T EVIS i - AmericanRadioHistory.Com

further condenser failures have occurred.
The addition of C, (Fig. 4)
also eliminated a slight genemotor ripple
which was previously apparent on
the carrier.
For the sake of clarity in the circuit
diagram (Fig. 5), several dual condensers
were given widely separated
numbers for their two sections. Obviously
if sections in separate functions
like C, -C.; were called "C, -C:.."
the reader's eye would have to travel
all over the diagram to follow the
parts list. Naturally. when the transmitter
is assembled it will be found
that the connections of the two sections
of any dual unit are adjacent,
or nearly so. Dual units were used
to save badly needed space, although
single units will probably fit fairly
easily in some places.
The antenna changeover relay, RL,,
Fig. 5. is a surplus item. Originally
designed for 12 -volt d.c. operation, it
was rewound for 6 volts. Those contacts
not required for antenna switching
are wired to the octal power plug
P. on the rear of the chassis and
can he used to disable the receiver
whenever the transmitter goes on the
air.
Crystal control may be employed
by plugging a crystal into the socket
on the front panel and then tuning
the v.f.o. dial slightly higher than
the crystal frequency. The v.f.o.- crystal
switch, S., should be thrown to crystal
and the final then tuned in the normal
manner. Since the setting of the v.f.o.
dial has some effect on the crystal's
frequency, the carrier can be "rub -
bered" a kilocycle or so when dodg-
ing QRM. On 20, 15, 11, and 10 meters,
40 -meter crystals should be used,
and for 80- and 40 -meter operation,
160 -meter crystals are required.
Since the crystal socket is wired in
the hot portion of the oscillator grid
circuit, whenever a crystal is in the
socket, the v.f.o. frequency will be
shifted slightly due to stray capacity
effects. As a consequence, during
v.f.o. operation it pays to remove the
crystal from the socket.
This nuisance will he minimized if
you can locate a BC -746A tuning unit.
The socket from a BC -746A is designed
to accommodate two crystals
and can be wired so that only two of
its four contacts are live. Then. when
using the v.f.o., a crystal may be
placed in the dead section of the
socket for storage.
Both the final tuning condenser and
the loading condenser have a rating
of 140 µµtd. They, too, are salvaged
from BC -746A tuning units. Since the
plate voltage is not very high, no
arcing difficulties should be experienced,
despite the small plate spacing
of these condensers.
After trying several different types
of final coil switching, a method employed
by W2AEF' was finally put to
use. The coils are mounted at right
angles to one another, and therefore
little loss is introduced into the circuit
as the coils are shorted when
switching from one band to another.
The 20- mete[- coil, L.., is tapped for
15 -meter operation and the 80 -meter
coil, L., is tapped for 40 meters. No
difficulty should be experienced in soldering
a tap to L.,, but working with
L., may not be so easy because of the
small spacing between turns. After
choosing the proper location for the
40 -meter tap, take a thin jackknife
blade and push the adjacent turns
aside far enough to allow room for
easy soldering. Once the tap is in
place, use the knife blade to readjust
the position of any turns which
may have been deformed by the pre-
vious operation. Make certain that
there are no shorts between turns and
then apply two or three coats of polystyrene
coil dope to the area sur-
rounding the tap. This will keep
shorts from developing while the
tapped coil is being wired in place.
Although the 10- and 20 -meter final
coils are self- supporting, the wire in
the 80 -meter one is not strong enough
for this purpose. Therefore, two
pieces of polystrene must be cemented
between the latter coil and the terminal
strips to which it is soldered in
order to provide the required amount
of support.
When wiring the final, be sure to
keep all plate leads well removed
from the grid circuit. Use the rear
deck of the bandswitch for plate coil
switching and be sure that the final
grid circuit switching takes place at
least two decks away. About the only
other wiring precaution that need be
observed is to make all bypass condenser
leads as short as possible. If
these steps are taken, no parasitic
or fundamental oscillation problems
should arise.
The transmitter can be aligned
without difficulty, providing you have
the proper test equipment at hand. An
accurately calibrated receiver, a 100 -
kc. frequency standard, a grid dip oscillator
covering all the bands from
160 through 10 meters, and a 0 -5 ma.
meter are "musts." Without these
items you'll find yourself floundering
helplessly, but if you have them, the
job can be performed with a minimum
of effort and wasted time.
First, switch to the 10 -meter band
and adjust C.:. until the oscillator just
hits 7 mc. with the plates of C,. fully
meshed. The grid dip oscillator should
be coupled to L and the slug of this
coil adjusted for resonance at 14.5 mc.
L; should then be peaked at 29 mc.
Tune C., until the v.f.o.'s fourth harmonic
falls on 29.1 mc. Peak L. and
L; for maximum final grid current.
Tune C,. to 28.9 mc. and repeak L:,
then tune to 29.3 mc. and repeak L.
Grid current will then hold fairly constant
from 28.6 mc. to 29.6 mc. It
should be at least 21_ ma. over most
of this range and shouldn't drop below
2 ma. at the extremes. If this is
not the case, readjust L: and L: until
optimum operation is achieved.
Throw S: to 11 meters and tune the
v.f.o. to 6775 kc. Either compress or
expand the turns of L, and L, until
maximum final grid current is obtained.
15 meters is next. Adjust L; to
approximately 7100 kc. by means of
the dipper. Tune the v.f.o. to the same
frequency and either squeeze or expand
the turns of L.. for maximum
deflection of the grid current meter.
On 20, tune the v.f.o. to 7100 kc.
and peak L... at 14,200 kc.
(Continued on pure 104)
Fig. 6. Top view of the transmitter chassis. Parts placement
is compact but not crowded. Note the right -angle mounting of
the output coils, and Bakelite strips on the modulator choke.
Fig. 7. Bottom view of the chassis. Slug tuned coils are so
arranged that adjacent units do not tune to same frequency.
thereby minimizing interaction and aiding unit's stability.
46 RADIO & TELEVISION NEWS