AL-800H Instruction Manual
7
Multimeter
The multimeter is the meter on the right. It continuously reads the forward
peak envelope power
on its left-
hand scale (FWD). This scale is calibrated in 100 watt steps up to 2 kW.
The right-hand scale of this meter serves multiple functions, including measurement of the high voltage
(HV), reflected power (REF), SWR, ALC voltage output (ALC), and relative ALC threshold (ALC SET).
These four metering functions are selected by the
"MULTIMETER"
switch. See MULTIMETER
FUNCTIONS for more information.
Multimeter Functions
HV
:
The multimeter will indicate the dc plate voltage of the PA tube when in the HV position. The correct
scale to use is the ACL / HV scale, which has a picket every 100 volts. Two zeros must be added behind
the numbers indicated on the meter scale (i.e. multiply by 100), so that "25"=2500 volts and "20"=2000
volts. Do not operate the amplifier if the high voltage is over 2700 volts with the amplifier on standby. See
the TRANSFORMER CONNECTIONS section on page 13 for information on correcting excessive high
voltage.
REF
:
The multimeter measures the antenna (or load) peak envelope reflected power in the REF position.
The full scale reflected power reading is 500 watts. This scale is marked every 10 watts below 100 watts,
and every 100 watts from 100 to 500 watts.
NOTE: The SWR of the load can be measured when the "MULTIMETER" switch is in the REF
position by observing the different red SWR curves. The forward and reflected meter
pointers will cross each other on, or near, the correct SWR curve.
ALC
: The multimeter measures the output voltage of the ALC detector when in the ALC position. The
full scale ALC reading is 35 volts and is read directly from the ACL / HV scale. The meter should flick
upwards occasionally during normal ALC action. See ALC METERING, CONTROLS, AND
ADJUSTMENTS operation on page 15 for more information.
ALC SET
:
The multimeter measures the
approximate
grid current that will produce ALC activity when
the meter switch is in the
"ALC SET"
position. One zero must be added to the reading on the ACL / HV
scale and the result divided by 2 for this function. For example, ALC action will begin at approximately 75
mA of grid current (typically between 60 mA and 90 mA) when the
"ALC SET"
control is adjusted until
the meter reads "15."
Wattmeter Circuit
The AL-800H wattmeter circuit uses an accurate directional coupler followed by a true peak detector
circuit. This circuit will accurately determine the true peak envelope power (PEP) of normal voice
waveforms. If the load SWR is high, the true power reading can be obtained by subtracting the reflected
power from the forward power reading.
