Potomac Instruments FIM-21 - Recalibration Process

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SECTION 4. MAINTENANCE ( Cont • )
4.9 Troubleshooting and Repair (Cont.)
6. If a radio station can be received, notice whether the variation
in field strength reading with modulation is excessive. Variations up to about
five percent may be caused by various normal conditions, but large variations
may indicate a faulty voltage regulator or integrated circuit Al02.
?. With the above indications as a guide, the unit may be opened
(see Sec, 4.5) and checked for internal faults. First, look for mechanical
faults, such as broken wires, switch proble1ns, and loose hardware. The most
likel y cause of circuit failure is a bad integrated circuit (IC), which are
all R CA Type CA3045. Type CA3046 may be substituted if the unit is not eJq:Josed
to temp eratures below freezing. A bad IC can easily be located without circuit
tracing by measuring the pin voltages and comparing the results with the chart,
Figure 4- 1. Replace IC
1
s in accordance with Sec.
4
,7,
Normally an IC can be
replaced without affecting the calibration of the uni t , but operation should be
carefully checked after replacement.
In the case of any serious malfunc tion, it is desirable to contact
the factory, as this may save considerable time and trouble and will be of
mutual benefit to both manufacturer and user. The repair itself is best
carried out at the factory, wh ere various special test fixtures are maintained
specifically for t h ese instruments, including an RF fie ld source of precisely-
known magnitude for calibration checks after repair.
4.10 Recalibration
For assurance that long-term changes in field strength readings
are due to factors other than the field strength meter itselfJ the instrument
should be recalibrated at regular intervals. The factory maintains a cali-
b rating field for this purpose that is he ld in close agreement with the
standard field 0£ the National Bureau of Standards, The factory recalibration
procedure includes a thorough check and readjustment of the unit, to insure
that it is working properly prior to the actual calibration, followed by
adjustments in the calibrating field on all ranges to bring the unit within
its original accuracy specifications. It is recorronended that this be done
at intervals of not more than two years in order to be assured of measurement
accuracy in critical situations. Recalibration can be done across for the
full frequency range or at a single specified frequency.
If at any time a question arises as to the calibration of the unit,
when it is opera ting normally, it may be returned to the factory for an "In-
coming Calibration". In this case complete measure.111ents are made on the unit
as it is received, without any readjustments, and a set of correction factors
is supplied that can be applied to readings already taken. These factors
include corrections for all ranges and for meter nonlinearity as well as for
absolute calibration.
4-8