Heathkit SB-1000 - Page 67

Page 70
OUTPUT
The tuned output circuit of the Amplifier is a pi
network consisting of Plate capacitor C27, load
capacitor C31, and coils L7, L8, and L9.
Band switch SW2E progressively shorts out the
unused portions of coils L7 and L8. The coil turns in
use are tuned to resonance by Plate capacitor C27.
Load capacitor C31 is tuned to complete the impe-
dance match between the tube and the load that is
connected to RF Out jack J2. On the 160-meter band,
fixed capacitors C28 and C29 are switched in paral-
lel with Load capacitor C31 to provide the addition-
al capacitance required on this band. On the 80 and
40-meter bands, capacitor C29 is switched in paral-
Jel with Load capacitor C31.
If a DC voltage is unintentionally applied to the
plate output circuit, RFC1 will provide a DC path to
ground, thereby short-circuiting the high voltage
supply, which will blow the fuses.
ALC Circuit
Control R2 allows you to set the ALC threshold to
match your exciter.
Capacitors C106 and C107 form an RF voltage
divider that couples some of the RF driving voltage
to diodes D103 and D104. When the RF driving
voltage exceeds the ALC threshold voltage, the
diodes rectify the negative half cycles. Capacitor
C105 filters and bypasses this voltage, while resis-
tor R103 provides isolation.
The negative voltage appearing at ALC jack J4 is
coupled back to the exciter to reduce its gain and
teduce "flat-topping" of voice peaks due to over-
drive.
Metering Circuits
GRID CURRENT METER
Refer to Pictorial 11-1 for a simplified drawing of
the grid current meter circuit.
Grid current meter M1 provides an indication of grid
current between 0 and 300 milliamperes. The grid
current meter is connected in parallel with shunt
Heathkit
resistor R203. The grid circuit return is through relay
K1 lugs 4 and 8, zener diode D2, to the center tap of
the filament winding of power transformer T1.
MULTIMETER
HV (High Voltage)
Refer to Pictorial 11-2 for a simplified drawing of
the high voltage meter circuit.
When you select this function, resistors R401 through
R404 form a_ voltage divider to measure the high
voltage. Resistors R402 through R404 form a series
multiplier resistance for meter M2, while resistor
R401 formsa current shunt for the meter.
IP (Plate Current)
Refer to Pictorial 11-3 for a simplified drawing of
the plate current meter circuit.
When you select this function, meter M2 indicates
the plate current drawn by the tube between 0 and
700 milliamperes. It is switched in parallel with
resistor R202 and measures the voltage drop across
it. The meter scale is calibrated to indicate the
plate current by using the fixed value of resistor R202
and the voltage across it.
PO (Power Output)
Refer to Pictorial 114 for a simplified drawing of
the power output meter circuit.
Capacitors C101, C102, and C103 on the ALC/power
circuit board form an RF voltage divider for a
portion of the RF output voltage to a rectifier circuit
formed by diodes D101 and D102. Capacitor C104
provides filtering and bypassing. The resulting DC
voltage then passes through resistor R101 to the
meter circuit board.
The DC voltage coming from the ALC/power circuit
board passes through control R204 to an amplifier
circuit formed by transistor Q201 and its associated
components. From here, the current passes through
multiplier resistor R206 to the multimeter, which is
calibrated to indicate peak output power between 0
and 2000 watts. Control R204 allows you to calibrate
the meter for this function.