Tektronix 502 series, Instruction Manual

Page: 22 / 114

The output impedance of the calibrator varies
with the output-voltage setting. It is as low as
10 ohms in the 1 MV position of the SQUARE-
WAVE CALIBRATOR switch and as high as 1.8
kilohms in the 10 V position. In the 100 V posi­
tion, the output impedance is approximately 170
ohms when the circuit is loaded by a resistance
of not less than four kilohms. When the switch
is at any position other than 100 V, the output
may be loaded with any impedance without
damaging the components. See Table 1 for the
approximate output impedance for each position
of the SQUARE-WAVE CALIBRATOR switch.
TABLE I
Approxim ate O utput Impedance o f C alibrator
Output SQUARE-WAVE CALIBRATOR
Impedance switch setting
:170 ohms 100 V
1.8 kilohms 10V
200 ohms 1 V
900 ohms
100 MV
100 ohms
10 MV
10 ohms
1 MV
*Do not load with less than 4 kilohms.
Trace-brightness (intensity) modulation
To couple markers or other signals into the crt
cathodes for trace-brightness modulation, dis­
connect the ground strap at the rear of the in­
strument and apply the signal between the CRT
CATH binding post and GND. This will simul­
taneously apply the signals to the upper and
lower beam cathodes. At normal brightness,
positive signals of about 25 volts will cut the
beams off. Always replace the ground strap
when you are not intensity modulating the
beams.
Graticule illumination
The graticule lighting control, labeled SCALE
ILLUM, can be adjusted to suit the lighting con­
ditions of the room. A green filter is supplied
with the instrument which can be used for in­
creased contrast. This filter should be mounted
next to the crt screen so it does not block the
light from the graticule lines.
The graticule of the Type 502 Oscilloscope can
be illuminated so that it appears to have either
red or white graticule markings. The markings
can be quickly changed from white to red or
from red to white by removing the graticule
cover and inverting the graticule. As a general
rule, white graticule lines are superior to red for
photographic purposes.
Direct connection to crt plates
In some applications, it may be desirable to
connect a signal directly to one or more sets of
CLIPS TO DEFLECTION PLATE P IN S '
AMPLIFIER
OUTPUT LEADS
---------
1 M.
1M.
0.1 /if
±
NOTE:
TO A V O ID G R O U N D IN G
LEADS USE INSULATED
SLEEVING WHERE
NECESSARY.
^0.1 /if
TO S IG N A L
SOURCE
Fig. 3 -9 . Typical circuit for ac coupling to the crt de­
flection plates.
crt deflection plates— bypassing the internal
oscilloscope amplifiers. This can be done in the
Type 502 if certain precautions are observed.
One of these precautions is to maintain the aver­
age dc voltage on the deflection plates at ap­
proximately +225 volts. If the average voltage
is not maintained at approximately +225 volts,
the crt display may be defocused.
For many applications, ac coupling of the sig­
nal to the deflection plates is permissible. It has
the advantage of allowing the use of frontpanel
controls to position the display and permits the
display of signals not having the required aver­
age voltage level. A diagram of this method of
coupling is shown in Figure 3-9. Here, the leads
from the oscilloscope amplifier are removed and
a resistor is connected between each lead and
its respective crt pin. A good value for this re­
sistor in most cases is 1 megohm. The vertical
deflection plate pins are located on the sides of
the crt neck and the horizontal deflection plate
pins are located on the top of the neck.
A convenient method for connecting the resis­
tors to the crt pins is to use clips removed from
standard miniature tube sockets. Before connect­
ing the resistors to the leads from the amplifier,
slip a piece of insulated sleeving (spaghetti) over
the wire. Then, after making the connections,
draw the sleeving back over the insulated resistor
pigtail. This will prevent the leads from acci­
dentally shorting to the chassis. The crt pins are
3-10
OPERATING INSTRUCTIONS — TYPE 502
®