Tektronix 511A, Instruction Manual

Page: 67 / 74

CALIBRATOR MODIFICATION
The three-range sine wave amplitude calibrator, for­
merly employed in the Type 51 1-A-AD oscilloscope, has
been replaced with a six-range square wave calibrator.
This improved circuit provides calibration accurate to
within ±2% of full scale.
The calibrator waveform is derived from the line
frequency voltage of a T1 secondary winding. The
positive peak is clipped or limited by means of the
diode connected section of a 12AT7, V16.1. The point
of limiting is determined by the setting of the CAL. ADJ.
potentiometer R75.4. Since the voltage on R75.4 is ob ­
tained from the +225 volt regulated supply, the limiting
point is not affected by line voltage variations over the
operating range of 105 to 125 volts. This limited wave ­
form
is applied
to the cathode follower section of VI 6.1,
which clips the negative peak and provides a low im­
pedance output.
The SIGNAL-CALIBRATE switch, S3, permits selection
of the SIGNAL INPUT, CAL. INPUT (which may be used
as an alternate signal input), or any of six calibrating
ranges from .3V to 100V peak to peak. Continuously
variable control for each range is provided by means
of the CAL. VOLTAGE potentiometer, R76.1.
Refer to Section II, "Calibration", for operating in­
structions which still pertain, except that the calibration
ranges now represent full scale readings of the CAL.
VOLTAGE control.
Do not attempt to check the calibrator accuracy with
an AC voltmeter. If inaccuracy is suspected, the output
should be measured by the comparison method. With
the INPUT CHAN. SEL. in the DIRECT position, connect
a 90 volt B battery to the SIGNAL INPUT, and note the
deflection of the CRT trace. Measure the battery volt ­
age with an accurate 100V scale DC voltmeter. Now
set the SIGNAL-CALIBRATE switch at 100V and the
CAL. VOLTAGE at the voltmeter leading and again
observe the trace deflection. If necessary, adjust R75.4
until
equal deflection is
obtained.
+ 225
CAL.
OUTPUT
CAL.
INPUT
J IN POWER SUPPLY
SIGNAL
CALIBRATE
28 0 V.A.C.
TO Tl
R76.2
13.2 K
R76.3
5.25 K
R76.4
1582
R76.5
416
R76.6
TERMINAL 16
TYPE 511-A, 511-AD