Tektronix 516 series, Инструкция по эксплуатации

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Maintenance — Type 5 1 6
used as cathode followers working into peaking circuits.
Shorted, or partially shorted, peaking coils are another com­
mon source; hot solder spilled on a peaking coil can cause
this condition.
An "overshoot" such as shown acccentuated in Fig. 5-4 (b),
is the result of excessive high-frequency peaking. This can
be caused by a type of tube distortion known as cathode
interface. The first thing to check, therefore, for this type of
distortion is the tubes. If tube replacement does not correct
the trouble, the variable peaking coils in the Vertical Am­
plifier may need adjusting. Refer to the Calibration section
of this manual for the adjustment procedure.
The "wrinkle" condition, illustrated in Fig. 5-4 (c), is pro­
duced by an improperly adjusted Delay Line. Refer to the
Delay Line Adjustment procedure in the Calibration section.
Improper Triggering.
If the Time-Base Generator trig­
gers normally in the EXT. and LINE position (+ or —), but
does not trigger properly in the INT. position, a defect in
the Trigger Pick-Off Circuit is indicated. In this case, the
trouble would be either a defective Trigger Cathode Follow­
er, V493, or a defective resistor in the circuit.
Troubleshooting the Time-Base Trigger
If the sweep cannot be triggered, and if the Trigger-
Pickoff circuit in the Vertical Amplifier has been eliminated
as the source of trouble, the Time-Base Trigger is at fault.
(This assumes that the trace can be turned on and off with
the STABILITY control.) The first thing to do is to replace
both tubes in the Time-Base Trigger and try to recalibrate
the circuit according to the procedure given in the Calibra­
tion section of this manual. If this does not correct the trouble,
then trouble in the circuitry is indicated.
To check the quiescent state of the circuit, set the red
TRIGGER SELECTOR control to AC, the black TRIGGER SE­
LECTOR control to —INT., and the TRIGGERING LEVEL con­
trol to 0. Next, connect a jumper wire between the junction
of R19, R20, and R21 and ground (see Fig. 6-6). This fixes
the voltage at the grid of V24B at ground potential. Then
measure the voltage at the plate of V24B (pin 1); it should
be about + 80 volts. If this voltage does not measure close
to + 80 volts, replace tube V24 again. Then, if necessary,
check for such things as off-value resistors, broken leads, and
poor switch contacts.
The next step is to measure the voltage difference be­
tween the two grids of V45 (pins 2 and 7). Since connecting
a voltmeter at the grid of V45A (pin 2) may produce an
adverse loading effect, it is recommended that this measure­
ment be made between the plate (pin 1) of V24B and the
grid (pin 7) of V45B. With the TRIG. LEVEL CENTERING
adjustment, you should be able to bring the voltage at the
grid of V45B to within 4 or 5 volts of the voltage at the
plate of V24B. This indicates that the hysteresis of the Trig­
ger Multivibrator can be set at the proper level with respect
to the grid of V45A.
If the voltage at the grid of V45B cannot be brought to
within 4 or 5 volts of the voltage at the plate of V24B with
the TRIG. LEVEL CENTERING adjustment, trouble in the Trig­
ger Multivibrator is indicated. In this case, replace V45
again; then, if necessary, check the rest of the circuit for
off-value resistors or other troubles.
Remove the jumper wire between the junction of R19, R20,
and R21 and ground.
Troubleshooting the Time-Base Generator
No Horizontal Sweep. If the Time-Base Generator is not
producing a sawtooth waveform when the STABILITY control
is set fully clockwise, 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 measuring the plate
voltage of the Miller tube, V I61A pin 6.
NOTE
A ll voltages in this section should be measured with
a 20,000-ohm s-per-volt voltmeter or a vacuum-tube
voltmeter.
The voltage reading obtained at the plate of V I61A will
probably be approximately +260 volts, or approximately
+30 volts. A reading of +260 volts indicates that the Miller
Runup Circuit has run up and has not been reset, while a
reading of +30 volts indicates that the Miller Runup Circuit
is not being allowed to run up. The condition that exists
will depend on the type of trouble. The two conditions of
plate voltage will be handled separately in the following
paragraphs.
High voltage at the plate of the Miller tube, V161A, in­
dicates that the tube is cut off. If this is the case, momen­
tarily ground the grid of the tube while monitoring the plate
voltage. If the tube is good, the plate voltage will drop to
about + 10 volts. Remove the ground as soon as the reading
is taken. If the Miller tube is found to be good, measure the
voltage at its grid. If this voltage is more than 20 volts
negative, V152B is probably not conducting. In this case,
check V I52, R147, and R148.
If the voltage at the grid of V I61A is not more negative
than —20 volts (it should be about —5 volts), measure the
voltage at the cathode of V I61B. If this voltage is approxi­
mately +200 volts, the Runup Cathode Follower stage may
be assumed to be operating correctly. If this voltage is
significantly lower than +200 volts, the stage is defective,
and its grid and cathode circuits should be checked.
If the Runup Cathode Follower is found to be operating
properly, measure the voltage at the cathode of V183A. If
this voltage is more positive than —45 volts, the trouble is
in the Sweep-Gating Multivibrator. Check the tubes and
resistors in this circuit. The voltage divider network in the
cathode circuit of V135B is particularly critical.
If the voltage at the cathode of V183A is more negative
than —55 volts, check the tube in the Hold-Off Circuit, the
Hold-Off capacitors, and the resistors in the cathode circuits
of the tube.
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 run-up operation. If this trouble exists
on only a few ranges of the TIME/DIV. control, the trouble
is probably an open timing resistor. If the trouble exists on
all ranges of the TIME/DIV. control, the trouble is probably
due to a defective Sweep-Gating Multivibrator.
5-8