PRODUCTION CHANGES STARTING
WITH
SERIAL NO. 1101
The
following infonnation
should
be
considered
as
a
supplement
to the
present
installation
and
ser-
vice
manual
of
the FM50A communications unit. Furure printings
of
the FM50A manual will
contain
this in-
formation. FM50A
units
starting
with
serial
number 1101
differ
from
previous
production
as
follows:
1.
The
transmitter
frequency
adjustment,
C183
is
accessible
from
the top
of
the
chassis.
2. The
IF
filter
ZF102
has
been moved
so
as
to
pennit
the
use
of
a
"three
crystal"
oven (for two
frequency
transmitter
application).
3.
Space
has
been provided on the
underside
for a two frequency
relay
and
second
channel
trans-
mitter
frequency adjustment.
4.
Chassis
removal from the
cabinet
has
been
simplified
by
the
elimination
of
the grommet, cup
washer
and
hold-down
screw.
The
screw
and
clip
fastening
the
rim
of the front
panel
to the
cabinet
have
also
been
eliminated.
5.
The
elimination
of
the items
called
for
in
step
4
was
made
possible
by the addition
of
a brace
from
the power transformer to the front panel
and
by
re-Iocation
of
the two
rear
lower
"chassis-to-cabinet"
mounting
screws.
*6. R175
in
the
cathode
circuit
of
V114A was
changed
from
3.3K
to
1K
(Y2
W.)
to
increase
microphone
sensi
tivity.
7. A
50
MFD,
25V
electrolytic
capacitor
was
added
across
terminals
11
and 13
of
the power
connec-
tor
J101
(positive
end
of
condenser
to tenninal 13) to provide
additional
transistor
protection
from
primary
power
sources
with poor
regulation
(DC
operation
only).
8.
R179, .47 ohms which
serves
as
a fuse
as
well
as
part
of
the
transistor
bias
circuit,
has
been re-
located
to the
outside
of
the component board to allow
easy
replacement
if
required.
*9. In
order
to
prevent
RF
feedback,
a 250
mmfd
mica
capacitor
(Underwood toothpick type)
was
ad-
ded to
the
speech
amplifier V114A pin 7 to ground,
part
number 23928-1.
The
following
notes
also
pertain
to
sets
below
serial
number 1101:
*10. In
early
production C158
was
not incorporated. C158 when
incorporated
was
orginally
a 7-45
MMF
wired
across
RI50
and
later
changed
from
terminal 3
of
Vll0
to ground. In
later
production (below
serial
#1101) C158
was
changed
to
a 5-25
MMF
(Hammarlund
part
#23038-3) to improve
neutralization
at
the
high
end
of
the
band.
11. In
early
production C189
consisted
of a
single
470
MMF
capacitor.
In
later
production an addi-
tional 470
MMF
capacitor
(C215)
was
added
in
parallel
to
improve
bass
response
of
the transmitter. Start-
ing
with
serial
#1101
these
capacitors
were
changed
to
a
single
.001 MFD.
*12. In
later
production
of
the
units
below
serial
number 1101, R194
(lK,
Y2
watt)
and C214 (4MFD
250V)
were
added
to the
standby
bias
circuit
of
the
transmitter
final
stage
to
eliminate
a weak
self-oscil-
lation
condition
during
standby.
This
change
also
eliminates
the
standby
plate
current
of
the final
stage.
*THESE CHANGES ARE RECOMMENDED FOR OLD SETS BROUGHT
IN
FOR PREVENTA TIVE
MAINTENANCE.
FREE DOWNLOAD
COURTESY OF N9SOR
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