Table of Contents
Advertisement
Triggering Checks
The internal trigger threshold is important as it determines
the display height from which a signal will be stably dis¬
played. It should be approx. 0.3-0.5div. for the HM203-7.
An increased trigger sensitivity creates the risk of response
to the noise level in the trigger circuit, especially when the
sensitivity of the vertical input is increased by depressing
the Y-MAG.X5 button. This can produce double-triggering
with two out-of-phase traces.
Alteration of the trigger threshold is only possible internally.
Checks can be made with any sine-wave voltage between
50Hz and 1 MHz. The AT/NORM, button should be in out
position (Automatic Triggering). Following this it should be
ascertained whether the same trigger sensitivity is also pre¬
sent with Normal Triggering (AT/NORM, button depressed).
In this trigger mode, a LEVEL adjustment is necessary. The
checks should show the same trigger threshold with the
same frequency. On depressing the -F/— slope button, the
start of the sweep changes from the positive-going to the
negative-going edge of the trigger signal.
As described in the Operating Instructions, the trigger fre¬
quency range is dependent on the trigger coupling
selected. For higher frequencies the HF coupling mode
must be selected. In this mode, triggering up to at least
40MHz (sine-wave) is possible. Internally the HM203-7
should trigger perfectly at a display height of approx.
0.5div., when the appropriate trigger coupling mode is set.
For external triggering (EXT. button depressed), the EXT.
TRIG, input connector requires a signal voltage of at least
0.3Vpp, which is in synchronism with the Y input signal. The
voltage value is dependent on the frequency and the trigger
coupling mode (AC-DC-HF-LF).
Checking of the TV triggering is possible with a video signal
of any given polarity. Use the TV: H position of the TV SEP.
switch for triggering on linear horizontal frequency (TIME/
DIV. switch to 20 or 10(xs/div.). The TV: V position is
required for fra/ne or vertical frequency (TIME/DIV. switch
to 5 or 2 ms/div.). With the -F/— button the correct slope of
the line sync pulse (front edge) must be selected. This slope
is then valid for both sync frequencies.
Perfect TV triggering is achieved, when in both display
modes the amplitude of the complete TV signal (from white
level to the top of the line sync pulse) is limited between 0.8
and 8div.
The display should not shift horizontally during a change of
the trigger coupling from AC to DC with a sine-wave signal
without DC offset.
If both vertical inputs are AC coupled to the same signal and
both traces are brought to coincide exactly on the screen.
when working in the alternate dual channel mode, then
no change in display should be noticeable, when the CH.I/
ll-TRIG.I/ll button is depressed or released or when the
TRIG, selector switch is changed from AC to DC position.
Checking of the line/mains frequency triggering (50-60 Hz)
is possible, when the input signal is time-related (multiple or
submultiple) to the power line frequency (TRIG, selector
switch to ~). There is no trigger threshold visible in this trig¬
ger mode. Even very small input signals are triggered stably
(e.g. ripple voltage). For this check, use an input of approx.
1 V. The displayed signal height can then be varied by turn¬
ing the respective input attenuator switch and its variable
control.
Timebase
Before checking the timebase it should be ascertained that
the trace length is 10.1 div. in all time ranges. If not, it can
be corrected with the potentiometer for sweep amplitude
(see Adjusting Plan). This adjustment should be made with
the TIME/DIV. switch in a mid position (i.e. 20^s/div.).
Prior to the commencement of any check set the time vari¬
able control to CAL. The X-MAG. xIO button should be in
out position. This condition should be maintained until the
variation ranges of these controls are checked.
Check that the sweep runs from the left to the right side
of the screen (TIME/DIV. switch to 0.1s/div.; X-POS.
control in mid-range). This check is only necessary after
changing the cathode-ray tube.
If a precise marker signal is not available for checking the
Timebase time coefficients, then an accurate sine-wave
generator may be used. Its frequency tolerance should not
be greater than ±1%. The timebase accuracy of the
HM 203-7 is given as ±3%, but it is considerably betterthan
this. For the simultaneous checking of timebase linearity
and accuracy at least 10 oscillations, i.e. 1 cycle every div.,
should always be displayed. For precise determination, set
the peak of the first marker or cycle peak exactly behind the
first vertical graticule line using the X-POS. control. Devia¬
tion tendencies can be noted after some of the marker or
cycle peaks.
The 20 and 10 ms/div. ranges of the TIME/DIV. switch can
be checked very precisely with line frequency (50Hz only).
On the 20 ms/div. range a cycle will be displayed every div.,
while on the lOms/div. range it will be every 2div.
If a precise Time Mark Generator is used for checking. Nor¬
mal Triggering (AT/IMORM. button depressed) and LEVEL
control adjustment is recommended.
The following table shows which frequencies are required
for the particular ranges.
Subject to change without notice
T3 203-7
Advertisement
Table of Contents
