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6
SECTION
TROUBLESHOOTING
6.1
FACTORY
REPAIR
Wavetek
maintains a
factory repair
department
for
those
customers
not
possessing
the
necessary
per-
sonnel
or
test
equipment
to
maintain the
instrument.
If
an
instrument
is
returned
to
the factory
for
alignment
or
repair,
a
detailed description of the specific prob-
lem
should
be
attached
to
minimize turnaround
time.
6.2
TROUBLESHOOTING TABLES
Table
6-1
gives
an
index
of
the troubleshooting tables
by
indications
of
common
problems.
The
tables
do
not
cover every
possible
trouble, but,
when
used
in
con-
junction with
circuit
descriptions
and
schematics,
will
be an
aid
in
systematically
isolating faulty
components.
6.3.2
Diode
A
diode (except
a
zener)
is
defective
if
there
is
greater
than
one
volt (typically
0.7
volt)
forward voltage
across
it.
6.3.3
Operational Amplifier
1.
The
"
+
"
and
"-"
inputs
of
an
operational
amplifier
will
have
less
than
15
mV
voltage
dif-
ference
when
operating
under normal
conditions.
2.
When
the output
of
the
amplifier
is
connected
to
the
"
-
"
input (voltage follower
connection), the
output should
be
the
same
voltage as the
"
+
"
input voltage;
otherwise, the operational ampli-
fier
is
defective.
6.3
TROUBLESHOOTING
INDIVIDUAL
COMPONENTS
6.3.1
Transistor
1
A
transistor
is
defective
if
more
than
one
volt
is
measured
across
its
base-emitter
junction
in
the
forward
direction.
2.
A
transistor
when
used as
a switch
may
have a
few
volts
reverse bias voltage
across base-
emitter
junction.
3.
If
the
collector
and
emitter voltages are the
same,
but the
base
emitter voltage
is
less
than
500
mV
forward voltage
(or
reversed
bias),
the
transistor
is
defective.
4.
A
transistor
is
defective
if
its
base
current
is
larger
than
1
0%
of
its
emitter current (calculate
currents
from
voltage
across
the
base and
emit-
ter
series
resistors).
5.
In
a
transistor
differential
pair
(common
emitter
stages), either
their
base
voltages are the
same
in
normal
operating
condition, or the
one
with
less
forward voltage across
its
base
emitter
junction
should
be
off
(no
collector current);
otherwise,
one
of
the
transistors
is
defective.
3.
If
the output voltage stays
at
maximum
positive,
the
"
+
" input
voltage should
be
more
positive
than
the
"
-
" input voltage, or
vice versa; other-
wise, the operational
amplifier
is
defective.
6.3.4
FET
Transistor
1
No
gate current should
be drawn by
the
gate
of
an
FET
transistor.
If
so,
the
transistor isdefective.
2.
The
gate-to-source voltage
is
always
reverse
biased under a normal
operating
condition;
e.g.,
the
source
voltage
is
more
positive
than the
gate voltage
for
2N5485, and
the
source
voltage
is
more
negative than gate voltage
for
a
2N5462.
Otherwise, the
FET
is
defective.
3.
If
the
device supplying gate voltage
to
an
FET
saturates, the
FET
has
too large
a
Vgs
(pinch
off)
for
the
circuit
and
should
be
replaced.
6.3.5
Capacitor
1
Shorted
capacitors
have
zero
volts
across
their
terminals.
2.
Opened
capacitor
can be
located
(but
not
always)
by
using
a good
capacitor
connected
in
parallel
with the capacitor
under
test
and
observing the
resulting
effect.
6-1
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