Änderungen vorbehalten / Subject to change without notice
8
Component testing
Out-of-circuit testing of individual components is
unproblematic. Qualitative results are obtained by
comparing with known good components
of the
same type and rating. This also applies to
semiconductors. For example, it is possible to quickly
determine which lead is the cathode on a diode or
zener diode with illegibly printed designations, to
identify NPN or PNP transistors, or to obtain emitter-
base-collector identification on unknown transistor
types. Even more important is the ability to quickly
obtain a good-bad evaluation of components in the
case of interruptions or shorts, which experience has
shown is what servicing jobs most often require.
It is urgently advised to exercise the usual
caution when handling individual MOS
components as regards electrostatic charges.
It is not quite as simple to obtain usable results when
performing in-circuit testing. As a result of parallel
connection of components with real and complex
characteristics - especially if these have relatively
low impedances at 50Hz - significant differences are
often obtained compared to out-ofcircuit testing of
individual components. If you must frequently test
circuits of the same type (service department), then
here, too, it is useful to
compare the trace pattern
with that of a functional circuit.
This is especially
quick and easy to do, since the second circuit used
for purposes of comparison need not (and must not!)
be placed under power. Using the test cables, simply
probe the corresponding pairs of test points and
compare the trace patterns obtained. At times, the
circuit under test will itself contain the circuit for
comparison, e.g. in the case of stereo channels and
symmetrical bridge circuits. In cases of doubt, one
lead of the component in question can be desoldered.
This lead is then connected with the red test jack of
the HZ65, in order to reduce hum. The black test jack
is directly connected with the oscilloscope chassis,
and is not sensitive to hum. Drum can also become
visible on the oscilloscope screen if the base or gate
of a single transistor is open, in other words it it is not
being tested at the moment (due to body effect).
The following should be stressed once again:
Tests with the HZ65 may only be performed on
components, modules or circuits in which no
currents are flowing.
Printed in Germany
140998-HE/sch
HAMEG GmbH · Industriestr. 6 · D-63533 Mainhausen · Tel. 06182/800-0 · FAX 06182/800-100
