33
Subject to change without notice
Testing Transistors
Three different tests can be made to transistors: base-emitter,
base-collector and emitter-collector. The resulting test patterns
are shown below. The basic equivalent circuit of a transistor is
a Z-diode between base and emitter and a normal diode with
reverse polarity between base and collector in series
connection. There are three different test patterns:
For a transistor the figures b-e and b-c are important. The
figure e-c can vary; but a vertical line only shows short circuit
condition.
These transistor test patterns are valid in most cases, but
there are exceptions to the rule (e.g. Darlington, FETs). With
the
COMPONENT TESTER
, the distinction between a P-N-P
to an N-P-N transistor is discernible. In case of doubt,
comparison with a known type is helpful. It should be noted
that the same socket connection (
COMP. TESTER
or ground)
for the same terminal is then absolutely necessary. A connection
inversion effects a rotation of the test pattern by 180 degrees
round about the center point of the scope graticule.
In-Circuit Tests
Caution!
During in-circuit tests make sure the circuit is dead. No
power from mains/line or battery and no signal inputs
are permitted. Remove all ground connections including
Safety Earth (pull out power plug from outlet). Remove
all measuring cables including probes between
oscilloscope and circuit under test. Otherwise both
COMPONENT TESTER leads are not isolated against
the circuit under test.
In-circuit tests are possible in many cases. However, they are
not well defined. This is caused by a shunt connection of real
or complex impedances - especially if they are of relatively low
impedance at 50Hz - to the component under test, often
results differ greatly when compared with single components.
In case of doubt, one component terminal may be unsoldered.
Component Tester (analog mode)
Storage Mode
This terminal should then not be connected to the ground
socket avoiding hum distortion of the test pattern.
Another way is a test pattern comparison to an identical circuit
which is known to be operational (likewise without power and
any external connections). Using the test prods, identical test
points in each circuit can be checked, and a defect can be
determined quickly and easily. Possibly the device itself under
test contains a reference circuit (e.g. a second stereo channel,
push-pull amplifier, symmetrical bridge circuit), which is not
defective.
Storage Mode
In contrast to analog mode, the storage mode offers the
following advantages:
One time events can be captured easily. Even very low
frequency signals can be displayed as a complete curve.
Narrow pulses with low repetition rates do not cause intensity
reduction. Documentation and processing of captured signals
is easily possible.
In comparison with analog mode, the disadvantages of
storage mode are:
The reduced X and Y resolution and a lower update rate.
Danger of alias signal display, caused by a sampling rate (time
base setting) which is relatively too low with respect to the
current signal.
The analog mode offers an unsurpassed faithful signal display.
With the combination of analog and digital oscilloscope,
HAMEG
