Heathkit IO-4550 - Page 12

Page 10
DC BALANCE (DC BAL)
The highly sensitive vertical amplifier input circuits in this
Oscilloscope, as in other sensitive equipment, may exhibit an
occasional unbalance caused by aging components and
temperature effects. Even though the DC BAL (balance)
control is not considered to be an operating control, you
should make it a habit to check the DC balance periodically
and readjust it when necessary. You will need a small
screwdriver to make this adjustment through the small hole
in the front panel.
To check the DC balance of either channel, set the input
switch (AC-GND-DC) to ground (GND) and obtain a trace
on the CRT. Turn the VOLTS/CM VARIABLE gain control
from fully clockwise to fully counterclockwise. If the trace
moves vertically, readjust the DC balance as follows:
1. Turn the VOLTS/CM switch to the 10 mV
Position.
(8x Fs News Se
fully Y
2. Turn the VARIABLE gain control
counterclockwise.
3. Center the trace on the screen.
4. Turn the VARIABLE gain control
clockwise to the CAL position.
fully
5. Adjust the DC BAL control to return the trace
to the centerline.
6. Repeat steps 2 through 5 until the trace does
not move when the VARIABLE gain control is
turned.
NOTE: If the trace does not move as the VARIABLE gain
control is turned, but moves when the VOLTS/CM switch is
changed, perform the “Vertical Amplifier Balance’ on Page
27.
NORMAL
OPERATING
CHARACTERISTICS
The following information is provided to help answer
Possible questions you may have about the operation of
your Oscilloscope.
Several minutes may be required for the trace to
stabilize when the Oscilloscope is first turned on,
especially on the more sensitive voltage ranges.
Random noise on the input signal may cause false
triggering, especially on the most sensitive voltage
ranges.
A baseline will automatically appear after a short 6
pause when the trigger LEVEL control is pushed in to
the AUTO position or when the input signal is
disconnected when automatic triggering is used.
In the EXT horizontal mode, the horizontal position of the
trace may change with different. horizontal
resistance.
driving
USING A 10 MILLIVOLT OSCILLOSCOPE
When you use an Oscilloscope as sensitive as this, you must
use special care to make reliable measurements. Keep the
following points in mind when you measure very low level
signals.
Placement of the ground clip may be critical if the
signal
source
ground
carries
an
appreciable
current.
Voltage
differences
of
several
millivolts
from
one
side
of
a
chassis
or
ground
foil
to
the
other
are
common.
Place
the
ground
clip
at
the
point
that
gives
the
least
error.
This
is
usually
nearest
the
signal
source.
You
may
have
to
move
the
ground
clip
when
you
measure
different
points.
Stray 60 Hz pickup may be hard to eliminate,
especially in high impedance circuits. Be sure to use
shielded test cables. Shield the signal source if
necessary.
Wideband measurements in the millivolt region are
more difficult because of the inherent noise (shot
noise and thermal noise) generated by electronic
components. This may appear as a widening of the
baseline or the baseline appearing out of focus. Noise
on the baseline that appears as “hash” or “‘spikes’’
may be’ caused by the electromagnetic pickup of
man-made noise such as ignition noise, appliance
noise, etc. Noise of any kind may cause erratic
triggering.