Wavetek 188 - Troubleshooting; Factory Repair; Troubleshooting Tables; Troubleshooting Individual Components

SECTION
U
TROUBLESHOOTING
6.1 FACTORY
REPAIR
Waveiek maintains
a
factory
reoair
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
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
Was),
the
transistor
is defective
4.
A
transistor
is
defective if its
base current
is
larger
than 10
%
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
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 connoctod
to
the
"
-
"
input
(voltage follower
connection), the
output
should be the
same voitago
as the
"
+
”
input voltage;
otherwise,
the operational
ampli-
fier is
defective
3 If the
output voltage
stays at maximum
positive,
the
"
+
"
input
voltage should
bo moro
positive
than
the
"
-
"
input
voitago. or
vice versa;
other-
wise.
the operational
amplifier
Is defective.
6.3.4
FET Translator
1
No gate current
should
be drawn
by the gate
of
an
FET transistor.
If
so.
the
transistor
is defective
2
The
gate-to-source
voltage
is always
revorse
biased
under
a normal
operating
condition;
e
g
.
the
source voltage
Is more
positive than
the
gate voltage
tor
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.
6-1