T EVIS i - AmericanRadioHistory.Com

= M. For center scale reading we
have
A= i. /i, =t /z and
RM
RM
X (1 -t) (R +M) R + M
Since R must limit current to give full
scale deflection when terminals are
open- circuited, we have
E
i, _ R+M
whence:
R= E -M (2)
i.
For the next lower range R must
limit the current to 10 i. with meter
shunted by S. Therefore
E MS R
10 i M + S 10
and similarly R, = R /100, etc.
The lowest range is determined by
the allowable battery current and the
sensitivity of the meter. For this in-
termittent use a current of 0.5 ampere
can be delivered by three flashlight
cells paralleled. However, new batteries
should never be paralleled with
old -the new ones will discharge
through the old ones.
Regarding calibration of the meter
scale itself, the easiest and most practical
method is to mark the scale directly
using resistors of known values,
preferably a decade resistance box;
however, if such is not available a
satisfactory calibration is possible
from formula (1) given previously,
noting that:
A = do
= RMX . . . (3)
R +M +X
for the highest range, i.e., where no
meter shunt is used.
If this method is used, the value of
R actually required to zero the meter
on the x10 range should preferably be
used.
-S-
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FM RECEIVER TUNING INDICATOR
By HARRY R. HYDER
the advent of FM broadcasting.
SINCE there has been a real need for an effective
tuning indicator for FM sets.
How many times have you thought you
had your set tuned properly, only to be
annoyed by ignition noise from some
passing ear. You rush to the set to retune.
but by the time you reach it. the
car is gone and you are as nu ch in the
dark as before. On weak stat' s particularly.
the "no noise" point is very
sharp.
A zero -center microammeter is fine
but extensive and it can only be used
with the Foster -Seeley type discriminator.
various types of `magic eye"
tutus suffer fr lack of sharpness and
other faults. N of them will work
with the gated -beum tube (6ßN6) type
of FBI detector.
The toting device to be described is
absurdly simple. is equally effective
with all types of FM detectors. and costs
practically nothing to build. Furthermore,
it is foolproof. since it attacks
the problem directly.
Fig. 1 shows a simple blocking oscillator,
using a war- surplus blocking oscillator
transformer. The output is
loosely capacity coupled to the antenna
input of the FM set. A few turns of wire
around the twin -lead is usually sufficient.
Plate voltage is applied to the
oscillator through a door -bell type
push -button switch. Just tune in the
station you want. push the button,
tune to the noise null, release the pushbutton,
and you are tuned in as well as
you can possibly be.
A blocking oscillator generating a
short pulse with a fast rise time puts
out a fairly uniform noise spectrum
throng' t the v.h.f. reg. . The fundamental
repetition rate of the oscillator,
which is in the audio region,
gives a tone to the noise.
You can get the same results by using
your electric razor, or an ordinary door
buzzer should work, due to the sparking
at its contacts. The blocking oscillator
has the advantage of being silent.
Both Utah and Raytheon transformers
have been used successfully. Although
it has not been tried. there is
no reason why one of the old style TV
blocking transformers should not work,
with suitable values of It and C.
Fig. 1. Two simple blocking oscillator circuits used to tune FM receivers.
(A) uses a triple-winding type of transformer and (B) a two -winding one.
B. 100 -250v
B
IDO-300V
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PUSH BUTTON SWITCH
PUSH BUTTON SWITCH
lyld
MEG
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pet
OUTPUT
MEG
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ud
4.
II
e22yy1d.
TO FM SET
TUNG -SOL
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A)
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ADJUST FOR
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ANT SMALL TRIODE
i
148
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