Induction coil T1 samples common-mode current flowing on the conductor under test.
The coil winding is resistively terminated at a low value to minimize circuit perturbation
and improve frequency linearity. Trimpot R3 adjusts the level of sampled signals, and is
used to calibrate the metering circuit against a known reference. Detector D2 drives the
meter movement directly through selected current-limiting resistors on the top three
ranges. On the lower three ranges, dual op-amp U1 buffers and amplifies detector output
prior to metering. Sensitivity control R1 expands the lowest measurement range--
extending meter sensitivity to the full gain limit of the amplifier. When SW1 is pressed
and released, a RC circuit holds dc-switch Q1 in conduction for approximately 15
seconds to permit hands-off measurement.
Maintenance
In Case of Difficulty:
Your meter is inherently trouble-free, but switches, controls, and
the meter movement may sustain damage if dropped or exposed to harsh vibration. Also,
avoid exposure to rain, snow, extreme heat, or other severe environmental conditions. If
you experience a problem, always check battery condition first. If battery replacement
fails to resolve the problem, you may contact MFJ Customer Service by phone at 601-
323-5869 or reach them via E-mail at [email protected]. Be sure to include a
complete description of the difficulty.
Calibration Instructions:
To check meter calibration, use the test set-up shown below:
HF Transceiver
In-Line
Wattmeter
>
7.000
MFJ-854
50-Ohm Dummy Load
50-watts should
indicate 1.0 Amp
of RF current into
a 50-ohm RF load.
50
Set the meter's
Range
selector to 1-A. For meaningful results,
your wattmeter must be
accurate and the 50-ohm load free of significant reactance.
To test, apply a 50-watt CW
signal to the load. You should observe a full-scale reading of 1.0-A (100 on the scale). If
results vary significantly from this reading, adjust trimpot R3 for the correct indication.
Trimpot R3 is located on the unit's pc board below the sense coil.
Important Warning: Avoid direct physical contact with this device when making
measurements with power sources of over 5 Watts. RF burns or permanent injury may
result from contact with high RF voltages.
If your meter is out of calibration and you lack the equipment or skills to perform the
calibration procedure, you may return it to the MFJ factory for service (see
In Case of
Difficulty
).
Содержание MFJ-854
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