T EVIS i - AmericanRadioHistory.Com

Fig. 2. All controls but the phone -c.w. switch are located on
the front panel. Note that several extra self -tapping screws
are used to fasten the panel, improving rigidity for the v.f.o.
Fig. 3. Inside the 6"'x9 -x5" box. The Bakelite strip mounted
on the choke provides a point for fastening choke to rear
panel, further bracing the chassis for assured v.f.o. stability.
6AQ5, and other familiar audio tubes
do not perform nearly as well as the
6L6 in this type of service. The size
of even a 6L6GA, however, is too large
to be practical in a compact transmitter.
Luckily, the little 6BQ6GT
is a miniature modulator in disguise.
While primarily designed for use as a
horizontal output tube in TV service,
the 6BQ6GT will do the work of a
6L6 modulator without taking up
nearly so much precious space.
Any single- button carbon mike with
a push -to-talk switch will work in
conjunction with the rig, nevertheless.
a chest mike containing a high
output Western Electric F -1 cartridge
is recommended. Driving with one
hand while holding a mike in the other
is a dangerous practice which can get
you into plenty of hot water if you
become involved in an accident. Wisdom
dictates the use of a mike that
permits you to keep both hands on
the wheel.
Although the author is no lover of
International Morse, provision for c.w.
operation is included in the rig for
use at those times when phone just
won't quite make the grade. There
is little reason to waste heater and
plate power in a modulator stage
while pounding brass and so S. the
"Phone/C.W." switch, opens the 6BQ-
6GT heater circuit at the same time it
shorts the modulation choke, CH,.
Since & is seldom used, it is relegated
to the rear panel.
Whenever possible, a crystal should
be employed during c.w. operation,
otherwise a slight chirp may be evident
on the signal. A class A isolating
stage between the 6BH6 and the 12-
BY7 would probably clear up this
trouble, but the added circuit complexity
seems hardly worthwhile
when one considers how seldom c.w.
is employed in mobile work.
The number of tuning controls is
held to a minimum by employing
broadband slug -tuned coils in the intermediate
stages. The plastic coil
forms visible in Fig. 7 were purchased
at a bargain sale and so exact duplicates
may be rather difficult to obtain.
However, ceramic forms of approxi-
44
mately the same dimensions and with
powdered iron slugs are listed in most
radio catalogues. The resistors across
some of the coils are needed to hold
the final drive down to a reasonable
level. On the lower frequency bands,
condensers are also placed across the
coils in order to increase the circuit
"Q." thereby reducing the amount of
reflected oscillator frequency shift
caused by tuning the final. Without
these condensers there just isn't sufficient
isolation between the oscillator
and the final.
At the time the transmitter was
constructed, the only suitable band -
switch that could be located was the
surplus unit shown in Fig. 7. Recently,
however, a new line of miniature
ceramic switches appeared on the
market. Centralab:s new switch model
#PA 2025 is recommended for use as
S -. Its small size will provide more
room under the chassis, thus simplifying
the wiring process.
No claims are made that this little
transmitter is completely TVI -proof.
Nevertheless, with the final operating
on 10 meters, a sensitive field strength
meter reveals that at a distance of 50
feet from the car, the second harmonic
is appreciably weaker than the signal
provided by a Channel 2 TV station
some 15 miles away.
The TVI precautions observed in the
construction of the unit include complete
shielding and the bypassing of
every lead coming out of the case. In
addition, a low -pass filter is installed
in the coax antenna feeder.
Without shielding, bypassing, and a
low pass filter, a 20- watter can ruin
reception for a block or more, even
where TV signals are strong. In a
fringe area, a poorly constructed
mobile is capable of blanking out a
whole community. Thus, it is obvious
that every rig must now be designed
with a view toward the reduction of
TVI.
The transmitter is housed in a 6" x
9" x 5" hammertone gray aluminum
box. As suggested in many articles
on TVI reduction, paint was removed
from all points of contact between the
front and rear panels and the case.
This was a rather messy, time consuming
job, and in this instance didn't
measurably cut down on harmonic
radiation. Unless you enjoy scraping
paint, you might as well forget this
one phase of construction.
Since quite a bit of heat is generated
within the case of the transmitter,
some method of ventilation
must be provided. A number of r4"
holes drilled in the rear panel will allow
a portion of this heat to escape
while still maintaining adequate TVI
shielding. It will also pay to put felt
feet on the bottom of the cabinet.
Then you won't have to worry about
scratching up the furniture while
operating away from the car at a
portable location.
The panel is illuminated by means
of a shielded pilot light assembly. For
some reason, this style of pilot socket
is available from only a few jobbers.
If you have difficulty finding one, ask
for E. F. Johnson Co.'s part no. 147-
330.
Mobile v.f.o.'s are often plagued
with carrier flutter resulting from
mechanical instability. The mechanical
stability of the rig being described
is comparable to that of similar commercial
units now being marketed.
All factors considered, the frequency
shift caused by mechanical shock is
relatively insignificant in this unit.
Oscillator microphonics will be held
to a minimum if the following points
are kept in mind: Use heavy aluminum
for the chassis. Employ several
extra screws to fasten the front and
rear panels to the case. Use a dual -
bearing oscillator tuning condenser.
Keep floppy wiring away from the
oscillator coils. Provide some means
of fastening the top of the modulation
choke to the rear panel. Support
the rear end of the bandchanging
switch, S:, with a small bracket running
to the chassis.
Frequency drift resulting from
changes in temperature can also
prove annoying. Be sure to use silver
micas or zero temperature coefficient
ceramics for C,, C, and C.. Make certain
that the two oscillator coils are
wound on ceramic forms. If you have
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