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Spurious Responses
27
6
MC/S BREAK-THROUGH
5. When the receiver is tuned to 3
.
2 Mc/s the first v.f.o. frequency is 43
.
5 Mc/s. This
reaches the second mixer and combines with 37
.
5 Mc/s giving a stable 6 Mc/s which
may pass through the 2
.
3 Mc/s BPF where it combines with the second v.f.o. running
at 4
.
4 Mc/s giving 1
.
6 Mc/s which then follows normal paths. This can be tuned out
by L301.
1
MC/S HARMONIC BREAK-THROUGH
6. Switch R.F.RANGE Mc/s to WWIDEBAND 75-ohms. 1 Mc/s break-through re-
sponses appear at 0 and 1,000 on the KILOCYCLES scale at each setting of the
MEGACYCLES dial and are generally more prominent with wideband input. If the
response is dependent upon the setting of the MEGACYCLES dial, the 1 Mc/s spec-
trum is probably breaking through to the first mixer stage. If the response is inde-
pendent of the MEGACYCLES dial setting, it is due either to break-through of the
second and/or third harmonic to the second or third mixer stage. Remove second
mixer valve to eliminate this stage and so determine in which stage the break-through
occurs.
FIRST V.F.O. HARMONICS
7. Spurious responses may occur at 4
.
5, 5
.
5 and/or 17
.
5 Mc/s, if C42A and/or C194A are
open circuit. These responses are caused by the harmonics of the first v.f.o. breaking
through to the second mixer stage and beating with the harmonics of the 37
.
5 Mc/s
heterodyne voltage.
B.F.O. HARMONICS
8. These responses may be detected at 100 kc/s intervals between 1 and 1
.
5 Mc/s when
the B.F.O. frequency is 100 kc/s and the receiver aerial input is tuned.
SECOND V.F.O. BREAK-THROUGH
9. Responses may occur at 3
.
8 and 4
.
3 Mc/s with tuned aerial input. Ascertain that the
first and second v.f.o. are not in contact, that the v.f.o. chassis is well bonded to the
main chassis and the fixing screw are tight.
NOTES:
A failure in any one of the following capacitors C66, C92, C96, C97, C98, C103
or C104 may result in increased ‘end of band’ responses. These responses will
disappear when the MEGACYCLE dial is detuned. The failure of C117, C327,
C207, C208 or C214 can result in ’end of band’ responses, or B.F.O. harmonic
break-through. Detuning the MEGA- CYCLES dial will have no effect.
1
.
7
MC/S BREAK-THROUGH
10. Responses may occur at 1
.
7 and 3
.
4 Mc/s with tuned aerial input due to radiation
from the 1
.
7 Mc/s crystal oscillator. Ascertain that bonding is effective between the
1
.
7 Mc/s oscillator/mixer chassis and the first V.F.O. chassis.
Содержание RA-117
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Страница 24: ...Detailed Circuit Description 24 at maximum is the correct calibration maintained...
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