CLEANING AND LUBRICATING

TM 11-6625-3017-14

2nd harmonic: (fl -f2):

2 kHz

3rd harmonic: (2f2 -fl):

12 kHz

and express them in dB's relative to 0 dB. Add +6 dB to

the 2nd harmonic reading and +9 dB to the 3rd harmonic

reading. The r. m. s. sum of the two levels (see below

for method of calculation) should be lower than -54 dB

(0. 2%. Distortion limits for other values of deviation and

Fig. 4-7. A.M. rejection measurement

modulation frequency are given in Sect. 1.2 - Data

Summary.

(3) Switch to F. M. - SET FREQ and adjust the

OSCILLATOR tune control to bring the meter reading to

Summation of dB levels

the SET mark.

The r. m. s. sum of two dB levels can be obtained

(4)

Switch to TUNE and adjust the LEVEL control to

by increasing the level of the larger one by an increment

give a reading just below full-scale on the

between 0 and 3 dB; the value of increment depends on

black arc.

the difference between the two levels as shown by the

graph. For example, if the two levels are -40 dB and -46

(5)

Turn the DEV RANGE switch to 5 kHz and the MAX

dB, giving a difference of 6 dB, the increment is 1 dB; the

MOD FREQ switch to 15 kHz.

sum of the two levels is therefore -40 +1 dB = -39 dB.

(6)

Switch to DEV+ and DEV- in turn and check that

the residual reading on the meter is less than about

Fig. 4-6. Summation of dB levels

150 Hz.

4.4 CLEANING AND LUBRICATING

Rotary switch contacts

These should be cleaned once or twice a year,

depending on usage, with benzine or white spirit (not

carbon tetrachloride). After cleaning wipe the contacts

with a suitable lubricant such as a 1% solution of

petroleum jelly in white spirit.

4.3.7

A.M. rejection

Oscillator unit

Test equipment:- a, f and m.

Excessive lubrication must be avoided but the

moving parts should be cleaned and lubricated at least

(1) Set up the equipment as shown in Fig. 4-7. Adjust

twice a year:

the signal generator to give a c. w. output of 100 mV at

any convenient frequency. Adjust the I. f. oscillator

(a)

Worm shaft: Use Rocol anti-scuffing paste.

frequency to 1 kHz at a level which gives 80%

modulation as measured on the TF 2300A.

(b)

Carriage slide: Use Aeroshell 4 oil.

(2) On the TF 2300A set the Function switch to TUNE

(c)

Scale drum mounting: Apply a few drops of

and the OSCILLATOR TUNE control to give maximum

Aeroshell 4 oil to the felt washer on the spigot

meter reading. Then adjust the LEVEL control for a

mounting.

reading within the black arc.

(d)

Range switch bevel gears: Use Rocol anti-

scuffing paste.

Note:

Sealed bearings at end of worm shaft will not

require lubrication within the life of the instrument.

4-8