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F ig. 2 .
GAG7 Amplifier.
The function switch allows complete station
control with o ne knob. In standby
position the cathode of the crystal oscillator
is lifted from the gro und. In the take-over,
spot, posit ion the exciter is switched o n by
SW2-A while SW2-B shorts out the loop
across the T U o u tput. This prevents the
receiver from keying the transmitter (if
printer and keyboard are in series), and also
allows a q uick return of the printer to lower
case by flipping the take over switch and
punc hing th e letters key. In the third position
the " C" section of SW2 is used to
contro l a transmit relay. In position No. 4
the short across the co nverter output is
removed and the transmitter may be keyed
by an incoming signal.
As with any hetrodyne circuit, care must
be exercised not to tune u p o n a harmonic
o r wrong beat. The unit should be set up
in itially with a grid dip meter or absorption
wave meter. The crystal used must be chosen
so that the fourth or fifth harmonic of the
pto docs not fa ll o n o r ncar the wanted
frequency.
By doubling in the transmitter any unwa
nted is fu rther rem oved from the tuned
o utput of the transmitter.
200 ~ ~ !
(--~--1r- TO ~TA l STAGE: (;1'110
~
F ig. 3. Coupling coil to transm itter .
The average RTTY enthusist will d evote
long hours and careful planning to come u p
with the best possible TU. Then in a rush to
get the rig o n the air will. all too often, slap
a diode shifter o n any ex isting VFO - with
less tha n the desi red resul t.
Or. should he d ecid e to sh ift a cr ystal he
may spend hou rs grindi ng the rock to a nct
86
F ig. 2.
GAG7 A mpl ifier.
frequency only to find his sh ift sh y o f the
8 50 liz. Not to mcntion thc fa ct that the net
frequency may change about the ti me he
fin ally gets within to lerance.
In fact it was grinding my fifth crystal for
Air Fo rce Mars net opera tion in a little over
one year that made me decide to do so mething,
even if it were wrong.
Some Transmitters may require mo rc
d rive than that obtainable from the balanced
modulator. An amplifier st age becomes
necessary _ A 6 AG7 is a logica l choice. The
grid o f this tube is capacitively coupled to
o ne side o f coil L-l through a short length of
RG-58. Two o r three turns should be removed
fro m this side of L- I to maintain
balance. (If individual condense rs arc used
for C- l, this may be maintained by tu ning.)
The layout of the 6AG7 stage must be
made wit h care. Some physical separatio n is
desirable between the coil L-I , and the
6AG7 tube socket. The output co il, L-2,
must be placed above chassis, (If L- I is
below) with the plate lead going directly
topside from the tube pin. The 27 ,000 ohm
resistor insures complete stability.
Lo w im pedance output, either fro 111 the
balanced modulator, or the 6AG7 stage, if
necessary . a llows the exciter to be lo cated a
convenient distanccfro rn the transmitter.
A tu ned link co u pled inpu t co il shou ld be
used at the crystal stage of the transmitte r.
Of the th ree FSK kcyer units I have buill,
all have p roved a pleasu re to usc. It has been
found unnecessary to let the pto ru n all the
time, as it will be well within to leran ce fro m
a co ld sta rt. And a five minute warm up will
put you righ t on the butto n. The shifte rs
have been used days at a time without
touching the dial o n Air Force Mars circuits.
So if you are having problems with drift.
setting up and maintaining proper sh ift, o r if
you are simply tired of grinding rocks. the
0 · 17·A RT 13 pto sh ifter may be your
answer.
... W4 LLR