Tektronix 535A, Instruction Manual

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Maintenance—Type 535A
test points are located. Refer to Fig. 5-14 for a low-voltage
check point location.
Each regulated bus is identified by a color-coded wire.
The —150-volt bus is coded brown, green and brown on a
black wire; the +100-volt bus is coded brown, black and
brown on a white wire; the + 225-volt bus is coded red, red
and brown on a white wire; the +350-volt bus is coded
orange, and green and brown on a white wire; and the
500-volt bus is coded green, black and brown on a white
wire.
If any of the supplies fail to regulate the first thing to
check is the line voltage. The supplies are designed to
regulate between 105 and 125 volts with the design center
at 117 volts, or between 210 and 250 volts with the design
center at 234 volts, rms, 50-60 cycle single phase ac.
When possible, check the resistance values between the
outputs of the Low-Voltage Power supplies and ground. Re
fer to your schematic diagram for the approximate resis­
tance values.
If the resistance values between the regulated buses and
ground check out, check the tubes (if this has not already
been done). Then make sure that the line voltage is set
near the design center for your instrument (117 or 234 v) and
check the rms voltage across the secondary winding for each
supply; the nominal value of each secondary voltage, when
the line is set to the design center, is indicated on the
circuit diagram. If the secondary voltages are all correct,
check the operation of the bridge rectifiers. This can be
done by measuring the rectified voltage at the input to
each regulator. These values are also indicated on the cir­
cuit diagram. Then check for off-value resistors, especially
in the dividers, and for open or leaky capacitors.
The material that follows may be used as a quick index
for troubleshooting the regulator circuits.
If the output voltage is high with excessive ripple, check:
a. For high line voltage.
b. For open voltage-regulator tube
c. The ampliifer tubes in the regulator circuit.
d. For insufficient loading
If the output voltage is high with normal ripple, check:
a. For proper resistance values in the dividers refer to
to the schematic for your instrument to determine the
location of the resistors involved. Since these are gen­
erally precision resistors (+1% tolerance) the use of a
good bridge is recommended in checking the value.
If the output is low with excessive ripple, check:
a. For low line voltage.
b. For shorted voltage-regulator tube.
c. The series tubes in the regulator circuit.
d. For excessive loading.
e. Open or leaky filter capacitors.
f. Defective rectifiers.
If the output is low with normal ripple, check:
a. The resistance values in the dividers.
b. The capacitors across the dividers.
NOTE
If any components in the —150-volt supply are
changed, or if the setting of the —150 Adj. con­
trol is changed, it will be necessary to recalibrate
the instrument.
TROUBLESHOOTING THE CALIBRATOR
Asymmetrical Output
If the output square wave is not symmetrical (the positive
portion has a duration different from that of the negative
portion) the two tubes in the Multivibrator circuit are being
held cutoff for equal periods. This will normally be caused
by a defective tube. If tube replacement does not correct
the waveform the circuit components must be checked. The
triode in the Multivibrator is held cutoff for an interval
determined by the discharge of C871 and the pentode is
held cutoff for an interval determined by the discharge of
C874. A change in the value of either capacitor, or in the
value of the resistor through which they discharge, could
produce an asymmetrical waveform.
In addition, the time needed for these capacitors to dis­
charge a given amount is affected by the potential toward
which they discharge; this would be the voltage at the plate
of the triode in the case of C874, and the voltage at the
screen of the pentode in the case of C871. Since these
voltages are affected by the value of R870 and R875, these
resistors should be checked. The resistors in the plate cir­
cuit of the pentode should also be checked, since they will
affect the plate-to-screen ratio of the pentode.
Incorrect Output Voltage
The amplitude of the output square wave is determined
almost entirely by the resistance values in the divider in the
cathode-follower stage. A quick check of the resistance
values can be made by turning off the Calibrator and mea­
suring the voltage at the Cal. Test Pt; if this point does not
measure exactly +100 volts the output voltages when the
Calibrator is turned on will not be correct.
The Cal. Adj. control R879 will vary the voltage at the
test point over about a 10-volt range. If this voltage cannot
be set to e100 volts, and if the tubes have been
replaced, then one or more of the precision resistors in the
divider have changed in value.
TROUBLESHOOTING THE CRT CIRCUIT
The intensity, focus, geometry and calibration of the crt
display depend on proper operation of the high-voltage
supplies in the CRT Circuit.
5-12