A Modulated Oscillator - Eddystone Short Wave Manual

EDDYSTONE SHORT WAVE MANUAL
33
A MODULATED OSCILLATOR
4.5 METRES TO 8 METRES.
General lay out showing disposition of
Components.
This oscillator will be of great value to the
ultra short wave experimenter in testing and
lining up ultra
short wave receivers.
It
radiates
a continuous note
at
a constant
level over a variable frequency band of 67 to
37.5 Mcfs from 15° to 100° on the dial.
The oscillator valve functions in the usual
way, an electron -coupled circuit being used.
A time constant arrangement, consisting of a
suitable value grid leak and condenser
is,
however, incorporated which breaks the oscil-
lations up into groups of about 300 per second,
thus giving an audible output. The high
tension supply necessary
is
60 volts, and
it should be noted that a variation of high
tension
will
slightly
alter the pitch of the
audible note. It is also interesting to note that
in this case a triode valve has been successfully
used as electron -coupled oscillator, although
it
is general practice to use a screened grid
valve for this purpose. The present triode
arrangement works extremely well and there
is a considerable saving in cost between the
price of the two valves.
The frequency range, which gives a com-
plete overlap each side of the 5 -metre amateur
band and the television wavebands, is covered
by a 40 m.mfd. condenser tuning a 5 -turn
silver plated coil. A tapping from one side
of the filament is taken to the centre of the coil.
The high frequency choke used in the filament
lead must be of low resistance so that no
appreciable drop of filament voltage occurs.
The Cat. No. 1021 H.F.C. is specially made for
this purpose.
The unit is mounted on a small panel and
baseboard, an illustration of the lay out being
given, and the assembly is one which can be
completed without difficulty in a very short
time.
EDDYSTONE PARTS.
Price
1 4 -pin Frequentite Valveholder, No. 949
..
1/5
1 Ultra S.W. Choke, No. 1021 1/3
2 2 -way Frequentite Terminal Saddles, No.
1046, 1/- each ..
'2/0
1
111' Insulating Pillar, No. 1029
..
41d.
1 Microdenser. 40 m.mfd., No. 900
..
4/3
1 5 -turn Coil and Base, No. 1050/1
..
2/7
1 Pointer Knob and Dial, No. 1027 ..
1/3
MISCELLANEOUS PARTS.
2 Fixed Condensers, .006 mfd., Dubilier or
T.C.C., 1/6 each
..
1
watt Resistor, 70,000 ohms, Mullard,
Erie or Dubilier
1 Baseboard, 6" x 4", r Plywood
.
.
1 Panel, 5" x 4" x f", Wood or Bakelite
Connecting Wire, Screws, etc.
1 Osram Valve, type HL2
Co+No.lo21
CIRCUIT DIAGRAM.
HT.+