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Alignment Procedures
40
(2) Check the calibration of the v.f.o. at 100 kc/s intervals; if the error exceeds 1
kc/s, adjust carefully the plates of the rotor of the variable capacitor in order to
correct the calibration.
1
.
6
Mc/s Rejection Filter
26. Disconnect lead to SK300A and apply 1
.
6 Mc/s signal generator output to SKT300A.
Set METER switch to S-METER and adjust core of L302 for minimum meter deflec-
tion.
B.F.O.
27. Set the SYSTEM switch to the CHECK B.F.O. position. Switch the meter switch to
R.F. LEVEL. Switch the B.F.O. on and set the B.F.O. frequency control knib to zero.
Adjust C199 as necessary to obtain zero-beat. Observe that the meter reads at least
100
µ
A.
28. If the B.F.O. frequency control knob has been removed, adjust the frequency capacitor
for zero-beat with the identification mark on the shaft uppermost. Replace the knob
so that the pointer indicates zero.
37
.
5
Mc/s Filter and Amplifier
29. Remove the 1 Mc/s crystal, second mixer valve V9 and first v.f.o. valve V5. Check
that all the screening covers are in place. Connect a suitable valve voltmeter, shunted
12pF, to TP3. Inject an accurate 37
.
5 Mc/s signal at TP1. Ensure that the valve
voltmeter and signal generator leads are short to avoid regeneration. Adjust L50,
C90, C81, C72, C63, C55, C45, C35, C24, L28 and L33 in that order, several times, to
obtain maximum output. The input required to produce 1V should be approximately
2.5mV. The 6dB bandwidth of the 37.5 Mc/s chain should lie between 229-300 kc/s.
The bandwidth at 40dB should not exceed 750 kc/s. The mean of the frequencies
corresponding to the 6dB points should not deviate from 37
.
5 Mc/s by more than 20
kc/s and more than 25 kc/s at 40dB banwidth.
30. C108 is adjusted to avoid interaction between the 37
.
5 and 40 Mc/s filters and should
not normally require further adjustment. Fit the 1 Mc/s crystal, the second mixer and
the the first v.f.o. valve.
1 Mc/s Oscillator
31. Connect the valve voltmeter to the 1 Mc/s output plug PL2 and adjust L2 for maxi-
mum output (2 – 3 volts). C2A may be adjusted to ”pull” the crystal to the correct
frequency; howewer, adjustment of crystal frequency should not be attempted unless
a standard is available having an accuracy of better than one part in 10
7
.
Second Mixer Drive Level
32. Remove the second mixer valve V9. Connect the valve voltmeter, shunted to 12pF, to
TP3. Tune through each megacycle calibration point and check that the level output
lies between 2 and 10V. To equalize the drive at 28 and 29 Mc/s carefully adjust C7.
First V.F.O. Calibration
33. Slacken off the mechanical end-stop until it is inoperative. Set C76 to maximum ca-
pacity and ensure that the calibration mark at the zero end of the MEGACYCLES
dial coincides with the cursor. Tighten end-stop after moving the scaale free from the
Summary of Contents for RA-117
Page 1: ...Racal RA 117 Radio Receiver...
Page 24: ...Detailed Circuit Description 24 at maximum is the correct calibration maintained...
Page 32: ...32...
Page 70: ...Illustrations 70...
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