, with resistors R1 and
R2, capacitor C3, and silicon diode D17 forms a half-
wave rectifier circuit. This circuit provides antenna
relay control voltage and cutoff bias voltage for the
grids of the amplifier tubes.
INPUT CIRCUIT
Tubes V1 and V2 are connected in parallel in a
cathode-driven (grounded grid), class B configura-
tion. Driving power for each band is coupled through
a broad-band network, consisting of a coil and as-
sociated capacitors, and through capacitor C16 to the
cathodes of tubes V1 and V2. Coils L1 through L4 with
their associated capacitors are used to make up these
impedance-matching networks, which reduce distor-
tion and increase efficiency.
The correct input network, as well as the correct out-
put circuit coil tap, for each band is selected with the
Band switch. (The coils in the input networks are
factory aligned and require no further adjustment.)
To keep the capacitance of the transformer filament
winding from shunting the RF driving power to
ground, the filament winding is isolated from the
cathode circuit by a bifilar-wound filament choke.
Also, this choke provides a cathode current path to
ground.
OUTPUT CIRCUIT
High voltage is applied to the plates of tubes V1 and
V2 through RF choke RFC1 and parasitic chokes PC1
and PC2. Tuning capacitor C25 is connected on the
input side of final coils L5 and L6. C26 is switched in
parallel with the Tuning capacitor on the 80 meter
band.
Loading capacitor C28 is on the output side of final
coils L5 and L6. Capacitor C27 is switched in parallel
with the Loading capacitor on the 80 meter band.
Output power is applied through antenna change-
over relay RL1 and through the SWR (standing wave
ratio) bridge circuit to the RF Output connector. The
SWR bridge consists of L7, L8, L9, capacitors C20 and
C23, resistors R19 and R20, and crystal diodes CR1
and CR2.
ANTENNA CHANGE - OVER AND
CUTOFF BIAS CIRCUITS
Antenna change-over relay RL1 is controlled by a
VOX (voice operated transmitter) relay in the exciter
used with the Linear Amplifier. The relay in the exci-
ter is connected to the Antenna Relay jack of the
Linear Amplifier. When transmitting, the VOX relay
grounds the Antenna Relay; when receiving, the An-
tenna Relay is ungrounded.
While receiving, cut-off bias voltage from the power
supply bias circuit is applied through the coil of relay
RL1, and through resistor R15 and choke RFC2, to the
grids of tubes V1 and V2. The Antenna Relay jack is
ungrounded and no current 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.
SS
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