SPURIOUS RESPONSES


ORIGINS OF SPURIOUS RESPONSES

1. In a high sensitive receiver, precautions against internally generated
spurious responses are essential. To this end, various sections of the
receiver have been carefully screened and the power supplies filtered.

2. Any reduction in the screening efficiency or the failure of any
filtering component may results in spurious signals being generated.
It is therefore essential to ensure that the bonding surfaces are clean and
that all securing screws are tight. Spurious responses in the receiver may
occur from the following main causes:-

(1) 37.5 Mc/s break-through from the second mixer V9 to the third
mixer V25.

(2) Break-through of 1 Mc/s harmonics.

(3) Break-through of b.f.o. harmonics.

(4) Responses at 3.800 and 4.000 Mc/s due to second v.f.o. break-
through.

(5) Responses of 1.7 and 3.4 Mc/s due to 1.7 crystal oscillator
break-through.

(6) Response of 3.2 Mc/s due to 6 Mc/s break-through.



CHECKS FOR SPURIOUS RESPONSES

3. Spurious responses are measured relative to receiver noise in the
following manner:-

When response is located, the receiver is de-tuned from it just
sufficiently to render the beat inaudible. The a.f. gain is then
adjusted to provide a convenient noise reference output (1mW)
and the receiver retuned to the spurious signal for maximum
output. The dB rise in audio output is a measure of the spurious
signal level relative to receiver noise.


Standard conditions of test:

No connection to aerial socket
System switch to MAN.
R.F./I.F. Gain at MAX.
B.F.O. on
3 kc/s bandwidth
R.F. ATTENUATOR at MIN.
Set V.F.O. switch to INT.


37.5 MC/S BREAK-THROUGH TO THIRD MIXER

4. Switch R.F. RANGE Mc/s to WIDEBAND 75-ohms. This response will be
indicated as a beat note which varies rapidly in frequency with respect
to the KILOCYCLES scale, i.e. a change of 1 kc/s on the scale results in a much
larger change in the note. It will also move along the KILOCYCLES scale if the
MEGACYCLES dial is adjusted slightly. This response may be eliminated by
adjusting the 37.5 Mc/s strap (L300 at second mixer anode).


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
givin 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
responses 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
spectrum is probably breaking through to the first mixer stage. If the response
is independent 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.


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