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Page 23
CIRCUIT DESCRIPTION
Refer to
the
Schematic
while
you
read
this
Circuit
Description.
Toroid coil L1 is a current pickup element. When power is
transmitted into the load resistor, R1, an AC field is present
around the wire that passes through the eyelet in coil L1.
The AC field induces current into L1 and produces a voltage
that is rectified by diode D3. Then it is adjusted by resistor
R6 to a calibrated level, and applied to the meter to indicate
the amount of power present in the load. Capacitor C8 and
resistor R5 filter and decouple the voltage. Resistors R7 and
R8 form a voltage divider for the two power ranges. Resistor
L I
PICKUP
COIL
INPUT
-r
CIRCUIT
BOARD
R3 forms a load across L1 to reduce the Q of the coil circuit
for a broader frequency range.
Another winding on coil L1 picks up reverse current. This is
effectively an SWR bridge and is used only to null out the
capacitive effects in coil L1 through capacitors C1, C2, C3,
and C4.
Capacitors C5 and C6, diode D1, and resistor R9 form a
frequency
sensitive
RF
voltmeter.
At
40
meters,
it
accurately indicates actual power on the meter.
Ferrite
beads L2, L3, and L4 prevent RF from traveling through the
cable into the meter.
C3
100
4
C.5
7.
e
e
_I —
2
—
18.5
NULL
DI
R2
CI
C2
TRIMMER
6
3300
150
150
56-20
100
V
V
R9
68
02
IW 10%
03
58-20
56-20
CC
C8
POWER
CALIBRATE
JUMPER
9 VOLT
BATTERY
O05
1
501'10%
,75W
NOTES:
SCHEMATIC OF THE
HEATHKIT
RF LOAD WATTMETER
MODEL HM -2103
I.
ALL RESISTORS ARE 1/2 WATT 5% UNLESS OTHERWISE NOTED.
RESISTOR VALUES ARE IN OHMS (k•1000).
2 ALL CAPACITORS LESS THAN 1 ARE IN MF, GREATER THAN 1
ARE IN pf.
3 FUNCTION SWITCH SHOWN IN 200 POSITION.
4.\:7
THIS SYMBOL INDICATES CIRCUIT BOARD COMMON.
5
=
THIS SYMBOL INDICATES CHASSIS GROUND.
6
I
—
I THIS SYMBOL INDICATES AN EXTERNAL CONNECTION
TO THE CIRCUIT BOARD.
THIS SYMBOL INDICATES A FERRITE BEAD.
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