In-Circuit Tests; Function Of The Controls; Power; Adjust - Hameg HM400 Handbuch

F u n c t i o n o f t h e c o n t r o l s
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.

Function of the controls

1

POWER

Pushbutton switch with indications of off (0) and on (I) positions. After
turn-on all LEDs will light up, the instrument performs a self-test.
As soon as this has been successfully completed, the oscilloscope
will switch to normal operation, all settings which were valid before
switching off will be resumed.
2 ADJUST – / +
Allows to change the value of diverse settings selected by SELECT
3

Indication LEDs

INTENS
The LED will light if the function intensity adjustment was selected
7
by SELECT . With the buttons ADJUST – / +
38 Subject to change without notice
decreased resp. increased. It is recommended to set the intensity no
higher than needed for easy viewing, this depends on signal parameters,
oscilloscope settings and the ambient light conditions.
FOCUS
The LED will light up if the function focus adjustment was selected by

SELECT

The focus adjustment depends on the intensity, the lower the intensity,
the better the focus. Also, the focus depends on the location of the trace
on the screen, the best focus is always in the center and it decreases
towards the edges. A reasonable focus setting is hardly possible with
only the trace on screen. Due to the interaction between intensity and
focus, the best procedure is this:
1. Apply a sine wave signal which covers the whole screen.
2. Set the intensity.
3. Adjust the focus for a uniform well focussed display over most of
Please note that the display of signals with a low rep rate at high
sweep speeds will ask for a higher intensity setting, this will entail a
readjustment of the focus.
TRACE
The LED will light up if the function trace rotation was selected by SE-
LECT
adjusted. Due to the earth's magnetic fi eld the trace may not be parallel
to the graticule lines, adjust for exact parallelism. See also under „Trace
Rotation" in the chapter „First time operation".
4
This button allows to select and change the settings related to the trace
in conjunction with the buttons ADJUST – / +
buttons shortly the functions intensity, focus and trace rotation will be
enabled in turn as indicated by the associated LEDs.
5
These knobs control the trace position of channel 1 respectively cannel
2. In ADD mode both position controls will be effective. In XY mode the
Y position control is disabled; the X position is always controlled by the
knob HORIZONTAL position
Measurement of DC voltages
If there are no voltages present at the Y amplifi er inputs
CH2, or if GND was selected, the trace positions correspond to zero
volts, however, they are also influenced by the position controls!
(The trace will be visible only if automatic triggering AUTO
selected.)
In order to measure a DC voltage, fi rst the input must be disconnected
or switched to GND. Then it is up to the user to choose a reference
trace position with the respective position control, i.e.
Then the DC voltage can be connected resp. the input switched from
GND to DC; the trace will move by an amount depending on the polarity
and value of the dc voltage and the sensitivity (and probe) selected. The
same is true for measuring the dc content of any signal.
In regular operation, the position controls will mostly be set to the
screen center representing zero. It should be kept in mind, that it can not
be just assumed that a trace position at the screen center corresponds
to zero volts, this has to be checked by switching to GND.
1
POWER POWER
4
CH1 CH1
CH2 CH2
2
the intensity may be
4
. With the buttons ADJUST – / +
the screen area.
4
. With the buttons ADJUST – / +
SELECT
POSITION 1 + POSITION 2, CH1 + CH2
13
.
2 3 4 8

ADJUST

SELECT
!
INTENS
+ +
FOCUS
TRACE
SAVE push long
RECALL
POSITION 1 POSITION 2
1
1–6
AUTOSET
2
5 6 7 5
2
the focus can be changed.
2
the trace rotation can be
2
. By pressing one of this
29 CH1 resp.
8 was
5 for channel 1.
11
HM400
40 MHz ANALOG OSCILLOSCOPE
AUTO TRIGGER TRIG'd
LEVEL X-POSITION
NORM
SLOPE X-MAG
x10
9 10
12 13