Tektronix 516 series, Instruction Manual

Page: 33 / 132

Maintenance — Type 5 1 6
Although the Type 516 Oscilloscope is a complex instru­
ment, it can be thought of as consisting of six main circuits
in addition to the Calibrator Circuit. These are:
1. Power Supply
2. Crt Circuit
3. Time-Base Generator
4. Time-Base Trigger
5. Vertical Amplifier
6. Horizontal Amplifier
Whenever any trouble occurs in the instrument, first check
the voltages at the points shown in Fig. 6-4, at the —1675-
and -(-2325-volt checkpoints at the top rear of the instru­
ment, and at the center arm of the INTENSITY potentiometer,
R826. If all of these voltages are approximately as indicated
(the voltage at the center arm of the INTENSITY potentio­
meter should be about —1700 volts when the INTENSITY
control is set fully clockwise), proceed to the troubleshooting
procedures described under the appropriate symptom. If
the low voltages (—150, +100, and +300) are not as in­
dicated, refer to the paragraphs on Troubleshooting the
Power Supply. If the low voltages are as indicated but the
high voltages (—1675, +2325, and INTENSITY potentio­
meter voltage) are not, refer to the paragraphs on Trouble­
shooting the Crt Circuit.
WARNING
Be careful o f the power-supply voltages. The low-
voltage Power Supply can cause more harm than
the high-voltage supply (in the CRT Circuit) due to
the higher current capabilities o f the circuit. When
you reach into the instrument to measure the v o lt­
age, do not touch the metal frame. If possible,
stand on an insulated surface and use insulated
tools and measuring probes.
The following troubleshooting information is divided ac­
cording to the symtoms presented to the operator. Upon
detecting an apparent trouble, use the symptoms to deter­
mine which circuit is at fault. Then refer to the Circuit
Troubleshooting information, where the procedure for trou­
bleshooting within the circuit is given.
No Trace on the Crt Screen
Normally, you should obtain two traces on the crt screen
when the STABILITY control is set for a free-running sweep
(fully clockwise), the MODE switch is set to the ALTERNATE
position, the HORIZ. DISPLAY switch is set to NORM., and
the POSITION controls are set to about midrange. If one
of the two traces is missing under these conditions, there is
an unbalance in the vertical input channel for which there is
no trace. (The channel which controls the displayed trace
can be determined by noting which VERTICAL POSITION
control moves the trace up and down.) In this case, adjust
the DC BAL. control of the faulty channel. If the trace is
obtained, try to set the DC BAL. control according to the
procedure given in the Calibration section of this manual.
If a trace is not obtained, or if it is not possible to set the
control as described, turn to the instructions for trouble­
shooting the Vertical Amplifier later in this section of the
manual.
If only two spots are present when the controls are set
as described in the previous paragraph, and the spots can
be moved with the HORIZONTAL POSITION control, then
the trouble is in the Time-Base Generator. If the spots can­
not be moved with the HORIZONTAL POSITION control,
then the trouble is in the Horizontal Amplifier.
If no spot or trace at all is present on the face of the
crt, the trouble may be in the Power Supply, the Vertical
Amplifier, the Horizontal Amplifier, the Time-Base Genera­
tor, or the Crt Circuit. To determine which circuit is at fault
in this case, set all POSITION controls to midrange, the
HORIZ. DISPLAY switch to the EXT. position, and turn the
INTENSITY control clockwise. If a spot now appears on the
screen, either the Time-Base Generator or the first stage of
the Horizontal Amplifier is at fault.
If a spot does not appear, short the vertical deflection
plates together at the neck pins of the crt.
CAUTION
Be careful not to short the deflection plates to the
metal shield around the crt, or to the GEOM. neck
pin. Also be ready to turn the INTENSITY control
down quickly when a spot does appear so as not
to burn the crt screen.
If a spot now appears on the screen, either the Vertical
Amplifier or the Delay Line is at fault. If no spot appears,
remove the shorting strap from the vertical deflection plate
pins and short the horizontal deflection plates together. If
a spot now appears, the trouble is in the Horizontal Am­
plifier.
If no spot or trace appears during any of the previous
checks, a defect in the Crt Circuit, possibly in the crt itself,
is indicated.
Insufficient or No Vertical Deflection, or W ave­
form Distortion
These troubles are all caused by a defective Vertical
Amplifier and/or Delay Line. Refer to the instructions for
troubleshooting these circuits. If there is reduced deflection
sensitivity both vertically and horizontally, the trouble is
probably in the Crt Circuit.
Insufficient Horizontal Deflection
This condition can be produced by the Time-Base Genera­
tor or the Horizontal Amplifier.
The operation of the Time-Base Generator can be checked
from the front panel. Set the HORIZ. DISPLAY switch to the
NORM, position, the TIME/DIV. switch to .5 SEC, and adjust
the STABILITY control for a free-running sweep (fully clock-
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