Instruction manual

TM 11-6625-3017-14
OSCILLATOR TUNE and LEVEL controls, a meter
deflection in the black arc can be obtained. If so,
the local oscillator, mixer and i. f. amplifier (Al to
A3), most of the a. m. detector (All) and the peak
reading voltmeter (A10) are working.
If not, but the meter deflects when the input
frequency is changed to the i. f. of 1.5 MHz, then
the-local oscillator or mixer are suspect. A fault in
the local oscillator can be confirmed if the first
paragraph of test 2 gives a positive result when
using an external local oscillator.
(3) Turn the Function switch to F. M. SET FREQ. or
A.M. SET FREQ. (after setting up the meter
deflection as in step 2 above). Check that the
meter deflects to the SET mark when the
OSCILLATOR TUNE control is slightly readjusted.
If so, the limiter circuit (A4) is working.
Input attenuator
This can be checked by feeding a 1.5 MHz signal
into the EXT OSC socket when the sensitivity should be
approximately 20 mV input for a meter reading on the
bottom of the black arc when the function switch is set to
TUNE. If a 5 mV signal is now applied to the R. F.
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
or part of the mixer is faulty. It is more likely to be the
attenuator.
5.2.3 Internal checks
Having roughly localized the fault by the front-panel
tests it may be more accurately located by signal tracing
tests, using the inter-unit wiring diagram as a guide.
(1) First check the power supply unit output voltages at
A13C9 (-18 V) and C12 (-12 V) - see Fig. 4-2 for
location. If incorrect, reset as described in Sect.
5.4. 3. If the fuses are intact and there is no output
from the power supply unit when a. c, operated, try
changing to battery operation. If this restores the
output, a faulty transformer/- rectifier circuit is
indicated.
(2) If trouble is suspected between the calibrator unit
and the meter, turn the Function switch to SET CAL
and check the calibrator wave-forms from the
calibrator unit output to the meter. These should be
as shown in Sect. 5.3, although the frequency of.
the square wave modulation is not critical and may
differ from the nominal 2 kHz value illustrated
Having identified a faulty board, use the static
voltage information on the circuit diagrams as a
guide to locating the source of the trouble.
5.3 WAVEFORMS
The oscillograms illustrated in Fig. 6-1 show the
significant waveforms that occur between the output of
the calibrator unit (A12) and the OUT-PUT terminals.
The measuring points are indicated on the inter-unit
wiring diagram, FO 6-1, and the circuit diagrams.
Measurement should be made with an oscilloscope
of at least 5 MHz bandwidth and 20 mV/cm sensitivity,
such as M.I. type TF 2201 series. Contact with the
printed board edge connectors may be made from below
the chassis but, if more convenient, the Extension Board
TM 7926 may be used to allow access from above the
chassis.
5.4 REALIGNMENT
5.4.1 Introduction
This is a complete realignment procedure with the
steps arranged in a logical order. If the full procedure is
not required, for example following a component
replacement, individual steps may be performed
providing they do not interact with other adjustments.
Although the likelihood of such interaction is pointed out
wherever possible, it is recommended that you always
consider the effects of any readjustment by reference to
the circuit diagrams.
NOTE: Performance limits given in this section are for
guidance only and should not be taken as guaranteed
performance specifications unless they are also quoted
in the Data Summary section.
5.4.2 Test equipment required - see Table 5.2.
5-2