CIRCUIT DESCRIPTION
The B & K Model 501A utilizes a total of 8 inte
grated circuits for an equivalent discrete transistor
count of 204 devices.
Three of the IC's are digital and 5 are linear. Each
unit's function is explained in greater detail within
the following circuit analysis.
Refer to Figure 40, the -Block Diagram. Beginning
at the left, a dual-secondary transformer is used for
isolation from the line and to reduce the 117V AC for
low voltage rectification. The low voltage secondary
AC is rectified by diode bridge D5 through D8. It is
center-tapped to produce both positive and negative
voltages with respect to ground, each filtered by
large value electrolytics. The raw ±22 volts thus
obtained are regulated down to three different levels:
+15 Volts, -15 Volts and +5 Volts. Both the+ and
-15 volt regulators are simple zener diodes (D9 and
DlO) which power all operational amplifiers in the
unit. The +5 volt line is highly regulated by use of
an IC (ICl ) and series pass transistor
(Q4).
This IC
possesses virtually all the components necessary to
produce a highly stable and accurate de regulator,
such as the reference source, feedback amplifier,
and output transistor. In addition, it is internally
current-limited so a shorted output condition will not
cause damage. Resistor R l 9 establishes the limiting
value to about 200 mA. The +5 volt line is highly
regulated because it not only powers the digital IC's
but is used as the reference voltage for producing a
precise staircase waveform. Long term stability of
the overall regulator is better than 0.5 % .
Transistors Q5 and QS comprise a pulse former
that produces a narrow pulse each time the AC line
crosses zero. It is this zero crossing that synchronizes
the entire unit. All line noise is eliminated from
these pulses by a pulse shaper, IC2 a, b, and c, ar
ranged in a monostable multivibrator mode with
buffered output. The pulse shaper output is routed
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to a countdown chain composed of 3 flip-flops within
two IC packages (IC3 and 4). The second flip-flop of
IC4 is not used except for gating a different number
of display curves. The three flip-flops would nor
mally count to a maximum of
7
and then start over
if it were not for the reset gate and buffer Dl l, Dl2
and IC2d. The two diodes are factory inserted to
sense the sixth count of the flip-flops and cause the
entire chain to reset to zero (start over again). Only
5 curves are displayed by this arrangement because
the sixth count is never allowed to run its course. A
different number of curves may be displayed with
the unit by appropriately re-arranging the position
of the diodes as indicated under the "Maintenance
and Calibration" section of this manual.
The flip-flop outputs of the count chain drive 3
transistor switches (Q8, 9 and 10) each in series with
a precision resistor (1244, 45 and 46). The solid-state
switches short and open the resistors to the highly
regulated +5 volt line in a binary coded sequence
as determined by the state of each flip-flop. The
other end of the resistors are tied to the inverting
input of operational amplifier IC5 which acts as both
a summing device and current-to-voltage converter.
A negative-going voltage staircase appears at the
output of this amplifier. The level change between
each step is determined by position of the feedback
resistance, CALIBRATE trimmer R36, and is precisely
adjusted for 0.5 volt.
The reference staircase from IC5 is routed thru a
polarity reversal slide switch (front panel POLARITY
switch) which determined which input of the differ
ential amplifier, inverting or non-inverting, will re
ceive the staircase. However, it is inverted again
through the STEP SELECTOR switch in the "Volts
per Step" positions for F.E.T. testing.
The DIFFERENTIAL AMPLIFIER, RANGING RE
SISTORS and ERROR FEEDBACK BUFFER are all
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Figure 40. Block Diagram
32
