Tektronix 511 Instruction Manual page 31

high, but should become normal after several
hours operation. Occasionally, a Type NE2 lamp
will exhibit a tendency to oscillate. This condi­
tion is evidenced by a " spot" on the horizontal
trace, usually near the sweep starting point.
When a sine wave such as the Calibrating
Voltage is applied to the CRT vertical deflection
plates, the " spot" will assume the form of a
damped wave train. If this trouble is encoun­
tered, short circuit the neon lamps, one at a
time, until the oscillation disappears, thus iso ­
lating the defective lamp.
CAUTION--LEAKAGE
MAY RESULT IF THE NEON LAMP
LEADS ARE BENT TOO CLOSE TO THE
GLASS.
2. Sweep Generator.
sweep generator is indicated when the multivi­
brator output is normal (fig. 4-12) and the saw­
tooth at the plate of V9 (fig. 4-21) is not. If
the sawtooth amplitude is low (slow sweep
speed), V9 is known to be good, and circuit
voltages appear normal, remove V9 and measure
the voltage at terminal 4 of V9 socket, which
should be between +5 and + 30 volts. This mea­
surement is made with the SWEEP STABILITY
control fully counterclockwise.
Error in sweep speed on one range only maybe
due to a defective timing capacitor, C15A, B,C,
D or E. The capacity of C15E is selected from
-3% to +3% of its design center and this value is
stamped on its case. C15B,CandDare also s e ­
lected to + or-3%, and then so matched to C15E
that their capacity deviates from design center
by the same amount and in the same direction as
that of C15E. If it is necessary to replace one or
more units, observe the capacity of C15E and
select a replacement having the same percent
and polarity of deviation from design center. If
the exact capacity is not available, this may be
accomplished by employing a capacitor of slight­
ly lower than the required value and padding it
up to the proper capacity. To preserve linearity
on the slowest sweeps, use only a mineral oil-
filled capacitor for C15E.
If the indicated sweep speed is accurate with
the SWEEP SPEED MULT, dial set at 1.0 (in
instruments S/N1406 and up), but inaccurate
at other settings suspect a change in resistance
of R20D, R20D1-9 or a short or ground at
some point along the step control. In the event
R20D must be replaced it should be ordered
AT THE SEAL
Faulty operation of the
T Y P E 51 1 - M A I N T E N A N C E
from the factory and will be supplied with an
individually calibrated dial.
3. Multivibrator. It is important that the multi­
vibrator output be of normal amplitude and free
from frequency modulation. Low amplitude of the
negative square-wave output will be evidenced
by short sweep length, and is usually caused
by a poor 6AC7, V4. This condition is more
likely to appear when the Type 511A is operated
at low line voltages, and therefore reduced
heater temperature of V4.
Frequency modulation of the multivibrator
output at line frequency is sometimes introduced
by a defective 6AG7, V5. This condition makes
it difficult to synchronize high frequency s ig ­
nals. If poor synchronizing is encountered,sev­
eral type 6AG7v s should be tried as new tubes
often have this defect.
Poor tracking of the C, C l -9 and D, Dl-9
sections of R20, the SWEEP SPEED MULT,
control, may also cause short sweep length.
A resistance check should reveal any trouble
of this nature.
Trigger Circuits
Faulty triggering of the sweep may be caused
by insufficient amplitude of the trigger impulse
or the introduction of spurious signals to the
multivibrator.
If triggering is erratic, it is desirable to
first eliminate the possibility that instability
of the signal being observed is the cause.
Possible internal causes are: arcing in the high
voltage supply, introduction of a 60-cycle jitter
due to defective tubes V16, V17, VI, V2 or V3,
excess power-supply ripple, and insufficient
trigger amplifier gain.
The Type 511A should trigger on signals of
.15 volt or less applied to the TRIGGER INPUT
except on the 1 microsecond range, which may
require slightly larger trigger input. The trig­
ger sensitivity may be quickly measured as
follows:
Connect a short wire between the CAL. OUT­
PUT and the TRIGGER INPUT binding posts,
set the sweep speed at 10,000 microseconds,
the SIGNAL-CALIBRATE switch at .3 v (X.l,
S/N455-2405), the CAL. VOLTAGE at 0, the
TRIGGER SEL. at + EXT., the TRIGGERAMPL.
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