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Silicon
Bipolar Transistor
Checking (Q11)
To
check
this
transistor
accurately,
you
should
use a
transistor
checker.
However,
if
one
is
not
available,
you
can use an
ohmmeter
to
determine the
general
condition of
this transistor.
The ohmmeter
must have
at
least
1
volt
DC
at
the
probe
tips
to
exceed the threshold of the diode
junctions
in
the
transistor
being
tested.
Most vacuum
tube and
digital
voltmeters
meet
this
requirement.
To
check
a
transistor
with
an ohmmeter,
proceed
as
follows:
1
.
Remove
the
transistor
from
the
circuit.
2.
Set the
ohmmeter on
the
X1000
range.
3.
Connect one
of the
ohmmeter
test
leads
to
the
base
(B)
of the
transistor.
Touch
the other
meter
lead
to
the emitter
(E)
and then
to
the
collector
(C).
Both
readings
should be the same, but
may
be
either
high
or
low.
If
one
reading
is
high
and
the other
low, the
transistor
should
be
replaced.
(Identify
the
transistor leads
on
the
"Identification
Chart"
on
Page
66
of
the Manual.)
4.
Repeat
step
3
with the
test
leads reversed.
Page
59
NOTE:
In
the unusual case
when
the readings
are
all
low, or
all
high,
no matter
which
ohmmeter
lead
is
connected
to
the
base,
the
transistor
should be
replaced.
MOSFET
Checking (Q21)
An
insulated-gate
type
MOSFET
is
used
at
Q21 on
the detector
circuit
board.
Usually
any
defect
in
this
device
is
found
to
be an
internal
short
between
the
source
(S)
and
one
of the
gates
(G1) or
(G2).
They
can be
checked
in
the
circuit
with
a high
impedance
input voltmeter (10
megohms
or
greater).
An
abnormally
low
source voltage
may
indicate
an
internal
short
circuit.
CAUTION:
If
you
ate
going
to
remove
the
MOSFET
from
the
circuit
board,
first
wrap
a small
wire
around
all
four
leads
to short
them
together.
This
is
required
because
some
soldering irons
have an
AC
voltage
at their tips
of
an amplitude
high
enough
to short
out
the protective diodes
in
the
MOSFET.
Damage
often
occurs
when
an attempt
is
made
to clean the
leads
of
solder
without
the
presence of the
shorting wire.
During normal
installation
of these devices,
previously
installed
parts
provide the necessary protection
-
a
path
to
ground
so
the shorting wire
is
not
necessary.
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Troubleshooting
