Racal Instruments RA-117 User Manual page 41

stop. Check that the mechanical stops operate before the capacitor end-stops become
effective at both ends of the band.
34. To re-adjust the first v.f.o. calibration, a heterodyne wavemeter should be employed.
This is Coupled very loosely to V7 by placing its input lead in the vicinity of the valve
base. The 1 Mc/s crystal, V12 and V27 should be removed.
35. Set the wavemeter to 40.5 Mc/s and the MEGACYCLES dial to zero. Adjust L36 for
zero-beat. Change the wavemeter setting to 69.5 Mc/s and the MEGACYCLES dial
to 29. Adjust C77 for zero-beat. Repeat adjustment as necessary. Check the frequency
calibration at 1 Mc/s intervals and ensure that the megacycle positions are reasonably
central on the scale markings. Remove the first mixer valve V7 and connect the valve
voltmeter, shunted 12pF, between TP2 and the chassis. Check that the valve voltmeter
indicates at least 1.5V over the range. Refit the 1 Mc/s crystal, V12 and V27.
Antenna Circuit
36. Remove the first V.F.O. valve V5 and the first mixer valve V7 and set the receiver
controls as follows:-
37. Remove the screening cover from around C18A/B and connect a 1 kilohm resistor
across the secondary section (C18B rear section). Set the R.F. TUNE control approx-
imately 7 8ths of its travel in a clockwise direction.
38. Connect the valve voltmeter, shunted to 12pF, between TP2 and chassis. Connect
the output of the signal generator to the aerial input socket. Set the generator for a
frequency of 1 Mc/s.
39. Remove the top core From the transformer L8 and adjust the primary core for a
maximum deflection on the valve voltmeter. (The position of this core should be such
that it tunes at a point nearest the bottom of the transformer).
40. Remove the 1 kilohm resistor from the secondary section and connect it across the
primary section of C18.
41. Refit top core (secondary) and adjust it for a maximum deflection on the valve volt-
meter.
42. Remove the 1 kilohm resistor from the primary of C18.
43. Reset the signal generator frequency to 2 Mc/s and adjust the R.F. TUNE control
(C18) for maximum output on the valve voltmeter then adjust the trimmer capacitor
C233 for a maximum deflection on the valve voltmeter also check for symmetrical
response.
44. Repeat the above procedure for the R.F. RANGE switch settings and frequencies listed
in Table 1 below. Check that the maximum voltage input to give 0.5 volts output is
as shown in Table 2 below.
Alignment Procedures
R.F. ATTENUATOR
R.F. RANGE MC/S
SYSTEM switch
R.F./I.F. GAIN
MIN.
1 – 2 Mc/s
MAN.
MAX.
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