Tektronix 502A

Maintenance — Type 502A
Next, measure the grid to cathode voltage of the Hold-
Off Cathode Follower, V183A and V145B. Both of these
readings should be approximately — 4 volts if the cathode
followers are operating correctly. If either or both of these
voltages are incorrect, check the hold-off capacitor and
the resistors in the cathode circuits of the two stages.
If the previous checks gave correct results, the voltage
at the grid of V135A should be sufficient to reset the
multivibrator (more positive than — 35 volts). If this is so,
and the multivibrator still does not reset, the trouble must
lie in that stage. Check the voltages at the plates of V135A,
V135B, and V145A, and the resistors in the cathode circuit
of V135B.
Miller Stage will not Run-up
Low voltage at the plate of the Miller tube indicates
that the tube is conducting quite heavily and is not being
allowed to perform its normal runup operation. If this
trouble exists on only a few ranges of the TIME/CM switch
the trouble is likely to be an open timing resistor. If the
trouble exists on all ranges of the TIME/CM switch, the
trouble is probably due to a defective Sweep-Gating
Multivibrator stage.
To check the Sweep-Gating Multivibrator, monitor the
voltage ot the junction of R114 and R1 16 and adjust the
STABILITY ADJUST Control for a reading of — 70 volts on
the voltmeter. With this voltage, the Sweep-Gating Multi 
vibrator and the sweep should free run. If the multivibrator
does not switch, check the resistances which make up the
stage.
If the voltage at the junction of resistors R1 14 and R116
remains relatively constant as the STABILITY ADJUST Control
is rotated, a defect in cathode follower V145B may be
causing the stage to effectively regulate the voltage at
this point. If such is the case, the probable cause of this
condition is a shorted holdoff capacitor. If the voltage
does not adjust to the proper level, check the resistors in
the grid circuit of V135A.
For nonlinear sweep
A nonlinear sweep voltage will be generated if the
current charging the Timing Capacitor does not remain
constant. If the nonlinearity exists at all sweep rates, a
defective Miller tube is the probable cause of the trouble.
If the nonlinearity occurs only at certain sweep rates, the
Miller tube or a leaky Timing Capacitor is the probable
cause.
For insufficient horizontal deflection
If the horizontal trace starts at the left-hand side of the
screen, but does not extend to the right-hand side, the
Hold-Off circuit is resetting the Sweep-Gating Multivibrator
before the sweep is complete. If the sweep cannot be
adjusted to normal length with the SWP LENGTH Control
R176, the resistance in the cathode circuit of V161B should
be checked.
Single Sweep will not perform properly
If D126 opens, the Time-Base Generator will still operate
normally when the MODE switch is in the NORMAL position.
However, when the instrument is set up for Single Sweep
operation, the circuit will not lockout at the end of the
sweep. This of course means that each incoming trigger will
produce another sweep across the crt.
If on the other hand D126 becomes shorted or leaky,
then the Time-Base Generator will not produce a sweep
even though the Single sweep circuit seems to be resetting
all right. If this condition exists the plate voltage of V135A
should be checked. It should normally be approximately
+ 138 volts when the Time-Base Generator is producing a
sweep, however when the Time-Base Generator is not pro 
ducing a sweep the plate voltage of V135A will normally
be approximately +4 volts.
D124 being opened will also cause the Single Sweep
circuit not to lockout, even though the READY light seems
to operate normally.
When D124 is shorted or leaky it will cause the Time-
Base Generator to free-run since the grid of V135A is pulled
negative. The TRIGGERING LEVEL Control will have little
or no effect upon the free-running Time-Base Generator.
TROUBLESHOOTING THE HORIZONTAL
AMPLIFIER
For no spot or trace
If both beams are visible when the horizontal deflection
plates are externally shorted together, but disappear when
the short is removed, the horizontal amplifier is in a state
of unbalance. The procedure for troubleshooting the Hori 
zontal Amplifier is somewhat the same as that explained
previously for troubleshooting the Vertical Amplifiers for
unbalance. Corresponding points on opposite sides of the
Horizontal Amplifier circuit should be progressively shorted
together until a point is reached where the beams do not
return to the face of the crt. The stage immediately following
this point is the one which is defective. Check for open
resistors in the cathode and plate circuits.
For insufficient or no horizontal deflection
If the gain of the Horizontal Amplifier decreases, the
trace will not extend to the left and right sides of the
crt screen. In addition, the sweep timing will no longer
be calibrated on any range of the TIME/CM switch. If the
change in sweep timing and sweep length is slight, it may
be possible to correct this trouble by adjusting the high
voltage and the X 1 CAL and X20 CAL Controls according
to the instructions contained in the Calibration Procedure.
If the decrease in gain of the Horizontal Amplifier is
more pronounced, or if there is no sweep at all, check for
components which will affect the gain but not the balance
of the amplifier. The input cathode follower, V 1 83B, should
be checked as one of the first steps. Also, the common
resistances in the cathodes of the amplifier stages should
be checked.
4-8