Subject to change without notice
24
Of course, the quality of the transmission performance is
not only dependent on the vertical amplifier. The input
attenuators, located in the front of the amplifier, are
frequency-compensated in each position. Even small
capacitive changes can reduce the transmission performance.
Faults of this kind are as a rule most easily detected with a
square-wave signal with a low repetition rate (e.g. 1kHz). If a
suitable generator with max. output of 40Vpp is available, it
is advisable to check at regular intervals the deflection
coefficients on all positions of the input attenuators and
readjust them as necessary. A compensated 2:1 series
attenuator is also necessary, and this must be matched to
the input impedance of the oscilloscope. This attenuator can
be made up locally. It is important that this attenuator is
shielded. For local manufacture, the electrical components
required are a 1M
Ω
±
1% resistor and, in parallel with it, a
trimmer 3-15pF in parallel with approx. 12pF. One side of
this parallel circuit is connected directly to the input connector
of CH.I or CH.II and the other side is connected to the
generator, if possible via a low-capacitance coaxial cable. The
series attenuator must be matched to the input impedance
of the oscilloscope in the 5mV/div. position (variable control
to CAL., DC input coupling; square tops exactly horizontal;
no ramp-off is permitted). This is achieved by adjusting the
trimmer located in the 2:1 attenuator. The shape of the square-
wave should then be the same in each input attenuator
position.
Operating Modes: CH.I/II, DUAL, ADD,
CHOP., INVERT and X-Y Operation
In DUAL mode two traces must appear immediately. On
actuation of the Y-POS. controls, the trace positions should
have minimal effect on each other. Nevertheless, this cannot
be entirely avoided, even in fully serviceable instruments.
When one trace is shifted vertically across the entire screen,
the position of the other trace must not vary by more than
0.5mm.
A criterion in chopped operation is trace widening and
shadowing around and within the two traces in the upper or
lower region of the screen. Set TIME/DIV. switch to 0.5ms/
div., set input coupling of both channels to GD and advance
the INTENS. control fully clockwise. Adjust FOCUS for a sharp
display. With the Y-POS. controls shift one of the traces to a
+2div., the other to a -2div. vertical position from the hori-
zontal center line of the graticule.
Do not try to synchronize (with the time variable control) the
chop frequency (0.5MHz)! Check for negligible trace widening
and periodic shadowing when switching between 0.5ms/div
and 0.2ms/div.
It is important to note that in the I+II add mode (ADD LED
lights) or the I-II difference mode (one INV LED lights) the
vertical position of the trace can be adjusted by using both
the Channel I and Channel II Y-POS. controls.
In X-Y Operation (XY LED in TIME/DIV sector lights), the
sensitivity in both deflection directions will be the same. When
the signal from the built-in square-wave generator is applied
to the input of Channel II, then, as with Channel I in the
vertical direction, there must be a horizontal deflection of
4div. when the deflection coefficient is set to 50mV/div.
position (variable control set to its CAL. position). The check
of the mono channel display is unnecessary; it is contained
indirectly in the tests above stated.
Triggering Checks
The internal trigger threshold is important as it determines
the display height from which a signal will be stably displayed.
It should be approx. 0.3-0.5div. for the HM304. An increased
trigger sensitivity creates the risk of response to the noise
level in the trigger circuit. 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 1MHz. The instrument should be in automatic peak
(value) triggering (NM LED dark) and the TRIG. LEVEL knob
in midrange position. It should be ascertained whether the
same trigger sensitivity is also present with Normal Triggering
(NM LED lights). In this trigger mode, TRIG. LEVEL
adjustment is absolutely necessary.
The checks should show the same trigger threshold with
the same frequency. On depressing the ± (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
frequency range is dependent on the trigger coupling
selected. For lower frequencies the LF coupling mode can
be selected. In this mode, triggering up to at least 1.5kHz
(sine-wave) is possible. Internally the HM304 should trigger
perfectly at a display height of approx. 0.5div., when the
appropriate trigger coupling mode is set.
For external triggering (EXT LED lights), the TRIG. INP.
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-L or TV-F setting for video sync pulse separation.
With the ± (SLOPE) button the correct slope of the sync
pulse (front edge) must be selected and a suited TIME/DIV
setting must be chosen. The 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
6div and sync pulses of more then 0.5 div. height. The display
should not shift horizontally during a change of the trigger
coupling from AC to DC when displaying 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 switching from
TRIG I to TRIG II or when the trigger coupling is changed
from AC to DC.
Checking of the line/mains frequency triggering (50-60Hz) is
possible, when the input signal is time-related (multiple or
submultiple) to the power line frequency ( ~ LED lights). There
is no trigger threshold visible in this trigger mode. Even very
small input signals are triggered stably (e.g. ripple voltage).
For this check, use an input of approx. 1V. The displayed
signal height can then be varied by turning the respective
input attenuator switch and its variable control.
Timebase
Before checking the timebase it should be ascertained that
the trace length is approx. 10div. in all time ranges. If not, it
can be corrected with the potentiometer X x1. This
adjustment should be made with the TIME/DIV. switch in a
