PacComm TINY-2 - Page 137

ADJUSTMENTS
Crystal
Oscillator
Adjustment
3.
If
the
proper
equipment
is
not
available
as
described
above,
make
the
deviation
adjustment
by
tuming
the
adjustment
screw
on
the
trimpot
until
no
increase
in
output
level
is
heard
at
the
monitoring
receiver.
Then
rotate
the
adjustment
screw
of
the
trimpot
in
the
opposite
direction
until
the
audio
signal
on
the
monitoring
receiver
is
slightly,
but
noticeably,
reduced
from
the
maximum
level.
high
drive
level
causes
over-deviation,
unnecessary
interference,
and
makes
your
signal
tess
copyable.
Listeners
may
also
notice
some
low-level
packet
tones
when
using
the
radio
for
voice
work.
4,
The
transmitter
deviation
is
now
set
to
approximately
the
correct
level.
Type
the
command
"Q"
to
unkey
the
transmitter.
Modem
Receive
Bias
Adjustment
Trimpot
(R9
100K)
is
the
modem
bias
adjustment.
The
factory
proce-
dure
below
is
used
to
adjust
the
modem
bias.
It
normally
results
in
a
value
of
2.65
to
2.70
vde
on
pin
7
of
the
TCM3105
modem
chip
(U16).
The
setting
does
not
seem
to
drift
over
time
but
may
need
to
be
changed
if
a
different
modem
chip
is
substituted.
If
it
is
necessary
to
adjust
the
modem
bias,
the
following
procedure
should
be
used.
1.
Prepare
to
perform
an
audio
loopback
by
jumpering
pins
1
and
4
of
J2,
the
radio
connector.
2.
Give
the
commands:
CAL
<CR>,
KD
<CR>
to
provide
a
‘dithered’
transmit
signal
for
loopback
demodulation.
When
the
adjustment
is
completed,
type
’Q’
to
discontinue
the
tone.
NOTE:
The
watchdog
timer
will
cause
the
PTT
circuit
to
reset
approximately
one
minute
after
the
’KD’
keys
are
depressed.
If
you
need
a
longer
adjustment
time,
type
’Q’
<CR>
to
return
to
the
cmd:
prompt
and
restart
the
procedure.
3.
Observe
the
signal
output
on
pin
8
of
the
TCM3105
(U16)
with
an
oscilloscope.
It
may
be
necessary
to
increase
the
modem
output
level
by
adjusting
trimpot
R12
to
get
a
strong
enough
signal
for
decoding.
The
signal
should
be
a
square
wave.
Adjust
R26
to
cause
the
positive
and
negative
portions
of
the
square
wave
to
be
of
equal
duration.
4.
Enter
"Q",
remove
the
jumper
from
J2
and
readjust
R12
if
needed.
Carrier Detect Level
Trimpot
R26
should
be
adjusted
to
cause
the
DCD
LED
to
illuminate
when
a
weak
input
signal
to
the
modem
is
present
and
to
go
out
when
no
signal
is
present.
See
the
section
on
audio
input
levels.
Crystal
Oscillator
Adjustment
:
You
may
adjust
the
oscillator
with
the
variable
capacitor
C17,
to
within
aS
KHz
range
(typical)
from
its
4.9152Mhz
nominal
frequency
to
move
a
harmonic
a
few
kilohertz
off
of
a
favorite
operating
frequency.
The
setting
of
this
frequency
is
not
critical.
A
30khz
error
at
the
oscillator
frequency
will
result
in
only
a
.6%
error
at
1200
baud.
16
TINY-2/MICRO-2
Technical
Ref.,
2
Ed
©
1989,
PacComm,
inc.