Instruction manual

TM 11-6625-3017-14
SECTION 4
MAINTENANCE
4.1 INTRODUCTION
This section contains information for keeping the
equipment in good working order and for checking its
overall performance.
CAUTION This instrument uses semiconductor
devices which, although having inherent long term
reliability and mechanical ruggedness, are susceptible to
damage by overloading, reversed polarity and excessive
heat or radiation. Avoid hazards such as reversal of
batteries, prolonged soldering, strong r. f. fields or other
forms of radiation, use of insulation testers or
accidentally applied short circuits. Even the leakage
current from an unearthed soldering iron could cause
trouble. Before shorting or breaking any circuit, refer to
the circuit diagrams to establish the effect on bias
arrangements of the transistors.
Screw fasteners
Screw threads used on this instrument are of the
following sizes - 8BA, 6BA, 4BA, 2BA and 1/4 BSF.
Cruciform headed screws are of the Phillips
Pozidriv pattern; to avoid damaging them a Pozidriv
screwdriver should be used.
4.2 ACCESS AND LAYOUT
The main case assembly is held in position by the
rear panel. This panel may be removed after unscrewing
the four coin-slotted screws across the corners; the
mains lead is held to the rear panel by a clip and will
come away with it if the mains plug is withdrawn. The
instrument can now be slid forward, out of the case;
alternatively, it can be placed face-downwards on a soft
surface and the case lifted off.
Rack-mounted models have top and bottom dust
covers, each attached by four screws.
Figs. 4-1 and 4-2 show the general location of all
units above and below the chassis. Components may be
identified by markings on the boards and chassis. For
replacement of sub-assemblies and components, see
Sects. 5. 5 and 5. 6.
4.3 PERFORMANCE CHECKS
The tests in this section may be used as a routine
maintenance procedure to verify the main performance
parameters of the instrument. All tests can be
completed without removing the case, except where
some internal readjustment is indicated. Tests may be
done at any convenient frequency within the carrier
range of the modulation meter provided that this
frequency is also within the range of the test equipment
items a, b, c, d and g.
4.3.1 Test equipment required
See Table 4. 1.
4.3.2 R.F. input and i.f. output
Test equipment:- a, d, e and 1.
This procedure verifies the sensitivity of the
modulation meter and the adjustment of the local
oscillator.
R.F. input resistance
(1) Turn the LEVEL control mid-way. Connect the
multimeter, set for resistance measurement,
between the centre pin of the R. F. IN socket and
earth and check that the reading is 50
Ω ±
10
Ω
R.F. sensitivity
(2) Adjust the signal generator to give a c. w. output of
about 50 mV and connect it to the R. F. INPUT
socket of the modulation meter and to the
voltmeter, keeping the leads as short as possible.
4-1