Tektronix 502A, Instruction Manual

Page: 52 / 110

Calibration Procedure —Type 502A
29. FEEDBACK BAL
Set
the TIME/CM switch of the Type 502A to 2 mSEC; the
Input Selector to B-DC and the SENSITIVITY switch to .2
mVOLTS PER CM. From the Type 105 apply 3 cm of 200
cycle square waves to the oscilloscope. Refer to figure 5-11
for the proper placing of attenuators and the capacitance
standardizer. Adjust for a stable display.
While observing the display adjust the FEEDBACK BAL
Control for an optimum square wave. Now apply the signal
to the A input connector and change the Input Selector
switch to A-DC. Check for a good square wave.
The final setting of the FEEDBACK BAL Control should be
made for an optimum square wave with the input applied
to either input channel. Disconnect the Type 105 from the
Type 502A.
30. TRIG. LEVEL CENT.
Place the HORIZ DEF PLATE SELECTOR Switch in the
TIME BASE AMP position, the UPPER BEAM SENSITIVITY
switch in the .5 VOLTS PER CM position, and the TRIG­
GERING LEVEL Control at midrange. Connect 50 millivolts
of Calibrator signal to input A of the UPPER BEAM vertical
amplifier and place the TRIGGER SELECTOR switch in the
UPPER AC position. Ground the junction of R15 and C13
(See Fig. 5-12) by placing a jumper between that point and
the chassis. Adjust the TRIG. LEVEL CENT Control so that
the sweep will trigger correctly with the TRIGGER SELECTOR
Slope Switch in either the + or — positions.
Fig. 5-12. Grounding the junction of R15 and C13. For adjustment
of the TRIG LEVEL CENT and DC TRIG controls, the junction of
R15 and C1 3 is grounded as shown.
31. STABILITY ADJUST
Disconnect the Calibrator signal, remove the jumper from
the junction of R15 and C13 to ground, place the
TRIGGERING LEVEL Control in the AUTOMATIC position
and set the TIME/CM control to .1mSEC. Connect a meter
from the wiper arm of the STABILITY ADJUST Potentiometer
to ground. Rotate the STABILITY ADJUST Control clockwise
from the fully counterclockwise position. Note the voltage
when the trace first appears on the face of the crt. Con­
tinue to rotate the STABILITY ADJUST Control clockwise
until the trace brightens and again note the voltage. Set
the STABILITY ADJUST Control so that the voltage reading
is midway between the two voltages previously noted.
Fig. 5-13. Adjustment of the STABILITY ADJUST control.
32. UPPER BEAM DC TRIG
Position the TRIGGERING LEVEL Control to midrange and
place a jumper from the junction of R15 and C13 to ground.
Place the UPPER BEAM SENSITIVITY switch in the .2 VOLTS
PER CM position and apply 50 MV of Calibrator signal to
input A of the UPPER BEAM vertical amplifier. Position the
UPPER BEAM trace under the upper horizontal centerline.
Place the TRIGGER SELECTOR switch in the UPPER DC
position and set the DC TRIG Control so that the sweep
will trigger on either the + or — positions of the TRIGGER
SELECTOR Slope Switch.
33. LOWER BEAM DC TRIG
Leave the jumper connected as in step 32. Place the
LOWER BEAM Input Selector switch in the A-DC position
and the LOWER BEAM SENSITIVITY switch in the .2 VOLTS
PER CM position. Connect 50 MV of Calibrator signal to
input A of the LOWER BEAM vertical amplifier. Place the
TRIGGER SELECTOR switch in the LOWER DC position and
5-10