SKM C450i24093017201
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PriGE 5
BULLETIN O.i-i' TASi-.'. ANI. ^ DIVISION/". ?. I. JUNE 1969
EQUIPMENT STORE NOTES.
Transistors.....
The equipment store still has available q quantity
of general purpose types ONLY. Price... 12o each over counter or
15o each posted.
Relays
\lso available ?ire some small relays. These opernte
very s'-ttisfaotoraly on 12 volts and. have four sets of change over
con-fcacts. AS already mentioned they are reasonably small and are
just the thing for those remote switching requirements..
oooOOOooo
J4'OR S;.LE. 2 Kr&esler Magnavox '/oafer Loudspeakers,
15 inch diameter, 25 v'/atfes RtaS each.
Tone resonance 35cps. ^70 pair.
G. H. Thompson
15 Proctors Rd.,
Dynyrne. Phone 24875.
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POTENTIO^ETER TRACKS..„
There are probably some readers v/ho are a little
hazy. as "bo the exact meanings of the terms "log" and "anti-log" e-bc. ,
this article I hope will help to clfarify the' subject a lit, tie.
The resistanoe la;/ 0:? taper is. the manner in ivhich the resistance
ohanges with rotation of the shaftr. The diag. shows the basic laws
'
found in the •vast majority of ra. dio applications. It should be noted,
hov/ever, that v'jhen a carbon track pot. includes a switch the first
20^ of the movemen+; is concerned '. vit;h the switch function; the
resistance variation is then condensed into the remaining 80?fcrot;at;ion.
Curve 1 shows the stiraightforword linear taper, This type of
resistance lq;'j is commonly used on voltage control po'fcentiometers, on
t. one controls anc.. other applications requiring straight division of
input voltageso
Curve 2 shows log la. v where the rosistanoe increases only grad-
ually during the early movement of the shaft, squeezing most of the
resistance var-" at ion into the final 2o% of the'-rotation, 'fihis type
of taper is roughly in accordance with the natural sensation of
ioudness(our ears follow a log la^ i-a their sensl't?'ivit;yt;o sound),
;ind such components are necessary for the conven-blonal type of vol-
ume control in order to provide an apparent linear increase in sound
output; as the shaft is totated; if a linear control is used in this
position almost all -bhe entire noticeable change in volume takes place
in the first few degrees of rotation.
Anti-log lai.tf tracks 'are the opposite of the log taper, providing
a big change in resistance in the first; half of the clockwise rotation.
These tracks are usefull in such applications as cathode gain controls
and for some biasing networks,
lie re the resistance la\'; is unkno';i?n it can be fairly readily found.
with the aid of an ohmraeter. First measure the full resistance value;
then the resistance vi/ith the shaft; turned half rfay. If the second
value is halfthe total, it is certainly linear; if only 10-2o^ of the
tota^-, then it is a log lauif component; and if about 80^ of the total,
then it is anti-log., Semi-log and linear tapered tracks have values
roughly midv/ay betweei. linear and log; to decide exactly which of these
two it will be necessary to d, ra\v a rough graph. However , neither of
these tapers are very comnon, although semi-log is used in some tone
control circuits.
NOTE... logqrithruic taper (sometimes called "?-iudio taper"^. and
antilogarithaic ("reverse taper")
Graph shovirlng the above curves is on page 2.
oooOOOooo
PAGE 4
BULLETIN O? TASkriNlA'N" DIVISION/ '7. I. A. JUNE 1969
TECHNICAL riRTIGLE. ........... Barr'y VK7ZBJ. ••
A practical '."/ay to obtain . f. m. with transmitters that use crystal
oscillators- is to employ the method shouirn in the diag. The junction
capacitance of GR1 is varied :by the incoming audio voltage. AS the
capacitance of GR1 varies, the oscillator frequency changes because
the crystal is"pulled" by the action of the' Varaotor diode. Only a
fevif volts of audio are needed to provide the necessary frquency swing.
A si'iiple faransistorized audio aKplifier of t>'/o or three stages is
usually sufficient; for this purpose. The ymmount of frequency swing
is controlled by the setting of the audio gain oontrol(devlation
control). This type of circuit is useful v/ith transmit-bers opera'fc-
irig at 50i-/iHz and above. The oscillator is followed by additional
frequenoy-multiplie. r stages, thus assuring ample frequency deviation
to provide a suitable f. m. signal. •„' '
This technique, though shown here with a Glass-A oscillator-
tripler, can be applied to other oscillator circuits too. It can be
used. with overtone , crystal oscillatox's as v/e 11, provided the order
of frquenoy multlpll. cat. lon in the transmlt. ter is high enough to
give ample frequency s. ';/ing at the carrier frequency*
A'.. Cilp I !\i>'>i. ;7
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1/} \/
Rl Is an R:,f isolating redistor. JR1 is a &' ':<v
small Varactor diode. In some acts. a ! c'^
high frequency small signal silicon diode
(or a VH^ silicon transistjr)can be used
for tne ScUue purpose.
NOTE. The supply rail is regulytu'd. by zenor diod OR2,