Component Test; In-Circuit Tests - Hameg HM400 Handbuch

C o m p o n e n t t e s t

Component test

The oscilloscope HM400 has a built-in component tester which is
activated by pressing the mode button COMP
is connected to the two contacts right and left below the screen. After
pressing the COMP button, the Y preamplifi ers and the time base will
be disconnected. While using the component tester, signals may be
present at the inputs as long as the unit under test is not connected to
any other circuit. It is possible to test components remaining in their
circuits, but in such cases all signals must be disconnected from the
three front panel BNC connectors! (See the following paragraph: „Test
in circuits"). With the exception of the SELECT button
2 3
buttons, and the X-POSITION
button all other controls will be disabled. Two cables with 4 mm plugs
are necessary to connect the unit under test to the component tester.
After completion of the component test pressing the COMP button again
is all that is needed to resume normal scope operation.
As outlined in the chapter Safety, all measurement con-
nectors are connected to the mains safety earth (in proper
operation). This implies also the COMP.TESTER contacts.
As long as individual components are tested, this is of no
STOP consequence because these components are not con-
nected to the mains safety earth.
If components are to be tested which are located in
circuits or instruments, these circuits resp. instruments
must be disconnected fi rst under all circumstances! If
they are operated from the mains, the mains plug of the
STOP test object has to be pulled out. This ensures that there
will be no loops between the scope and the test object via
the safety earth which might cause false results.
Only discharged capacitors may be tested!
The test principle is quite simple. A generator within the HM400 ge-
nerates a 50 Hz ±10 % sine wave which feeds the series connection
of the test object and a sense resistor. The sine wave voltage is used
for the X defl ection and the voltage drop across the resistor for the
Y defl ection.
If the test object has only a real part such as a resistor, both defl ection
voltages will be in phase; the display will be a straight line, more or less
slanted. Is the test object short-circuited, the line will be vertical (no vol-
tage, current maximum). If the test object is open-circuited or missing
a horizontal line will appear (voltage, but no current). The angle of the
line with the horizontal is a measure of the resistance value, allowing
for measurements of resistors between 20 Ω and 4.7 K.
Capacitors and inductors cause phase shift between voltage and current
and hence between the defl ection voltages. This will cause displays of
ellipses. The location and the form factor of the ellipse are determined
by the apparent impedance at 50 Hz. Capacitors can be measured
between 0.1 and 1000 μF.
– An ellipse with its longer axis horizontal indicates a high impedance
(small capacitance or large inductance)
– An ellipse with its longer axis vertical indicates a low impedance
(large capacitance or small inductance)
– An ellipse with its longer axis slanted indicates a relatively large
resistive loss in series with the impedance of the capacitor or
inductor.
With semiconductors the transition from the non-conducting to the
conducting state will be indicated in their characteristic. As far as is pos-
sible with the available voltages and currents the forward and backward
characteristics are displayed (e.g. with zener diodes up to 9 V). Because
this is a two-pole measurement, the gain of a transistor can not be de-
38 Subject to change without notice
46
. The unit under test
7
, the ADJUST
17 18
knob, and the X-MAG/x 10
termined, however, the B-C, B-E, C-E diodes can be measured. Please
note that most bipolar transistors can only take an E-B voltage of appr.
5 V and may be damaged if this is exceeded, sensitive HF transistors
take even much less! With this exception the diodes can be measured
without fear of destruction as the maximum voltage is limited to 9 V and
the current to a few mA. This implies, however, that a measurement
>
of breakdown voltages 9 V is not possible. In general this is no dis-
advantage because, if there is a defect in a circuit, gross deviations are
to be expected which will point to the defective component.
Rather exact results may be achieved if the measurements are com-
pared to those of intact components. This is especially true for semi-
conductors. The polarity of diodes or transistors can thus be identifi ed
if the lettering or marking is missing.
Please note that with semiconductors changing the polarity (e.g. by
exchanging the COMP.TESTER and ground terminals) will cause the
display to rotate 180 degrees around the screen center. More important
in practice is the quick determination of plain shorts and opens which
are the most common causes of requiring service.
It is highly recommended to observe the necessary pre-
cautions when handling MOS components which can be
destroyed by static charges and even tribo electricity. The
display may show hum if the base or gate connection of a
STOP
transistor is open, i.e. it is not being tested. This can be
verifi ed by moving a hand closeby.

In-circuit tests

They are possible in many cases but deliver rarely clear results. By par-
alleling of real or complex impedances – especially if those are fairly low
impedance at 50 Hz – there will be mostly great differences compared
to individual components. If circuits of the same type have to be tested
often (service), comparisons with intact circuits may help again. This is
also quickly done because the intact circuit has not to be functional, also
it should not be energized. Just probe the various test points with the
cables of the component tester of the unit under test and the intact unit
and compare the screen displays. Sometimes the unit under test may
already contain an intact portion of the same type, this ist e.g. the case
with stereo circuits, push-pull circuits or symmetrical bridge circuits.
In cases of doubt one side of the dubious component can be unsoldered,
and this free contact should then be connected to the COMP.TESTER
contact which is not identifi ed as the ground contact. This will reduce
hum pick-up. The contact with the ground symbol is connected to the
scope chassis and is thus not susceptible to hum pick-up.