REPLACEMENT OF SUB-ASSEMBLIES

TM 11-6625-3017-14

and Q6 in the discriminator (A5) and/or by selecting

5.4.10 Set l.f.

a new value for A5R13.

Test equipment: 1

5.5 REPLACEMENT OF SUB-ASSEMBLIES

(1)

Connect the crystal oscillator to the R. F. IN socket.

Oscillator

(2) Turn the oscillator RANGE switch to EXT and the

Function switch to TUNE.

Remove the instrument case, disconnect the plug to

the mixer unit and withdraw the lead and plug into the

(3)

Set the LEVEL control for a meter reading at the

oscillator compartment. Remove the chrome screws at

'top end of the black R. F. LEVEL arc.

each corner of the oscillator front panel and slide the unit

out. To remove completely, disconnect the supply plug

(4)

Turn the Function switch to F. M. SE.T FREQ and

from within the oscillator compartment.

When

adjust

A4L1

through

the

hole

the

withdrawn, all presets, coils etc., are accessible without

limiter/discriminator cover (see Fig. 4-2) to bring the

further dismantling.

meter reading to the SET mark.

R.F. attenuator and mixer.

5.4.11 L.F. and discriminator noise

It is not advisable to attempt any removal of these

Test equipment: 1, m, n and o

items, the former being a sealed unit. In all cases,

contact Marconi Instruments if repair appears necessary

(1)

Connect the 0. 014 [F capacitor, r.m.s. voltmeter

in this unit.

and oscilloscope across the OUTPUT terminals.

Printed circuit boards

(2)

Turn the oscillator RANGE switch to EXT.

The following are plug in circuit boards which, apart

(3)

Connect the 1.5 MHz crystal oscillator to the R. F.

from A13, can be removed by pressing a spring clip and

In socket and turn the MAX MOD FREQ switch to

pulling the board upwards:

15 kHz.

(1) 1st 1. f. amplifier (A7).

(4)

Turn the Function switch to SET FREQ-F. M. and

(2) 2nd 1. f. amplifier (A9).

check that the TF 2300A meter deflects to the SET

(3) A. M. detector (All).

mark.

(4) Peak reading voltmeter (A10).

(5) Calibrator (A12).

(5)

Turn the Function switch to TUNE and adjust the

(6) Power supply unit (A13).

LEVEL control to bring the TF 2300A meter reading

(This board is secured by a bracket which has to be

to the top end of the black R. F. LEVEL arc.

removed before the board can be unplugged. )

(6)

Turn the Function switch to SET CAL and adjust the

An extension board, type TM 7926, is stowed on the

r. m. s. voltmeter range switch to give a convenient

inside of the rear panel. By means of this board, any of

reference level. Note this reference level.

the above can be raised above its compartment for

examination while in the operative condition.

(7)

Turn the Function switch to DEV+ and the RANGE

switch to 5 kHz. Turn down the external voltmeter

I.F. amplifier

range switch and check that the reading is at least

52 dB below the reference level, i. e. -72 dB relative

The complete amplifier can be removed when

to the level of 50 kHz deviation.

necessary by unsoldering one lead and dis- connecting

two miniature plugs. Three screws retain the unit to the

(8)

If not, remove the yellow lead from the discriminator

chassis. Access for adjust- ment is provided by a

output, A5 pin 4 (see Fig. 5-3) and check that the

removable cover, thus enabling the inductors of the

noise level drops to -58 dB. A high noise figure can

tuned circuits to be adjusted through holes in the back of

be improved by selection of transistors Q3, Q4, Q5,

the circuit boards with a trimmer tool.

5-8