SECTION 4 MAINTENANCE

TM 11-6625-3017-14

SECTION 4

MAINTENANCE

units above and below the chassis. Components may be

4.1 INTRODUCTION

identified by markings on the boards and chassis. For

replacement of sub-assemblies and components, see

This section contains information for keeping the

Sects. 5. 5 and 5. 6.

equipment in good working order and for checking its

overall performance.

4.3 PERFORMANCE CHECKS

The tests in this section may be used as a routine

CAUTION This instrument uses semiconductor

maintenance procedure to verify the main performance

devices which, although having inherent long term

parameters of the instrument.

All tests can be

reliability and mechanical ruggedness, are susceptible to

completed without removing the case, except where

damage by overloading, reversed polarity and excessive

some internal readjustment is indicated. Tests may be

heat or radiation. Avoid hazards such as reversal of

done at any convenient frequency within the carrier

batteries, prolonged soldering, strong r. f. fields or other

range of the modulation meter provided that this

forms of radiation, use of insulation testers or

frequency is also within the range of the test equipment

accidentally applied short circuits. Even the leakage

items a, b, c, d and g.

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

4.3.1

Test equipment required

arrangements of the transistors.

See Table 4. 1.

Screw fasteners

4.3.2

R.F. input and i.f. output

Screw threads used on this instrument are of the

following sizes - 8BA, 6BA, 4BA, 2BA and 1/4 BSF.

Test equipment:- a, d, e and 1.

Cruciform headed screws are of the Phillips

Pozidriv pattern; to avoid damaging them a Pozidriv

This procedure verifies the sensitivity of the

screwdriver should be used.

modulation meter and the adjustment of the local

oscillator.

4.2 ACCESS AND LAYOUT

R.F. input resistance

The main case assembly is held in position by the

rear panel. This panel may be removed after unscrewing

(1)

Turn the LEVEL control mid-way. Connect the

the four coin-slotted screws across the corners; the

multimeter, set for resistance measurement,

mains lead is held to the rear panel by a clip and will

between the centre pin of the R. F. IN socket and

earth and check that the reading is 50 Ω 10 Ω

come away with it if the mains plug is withdrawn.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.

R.F. sensitivity

Rack-mounted models have top and bottom dust

(2)

Adjust the signal generator to give a c. w. output of

covers, each attached by four screws.

about 50 mV and connect it to the R. F. INPUT

socket of the modulation meter and to the

Figs. 4-1 and 4-2 show the general location of all

voltmeter, keeping the leads as short as possible.

4-1