Tektronix 502 series, Instruction Manual

Page: 44 / 114

Neither Beam Visible On CRT
If neither beam is visable, either both electron
guns are cutoff, or both beams are deflected
off the face of the crt. These troubles probably
will be caused either by the Horizontal Amplifier
or the CRT Circuit. To determine which circuit is
at fault, short the horizontal deflection plates
together at the neck of the crt being careful that
the pins are not shorted to the crt shield. If the
beams return to the crt face, (you may have to
readjust the vertical POSITION controls) the
trouble is due to an unbalanced horizontal am­
plifier. If the beams do not reappear, the trouble
is due to a defective crt circuit.
Insufficient Vertical Deflection, Waveform
Distortion, or Low Differential Input Rejection
Ratio
These troubles are all caused by a deflective
vertical amplifier. Refer to that section of Part II
of the troubleshooting procedure.
Insufficient or No Horizontal Deflection
Either of these conditions can be produced by
the Time-Base Generator or the Horizontal Am­
plifier. If the sweep is shortened, but the timing
is not affected, the trouble is probably in the
Time-Base Generator. If both the sweep length
and the timing are affected, the trouble is prob­
ably in the Horizontal Amplifier.
As an additional check to determine which
circuit is defective, place the HORIZONTAL DIS­
PLAY switch at 1 EXT VOLTS/CM and the
SQUARE-WAVE CALIBRATOR switch at 10V.
Connect the calibrator output to the EXTERNAL
input connector. Both beams should now be de­
flected horizontally a total of 10 centimeters. If
the deflection is correct, the trouble is probably
in the Time-Base Generator, however, the Hori­
zontal Amplifier input cathode follower stage,
V183B, should also be checked. If the deflection
is not correct, the trouble is in the Horizontal
Amplifier.
Nonlinear Horizontal Sweep
The linearity of the horizontal-deflection cir­
cuit can be checked by connecting a marker
generator or the calibrator output to a vertical
input of the oscilloscope. If the sweep is linear,
the markers or calibrator waveforms should be
spaced equidistant along the sweep. A nonlinear
sweep can be caused by either the Time-Base
Generator or the Horizontal Amplifier.
To check the Time-Base Generator, connect a
jumper from an input of the oscilloscope to pin
3 of V I61 being careful that the lead does not
touch the chassis at any time. Set the SENSI­
TIVITY control at 20 volts per cm, and rotate the
TRIGGERING LEVEL control fully clockwise to
the RECURRENT position.
Set the TIME/CM
switch at 1 MILLISEC. You should then obtain a
stable diagonal trace. This trace represents a
plot of the output from the Time-Base Generator
(applied to the Y axis) and the output of the
Horizontal Amplifier (applied to the X axis). If
the slope of this trace is constant, this indicates
that both voltages rise at the same rate and the
lack of linearity is due to a trouble occuring
in the Time-Base Generator. If the slope is not
constant, the horizontal amplifier is distorting the
constant slope sawtooth waveform from the Time-
Base Generator.
Improper Sweep Timing
If the timing is off in some, but not all, posi­
tions of the TIME/CM switch, one of the timing
resistors or timing capacitors has changed in
value. By comparing the switch positions in
which the timing is incorrect with the timing
switch diagram, you will be able to tell which
components are common to these positions.
If the timing is off in all positions of the
TIME/CM switch, the Horizontal Amplifier is
probably the circuit at fault although it is im­
portant that both the low and high voltage
power supplies be checked. Check to see if
the timing circuits can be recalibrated in accord­
ance with the instructions presented in the Re­
calibration Procedure. If the circuits cannot be
adjusted for correct timing, then refer to the sec­
tion on troubleshooting the Horizontal Amplifier
in Part II.
Improper Triggering
If improper triggering results from triggering
signals obtained from only one of the vertical
amplifiers, the trigger pickoff circuit of that am­
plifier is probably at fault. If the sweep triggers
incorrectly with signals obtained from both verti­
cal amplifiers as well as external triggering sig­
nals the trouble can be caused by either the
Time-Base Trigger circuit or the Time-Base Gen­
erator circuit. If the trace can be turned on and
off by switching the TRIGGERING LEVEL control
into and out of the RECURRENT position, the
Time-Base Trigger is probably causing the
trouble; if the trace cannot be turned off with
the TRIGGERING LEVEL control, the Time-Base
Generator is likely at fault.
5-6
MAINTENANCE — TYPE 502 ®