WAVEFORMS

TM 11-6625-3017-14

OSCILLATOR TUNE and LEVEL controls, a meter

(2)

If trouble is suspected between the calibrator unit

deflection in the black arc can be obtained. If so,

and the meter, turn the Function switch to SET CAL

the local oscillator, mixer and i. f. amplifier (Al to

and check the calibrator wave-forms from the

A3), most of the a. m. detector (All) and the peak

calibrator unit output to the meter. These should be

reading voltmeter (A10) are working.

as shown in Sect. 5.3, although the frequency of.

the square wave modulation is not critical and may

If not, but the meter deflects when the input

differ from the nominal 2 kHz value illustrated

frequency is changed to the i. f. of 1.5 MHz, then

the-local oscillator or mixer are suspect. A fault in

Having identified a faulty board, use the static

the local oscillator can be confirmed if the first

voltage information on the circuit diagrams as a

paragraph of test 2 gives a positive result when

guide to locating the source of the trouble.

using an external local oscillator.

(3)

Turn the Function switch to F. M. SET FREQ. or

5.3 WAVEFORMS

A.M. SET FREQ. (after setting up the meter

deflection as in step 2 above). Check that the

The oscillograms illustrated in Fig. 6-1 show the

meter deflects to the SET mark when the

significant waveforms that occur between the output of

OSCILLATOR TUNE control is slightly readjusted.

the calibrator unit (A12) and the OUT-PUT terminals.

If so, the limiter circuit (A4) is working.

The measuring points are indicated on the inter-unit

wiring diagram, FO 6-1, and the circuit diagrams.

Input attenuator

Measurement should be made with an oscilloscope

of at least 5 MHz bandwidth and 20 mV/cm sensitivity,

This can be checked by feeding a 1.5 MHz signal

such as M.I. type TF 2201 series. Contact with the

into the EXT OSC socket when the sensitivity should be

printed board edge connectors may be made from below

approximately 20 mV input for a meter reading on the

the chassis but, if more convenient, the Extension Board

bottom of the black arc when the function switch is set to

TM 7926 may be used to allow access from above the

TUNE. If a 5 mV signal is now applied to the R. F.

chassis.

INPUT socket a meter reading will be produced again on

the black arc. If there is no deflection when the 29 mV

signal is applied to the EXT OSC socket the attenuator

5.4 REALIGNMENT

or part of the mixer is faulty. It is more likely to be the

attenuator.

5.4.1

Introduction

This is a complete realignment procedure with the

steps arranged in a logical order. If the full procedure is

5.2.3

Internal checks

not required, for example following a component

replacement, individual steps may be performed

Having roughly localized the fault by the front-panel

providing they do not interact with other adjustments.

tests it may be more accurately located by signal tracing

Although the likelihood of such interaction is pointed out

tests, using the inter-unit wiring diagram as a guide.

wherever possible, it is recommended that you always

consider the effects of any readjustment by reference to

(1)

First check the power supply unit output voltages at

the circuit diagrams.

A13C9 (-18 V) and C12 (-12 V) - see Fig. 4-2 for

location. If incorrect, reset as described in Sect.

NOTE: Performance limits given in this section are for

5.4. 3. If the fuses are intact and there is no output

guidance only and should not be taken as guaranteed

from the power supply unit when a. c, operated, try

performance specifications unless they are also quoted

changing to battery operation. If this restores the

in the Data Summary section.

output, a faulty transformer/- rectifier circuit is

indicated.

5.4.2

Test equipment required - see Table 5.2.

5-2