FIELD SERVICE BULLETIN
7/10/81
WJ-8770-1
HF
Transportable Receiver
A discrepancy was found regarding
the
audio response measurement while
testing
the
production of
the
WJ-8770-1. A
different
set
of
data
sometimes
resulted when
the
BFO
had a positive or negative
offset
with
the
negative
offset
typically yielding an out of
tolerance
result
for
the
upper 3
dB
limit.
An investigation of
the
product
detector
linearity,
BFO
level and variation, audio
roll-off
etc.
was conducted. Some product
detectors
were found
to
be oscillating
at
approximately 100 MHz. Proper product
detector
gain/frequency response was
restored
with
the
spurious oscillation suppressed giving
symmetrical
data
well within specification.
The oscillation was suppressed
by
lowering
the
terminating
resistor
(A4A1A10
R52) on
the
product
detector's
carrier
input from 1
kn
to
lOOn. The source
resister
(A5A4
R19) on the BFO module was reduced from 510n
to
270S,
to
restore
the
proper drive level.
Approximately
25
receivers
were shipped prior to
the
incorporation of
these
changes, and will possibly have slightly reduced audio performance
in
the
CW
mode. Since
most
CW
operators
work in
the
400-2000
Hz
range,
there
should be
no
problem. The audio
response
in
other
reception
modes is not
affected
by
these
changes.
If
there
is a problem
relating
to this
in
the
field, and units
are
to be modified,
there
is
no
re-alignment
or adjustments involved with
the
resistor changes.
The
customer
may modify
the
units if he determines
the
above problem exists or
return
the units to the
factory
for repair. If
there
are
questions regarding this bulletin,
contact
your local Watkins-Johnson Company
Representative
or
the
factory
directly.
