Tektronix 503, Instruction Manual

Page: 31 / 147

Maintenance— Type 503
check the adjustment of C406 and C416 and the high fre­
quency compensation networks in the attenuators.
Refer
to the Calibration section of this manual.
NOTE
If tubes are replaced, phasing adjustm ents and
Common M ode Rejection must be re-checked.
Undershoot or rounding can be caused by insufficient
high frequency peaking and by tubes. If tube replacement
does not correct this type of distortion, then check the adjust­
ments of C406 and C416 and the divider networks in the
attenuators.
Low Common Mode Rejection Ratio. Low rejection
ratio is usually caused by tubes or transistors. If replace­
ment of tubes or transistors does not correct the trouble,
check for components which can affect the balance of the
circuit.
Horizontal Amplifier
If a spot is visible when the horizontal deflection plates
are externally shorted together, but disappears when the
short is removed, the Horizontal Am plifier is in a state of
unbalance.
When the HORIZONTAL DISPLAY switch is in the HORIZ.
AMPLIFIER position, the operation of the Horizontal Ampli­
fier is virtually identical to that of the Vertical Amplifier.
Therefore, the procedure for troubleshooting the Horizontal
Amplifier is the same as that explained previously for
troubleshooting the Vertical Amplifier.
Sweep Trigger Circuit
Switch the LEVEL control to FREE RUN and establish that
the Sweep Generator is functioning. To determine which
stage is defective, rotate the TRIGGER LEVEL control fully
counterclockwise to the AUTO, position. W ith no triggering
signal, the sweep should appear on the CRT. If the sweep
does not appear, either the Trigger M ultivibrator is inopera­
tive or the SWEEP STABILITY ADJUST is misadjusted. Refer
to the Calibration section for adjustment procedures for the
SWEEP STABILITY ADJUST.
A check on the Trigger Am plifier circuit may be made
as follows: with the LEVEL control still in the AUTO, posi­
tion, measure the voltage at the plate, pin 6, of V24B. This
voltage should be approximately + 9 8 volts. If the voltage
is incorrect, the trigger amplifier circuit is defective. The
trouble w ill probably exist in the tube, resistors, or switches
of the circuit. Measure the voltage at pins 2 and 7 of V24;
the voltage should be 0. If the grids measure 0 volts and
the voltage at pin 6 is incorrect, V24 is probably defective.
If the voltage measured at pin 6 of V24B is correct, rotate
the LEVEL control completely through its range while moni­
toring the voltage at this point. The voltage should vary
between approximately 70 and 130 volts.
An incorrect
voltage range indicates a defective amplifier or LEVEL con­
trol. If the voltage range is correct, the trouble is in the
trigger multivibrator. A trouble in the multivibrator will
probably be due to a defective tube or resistor. The voltage
divider network between the plate of V45A and the grid of
V45B is particularly critical.
Sweep Generator Circuit
Familiarity with the operation of the Sweep Generator
circuit is important before proceeding with any extensive
investigation of the circuit. For this reason, we suggest a
thorough study of that portion of the Circuit Description
that pertains to this circuit.
No Horizontal Sweep. If the sweep circuit is not pro­
ducing a sawtooth waveform when the TRIGGER LEVEL
control is in the FREE RUN position, some defect in the circuit
is causing the output to remain at some fixed voltage. A
clue to the cause of this trouble can be obtained by meas­
uring the plate voltage of the M iller tube, V160A.
NOTE
A ll voltages in this section should be measured
w ith a 2 0 ,0 0 0 ohms - per - v o lt voltm eter or a
vacuum -tube voltm eter.
The voltage reading obtained w ill probably be approxi­
mately +200 volts, or approximately + 3 0 volts. A reading
of 200 volts indicates that the M iller stage has run up and
has not been reset, while the reading of 30 volts indicates
that the M iller stage is not being allowed to run up. The
condition that actually exists w ill depend on the type of
trouble occuring in the circuit. The two conditions of the
plate voltage w ill be handled separately in the following
paragraphs.
+ 2 0 0 V at the plate of the M iller tube, V160A, indicates
the tube is cut off. This can result from any one of the fo l­
lowing conditions: (1) The Disconnect Diodes do not con­
duct, (2) Sweep-Gating M ultivibrator does not reset, and,
(3) Runup Cathode Follower (V152B) does not drive the
H old-O ff Circuit. The defective stage can be detected by a
series of systematic voltage measurements.
When an
improper voltage reading is obtained, this w ill indicate
the defective stage.
Check the voltage at the grid of the M iller tube, pin 2,
V160A. The static voltage at the M iller grid is determined
by conduction through the Timing Resistor, R160 (from
— 100-volt bus), the lower diode, D152, and resistor R147.
If the voltage reading is less than —4 volts, D152 is proba­
bly conducting normally and can be eliminated as a possi­
ble cause of the trouble. If the voltage is more negative
than approximately —20 volts, the diode is probably not
conducting. Check V152A, D152 and resistor R147.
Measure the voltage at the output of the Sweep Genera­
tor circuit (pin 8 of V160B). If this voltage is approximate­
ly + 150 volts, the Runup Cathode Follower stage may be
assumed to be operating correctly. If this voltage is low,
however, the stage is defective and its grid and cathode
circuits should be checked. If the grid voltage is low, B167
(neon) may possibly be open.
Next, measure the voltage at the cathode of V145B, pin
3. If this voltage is more positive than —55 volts the trouble
is in the Sweep-Gating Multivibrator. Check the tubes and
resistors in this circuit. The voltage divider network in the
cathode of V135B is particularly critical.
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