Page
274
r-~**.*-~***:*&
CHECKING TRANSISTORS AND DIODES
SILICON BIPOLAR TRANSISTOR CHECKING
Method
1
To check a transistor accurately, you should use a transistor
checker.
Howwer, if one is not available, you can use an
ohmmeter to determine the general condition of any of the
bipolar transistors in this kit. The ohmmeter used must have
at least 1 volt
DCg,at
probe tip to exceedthe
tieshhsf
the diode junctions in the transistor being tested.
~.
~~
To check a transistor with an ohmmeter, proceed as follows.
NOTE: Identify the transistor leads on the "Pinout
Diagrams" on
Page
276.
1.
Unsoldw and remove the transistor from the circuit.
2.
Set
the ohmmeter to the RXIMX) range.
3.
Connect one of the ohmmeter test leads to the base
(B) of the transistor. Touch the other meter lead to
the emitter
(El 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.
4.
Repeat step
3
with the test leads reversed.
NOTE: In the unusual
case
when the readings are all low or
all high, no matter which ohmmeter lead is connected to the
be, the transistor should be replaced.
MOSFET AND JFET CHECKING
Insulated gate
type
MOSFET's are used at Q501 on the
receiverllf audio circuit board, and at 0701 and Q704 on
the receiver front end circuit board.
JFET's are used at
Q1202 and 01203 on the VFOIbuffer circuit boards.
Usually, any defect in these devices is found to be an
internal short circuit between the source and the gate of the
JFET (between the source and one of the gates of the
MOSFET). These devices can be checked for Serviceability
by one of the two following methods.
1.
Remove the device from the circuit.
2.
Set your ohmmeter to the
R
X 10
K
range.
3.
Connect the common meter lead to the gate and the
hot lead to the source.
4.
If the device is good, a reading near infinity will
be
obtained. If you do not
get
this reading, reverse the
meter leads.
5.
If you do not get a reading near infinity with the
meter leads connected either way, the device should
be
replaced.
Method
2
1.
Remove the davice from the circuit.
2.
Set your ohmmeter to the R
X
10
K
range.
3.
Connect the gate to the source.
4.
Connect the common meter lead to the gate and
source and connect the hot lead to the drain.
5.
The meter reading should be in the lower part of the
meter scale.
6.
If the reading is not in the lower part of the meter
scale, the device should be replaced.
DIODE CHECKING
To check a diode, unsolder one end from the circuit board,
pull the lead up out of the circuit board hole, and proceed as
follows:
1.
Set the ohmmeter on
the
R
X
1000 range.
2.
Connect one of the ohmmeter test leads to the
cathode (banded) end of the diode. Connect the
other
test lead to the other end of the diode. Note the
reading. Then reverse the meter leads and take another
reading. One reading should be high and the other low
(at least
10:l). If both the readings are either low or
high, the diode should be replaced.
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