Tektronix 516 series, Instruction Manual

Page: 34 / 132

Maintenance — Type 5 1 6
wise). Connect a voltmeter between the SAWTOOTH OUT
connector and ground. If the voltage varies between zero
and +150 volts ± 1 5 % , as the Miller circuit runs up and
back, the Time-Base Generator is operating correctly. This
means that the trouble is in the Horizontal Amplifier. No
voltage variation at this point, or a variation of significantly
less than 150 volts, indicates trouble in the Time-Base Gen­
erator circuit.
Nonlinear Horizontal Sweep
The linearity of the horizontal deflection circuit can be
checked by connecting a marker generator or the Calibra­
tor output to the Vertical input of the oscilloscope. Set the
MODE switch to correspond to the Vertical Channel being
used, and adjust the Time-Base controls for a stable display.
If the displayed markers, or the square waves produced by
the Calibrator, are not symmetrically spaced across the crt,
a nonlinear sweep is indicated. This condition can be caused
by nonlinear amplification in the Horizontal Amplifier, or by
the generation of a nonlinear sawtooth in the Time-Base
Generator.
To determine which circuit is at fault, connect a jumper
wire from one of the Vertical input connectors to the SAW­
TOOTH OUT connector.
CAUTION
To avoid shorting the 150-volt sawtooth to the
chassis, connect the jumper wire to the input con­
nector before connecting it to the SAWTOOTH
OUT connector.
Set the appropriate VOLTS/DIV. switch to 20, and adjust
the STABILITY control for a free running sweep (fully clock­
wise). If the TIME/DIV. control is set to a rate of 1 mSEC or
faster, a steady diagonal trace will be seen on the crt. This
trace is the sweep portion of the sawtooth voltage produced
by the Time-Base Generator. If the slope of the trace is
constant, the nonlinearity is being produced in the Time-Base
Generator. If the slope of the trace is not constant, the non­
linearity is being produced in the Horizontal Amplifier. Re­
fer to the instructions for troubleshooting the Horizontal Am­
plifier.
Improper Sweep Timing
If the sweep timing is off in some, but not all, positions
of the TIME/DIV. control, 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/DIV. con­
trol, the Horizontal Amplifier is probably the circuit at fault.
However, it is important that the Power Supply voltages be
checked first. Check to see if the timing circuits can be cali­
brated in accordance with the instructions presented in the
Calibration section of this manual. If the circuits cannot be
adjusted for correct timing, then refer to the instructions for
troubleshooting the Horizontal Amplifier.
Improper Triggering
The most probable cause of poor triggering (as compared
with no triggering at all) is lack of calibration of the Time-
Base Trigger. The first thing to check in the event of poor
or erratic triggering, then, is the calibration of the Time-
Base Trigger. Procedures for calibration are given in Sec­
tion
6.
If the oscilloscope will not trigger at all, and yet a free-
running trace can be produced by turning the STABILITY
control fully clockwise, trouble exists in the Trigger Pickoff
circuit of the Vertical Amplifier or in the Time-Base Trigger
itself. If only the internal triggering is absent, the trouble
is in the Trigger-Pickoff circuit or in SW10A in the Time-
Base Trigger. If all triggering (internal, external, and line)
is absent, then the trouble is in the Time-Base Trigger.
If stable triggering cannot be obtained and the free-running
trace cannot be turned off with the STABILITY control, the
trouble lies in the Sweep-Gating Multivibrator of the Time-
Base Generator.
CIRCUIT TROUBLESHOOTING
This portion of the Troubleshooting procedure contains
information for locating a defective stage within a given
circuit. Once the stage at fault is known, the component or
components causing the trouble can be located by tube and
component substitution or by voltage and resistance meas­
urements.
Tube failure is the most prevalent cause of circuit failure.
For this reason, the first step in troubleshooting any circuit is
to check for defective tubes, preferably by direct substitution.
If replacement of a defective tube does not correct the
trouble, then check to see whether components, through
which the tube draws current, have been damaged. Shorted
tubes will often overload plate-load and cathode resistors.
These components can often be located by a visual inspection
of the circuit. If damaged components are not apparent, it
will be necessary to make measurements or other checks
within the circuit to locate the trouble.
Troubleshooting the Power Supply
Proper operation of every circuit in the Type 516 Oscil­
loscope depends upon proper voltages from the Power Sup­
ply. The voltages must remain within their specified toler­
ances for the instrument to maintain its calibration.
The regulated supply voltage busses are identified by
color-coded wires, following the standard color code. The
— 150-volt bus is coded brown-green-brown; the +100-volt
bus is coded brown-black-brown; and the +300-volt bus
orange-black-brown.
The widest stripe always identifies
the first color in the code.
No Output V oltage.
If there are no output voltages from
the Power Supply, note whether the graticule lamps are
lighted. If they are, then the trouble is in the Power Supply.
5-5