Second V.f.o - Racal Instruments RA-117 User Manual

from signal generators. A digital frequency meter should therefore be employed. The
equipment should be connected to SK8 or SK9. The exact frequency passing through
the circuit will be displayed on the indicator panel. Should the level of output at any
time during the alignment procedure be insufficient to drive the frequency meter, the
signal generator output can be increased to obtain the frequency check but must be
restored to the lower value for level measurements. When such increases are made,
the meter on the receiver panel should be switched to A.F. LEVEL to avoid damage.

14.4. Second V.F.O.

Minor Corrections
23. The variable capacitor has been carefully adjusted and should not be re-adjusted
unless absolutely necessary. Minor corrections can be made as follows:-
(1) Set the SYSTEM switch to CAL.
(2) Set the KILOCYCLES cursor in line with the MEGACYCLES cursor (i.e. central)
(3) Ensure that the B.F.O. switch is OFF.
(4) Rotate the R.F./I.F. GAIN to MAX.
(5) Set the BANDWIDTH switch to 3 kc/s.
(6) Set the KILOCYCLES scale to zero (0 kc/s) and adjust the capacitor C306 to
give zero beat note in the loudspeaker.
(7) Set the KILOCYCLES scale tothat zero beat point which is nearest to the 1000
kc/s position.
(8) Lock the drive sprocket.
(9) Adjust the position of the film scale to produce a correct calibration.
NOTE: When moving film scale relative to the sprockets, grip both sides of the film
scale in order to create a loop which will allow the film to slide round the drive
sprocket; the drive sprocket is on the left when facing the receiver and hence
movement of the film scale will have to be to the left.
(10) Repeat (6) to (9) until an adequate degree of accuracy is obtained.
IMPORTANT NOTE: The tuning slug of L55 has been sealed by the manufacturer and
Replacement of Variable Capacitor
24. The procedure described below should not normally be carried out unless the variable
capacitor C310 is being replaced. Before electrical adjustment, the following mechan-
ical points should be verified:-
(1) Check that the fixed and movable vanes of C310 are fully meshed.
(2) Check that the distance from the cursor to the extreme end of the scale adjacent
to the 1000 kc/s point, is approximately 71/2-in. Should this distance vary ap-
preciably from 71/2-in. Carefully lift the scale from the drive and move the scale
round the required position.
(3) Whenever the scale is replaced, endeavour to re-align by adjusting the film to the
correct position before trimming.
25. The procedure for electrical adjustment is carried out as follows:-
(1) Proceed as in 24. (1) above.
Alignment Procedures
must not be touched under any circumstances.
39