Section 3 Theory Of Operation - Tektronix PG 506 Instruction Manual

Section 3-PG 506 (SN 8040000 & up)
THEORY OF OPERATION
Primary Power
0
The 120 V dc supply is the main power source for the
Standard Amplitude system.
The + and - 16.5 V dc supply is the main power source
for the Digital Voltmeter circuitry, the Fast Rise stages,
and two operational amplifier circuits in the Standard
Amplitude system.
The -72 V, variable dc supply is the mai n power source
for the High Ampl itude section . This supply can vary from
about -10 V to about - 72 V, dependent upon operating
conditions.
All of the above dc supplies are produced by conven­
tional full-wave bridge rectifier circuits that are driven by
an inverter system that changes a dc voltage to ap­
proximately 25 kHz power in the primary and secondaries
of T130. Each supply is switched on or off, dependent
upon the operating modes.
The 5.2 V dc supply is derived from a 11 .5 V dc source
in the power module and is distributed mainly to the
Period Generator, Counter circuits, and certain logic
gates. This supply is also used as a return for the High
Amplitude circuits.
CR10 and CR11 , together with ClO, convert 25 V ac
(rms) from two transformer secondaries in the power
module to about 35 V dc. VR 1 0 sets and regulates the base
voltage of emitter-follower 0 15 to about 15 V, establ ishi ng
a fixed 14 V supply for the 25 kHz free-running mul­
tivibrator (090-0100).
The free-running multivibrator collector output has a
peak-to-peak amplitude of about 5 V, and the positive
swing is limited to about +5.8 V by CR86 and CR105. This
signal drives the bases of 085 and 0120 for the inverter
system . The feedback connections from the collectors of
080 and 0125, through CR80 and CR125, ensures that
both transistors are never on at the same time.
The maximum voltage swing at the collectors of 080
and 0125 is about twice the dc level established at the
junction of L35 and C36 in any operating mode. For the
High Amplitude mode, this dc level is dependent on the
@
setting of the PULSE AMPLITUDE control and the exter­
nal load that terminates the PG 506. With a 1 MO (unter­
mianted) load, the dc level will be about 3.5 V for the MIN
position and about 20 V for the MAX position.
Remote voltage sensing to regulate the -72 V variable
supply originates in the High Amplitude circuit and is
applied through CR27 to pin 4 of voltage regulator U20.
When in the Standard Amplitude or Fast Rise mode, the
j unction of L35 and C36 is about 20 V dc, with the -72 V
variable supply disconnected from the High Amplitude
circuitry.
Voltage regulation for the Standard Amplitude and Fast
Rise modes is dependent upon the peak voltage (about
10 V) developed across C75 by the half-wave rectifier
action of CR78, which obtains its ac voltage from a sense
winding of T130. The peak level across C75 is applied to a
voltage divider composed of R31 , R30, and R29. The
quiescent level set on pin 4 of U20 by the adjustment of
R30 determines the quiescent current through the NPN
seri es-pass transistor. Pi n 4 of U20 is the i nverti ng input
terminal for an internal comparator, and any voltage
change on pin 4 causes a voltage change in the opposite
direction on pin 10. A potential difference of 33.0 Vacross
the + and -16.5 V supplies is accomplished by adjusting
R30.
Current limiting for the 35 V dc input is controlled by
the voltage drop across R22. If pin 2 of U20 goes about
0.6 V more positive than pin 3, pi n 10 goes negative to limit
current through athe NPN series-pass transistor and the
load. CR22 protects a transistor internal to U20. C22
frequency compensates the voltage regulator . VR30 is not
normally on; it protects tI-,e supply from over-voltage
conditions if the potential difference across it exceeds
12 V.
U20 sets its own reference voltage of about 7 Von pi n 6,
with pin 5 being the non-inverting input to an internal
comparator. The reference voltage on pin 6 is divided
down by R40 and R42 to set a reference level of 5.2 Von
pin 2 of error amplifier U50.
Voltage regulation of the 5.2 V supply is accom plished
by comparing the voltage level on pin 3 of U50 with the
3-1