Subject to change without notice
36
on the test voltage, their zener breakdown forms a second
knee in the opposite direction. If the breakdown voltage is
higher than the positive or negative voltage peak of the test
voltage, it can not be displayed.
The polarity of an unknown diode can be identified by
comparison with a known diode.
Testing Transistors
Three different tests can be made to transistors: base-emitter,
base-collector and emitter-collector. The resulting test pat-
terns 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 out-
let). Remove all measuring cables including probes
between oscilloscope and circuit under test. Other-
wise both COMPONENT TESTER leads are not isola-
ted 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.
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.
Storage mode
