Heathkit HD-1250 - Silicon Bipolar Transistor Checking; MOSFET Checking

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.