Instruction manual

TM 11-6625-3017-14
5.4.10 Set l.f.
Test equipment: 1
(1) Connect the crystal oscillator to the R. F. IN socket.
(2) Turn the oscillator RANGE switch to EXT and the
Function switch to TUNE.
(3) Set the LEVEL control for a meter reading at the
’top end of the black R. F. LEVEL arc.
(4) Turn the Function switch to F. M. SE.T FREQ and
adjust A4L1 through the hole in the
limiter/discriminator cover (see Fig. 4-2) to bring the
meter reading to the SET mark.
5.4.11 L.F. and discriminator noise
Test equipment: 1, m, n and o
(1) Connect the 0. 014 [F capacitor, r.m.s. voltmeter
and oscilloscope across the OUTPUT terminals.
(2) Turn the oscillator RANGE switch to EXT.
(3) Connect the 1.5 MHz crystal oscillator to the R. F.
In socket and turn the MAX MOD FREQ switch to
15 kHz.
(4) Turn the Function switch to SET FREQ-F. M. and
check that the TF 2300A meter deflects to the SET
mark.
(5) Turn the Function switch to TUNE and adjust the
LEVEL control to bring the TF 2300A meter reading
to the top end of the black R. F. LEVEL arc.
(6) Turn the Function switch to SET CAL and adjust the
r. m. s. voltmeter range switch to give a convenient
reference level. Note this reference level.
(7) Turn the Function switch to DEV+ and the RANGE
switch to 5 kHz. Turn down the external voltmeter
range switch and check that the reading is at least
52 dB below the reference level, i. e. -72 dB relative
to the level of 50 kHz deviation.
(8) If not, remove the yellow lead from the discriminator
output, A5 pin 4 (see Fig. 5-3) and check that the
noise level drops to -58 dB. A high noise figure can
be improved by selection of transistors Q3, Q4, Q5,
and Q6 in the discriminator (A5) and/or by selecting
a new value for A5R13.
5.5 REPLACEMENT OF SUB-ASSEMBLIES
Oscillator
Remove the instrument case, disconnect the plug to
the mixer unit and withdraw the lead and plug into the
oscillator compartment. Remove the chrome screws at
each corner of the oscillator front panel and slide the unit
out. To remove completely, disconnect the supply plug
from within the oscillator compartment. When
withdrawn, all presets, coils etc., are accessible without
further dismantling.
R.F. attenuator and mixer.
It is not advisable to attempt any removal of these
items, the former being a sealed unit. In all cases,
contact Marconi Instruments if repair appears necessary
in this unit.
Printed circuit boards
The following are plug in circuit boards which, apart
from A13, can be removed by pressing a spring clip and
pulling the board upwards:
(1) 1st 1. f. amplifier (A7).
(2) 2nd 1. f. amplifier (A9).
(3) A. M. detector (All).
(4) Peak reading voltmeter (A10).
(5) Calibrator (A12).
(6) Power supply unit (A13).
(This board is secured by a bracket which has to be
removed before the board can be unplugged. )
An extension board, type TM 7926, is stowed on the
inside of the rear panel. By means of this board, any of
the above can be raised above its compartment for
examination while in the operative condition.
I.F. amplifier
The complete amplifier can be removed when
necessary by unsoldering one lead and dis- connecting
two miniature plugs. Three screws retain the unit to the
chassis. Access for adjust- ment is provided by a
removable cover, thus enabling the inductors of the
tuned circuits to be adjusted through holes in the back of
the circuit boards with a trimmer tool.
5-8