Power Supply; Inverter And Regulator - Tektronix 213 DMM Instruction Manual

The crt filament current is rated for operation within a plus
or minus 5 percent tolerance. Since the power supply is a
pulsed inverter type, direct connection of the filament to
the inverter transformer would result in supply current var-
iations greater than 5 percent. Inductor L537 provides a
series impedance which varies with the supply frequency
and keeps the supplied current within the required 5 per-
cent tolerance.
The cathode of the crt is the directly heated type and is
maintained at -1000 volts plus or minus 1 percent by the
high-voltage power supply.
FOCUS control R565 is used to compensate for differences
between individual crt's and has an adjustable range of about
200 to 340 volts more positive than the cathode. VR535
holds one end of R565 at -800 volts while current through
R530 and R532 set the other end of R565 at about -600
volts. When the INTENSITY control is set so the crt is cut
off, the focus circuit draws about 90 microamperes. The As-
tigmatism control R575 is used in conjunction with the
FOCUS control to adjust for the best spot definition.
Intensity control for the crt is provided by R560. Its opera-
ting potential is provided by the difference between these-
venth and eighth stage of high voltage multiplication which
is about 175 to 185 volts. When the intensity grid (pin 3,
V560) is approximately 105 volts more negative than the
cathode, the crt will be cut off. Since grid characteristics
vary from crt to crt, R535 is added as a means of limiting
beam current. When R560 is set for maximum intensity,
R535 is set for a maximum beam current of 200 microam-
peres. This is accomplished during calibration by measuring
the voltage across R527. With a beam current of 200 micro-
amperes and a focus current of 90 microamperes, there
would be a voltage drop of 0.29 volts across R527. To pre-
vent exceeding the beam current of 200 microamperes,
R535 is actually set for a 0.28 volt drop across R527.
The crt has an internal graticule; therefore, trace alignment
is accomplished internally with R570 and a coil around the
outside of the front portion of the crt. R570 sets the cur-
rent through the coil which establishes a magnetic field.
This field interacts with the crt beam and allows the trace
to be rotated on the crt axis about 1 degree beyond level
in either direction.
Blanking is accomplished with blanking deflection plates,
and R580 is used to properly center the beam when the crt
is unblanked. When blanked, the plate connected to the
Z-Axis Amplifier is at about+ 7 volts causing all beam cur-
rent to be absorbed by that plate. When unblanked, the
Z-Axis Amplifier plate os at about +4 7 volts as set by V R395.
Resistor R580 is also set at about +4 7 volts; therefore, the
beam is centered which causes unblanking.
Deflection is accomplished with internal deflection plates
driven directly by the vertical and horizontal output ampli-
REV. A, MAR. 1975
Theory of Operation-213 Service
tiers. Each graticule division of vertical deflection requires
about 12.8 to 14.1 volts difference between the plates,
while each graticule division of horizontal deflection re-
quires about 8.2 to 9.2 volts difference between the plates.
POWER SUPPLY
~ ~
The power supply consists of the batteries, an inverter, a
regulator, a low-voltage shutdown circuit, a low-voltage
rectifier-filter section, and a high-voltage power supply.
From the 2.4 volts provided by a pair of nickel-cadmium
cells, operating voltages of 0.6 volts ac, and +6.5, -6.5, +15,
+75, and -1000 volts de are produced. The +15 volts, which
is used throughout the instrument as a reference, is the most
critical, and is regulated within 1 percent. Whenever battery
voltage falls to a predetermined level, below which battery
damage may occur, a low-voltage shutdown circuit causes
instrument shutdown.
Inverter and Regulator
The inverter is a switching regulator type and consists of
0460, 0465, 0470, 0474, 0475, 0480, 0484, 0486,
0488, and T490. Refer to the detailed block diagram of
the power supply, Figure 3-8, for the following explanation
of switching regulator operation. Switch transistor 0488 is
turned off and on by the pulsed waveform duty cycle from
the astable multivibrator 0470 and 0475, which is con-
trolled by the +15 volt supply. When 0488 is turned on,
current flow through the primary of T490 causes an ener-
gy build up. When 0488 turns off, this energy build up col-
lapses which causes a flyback effect that transfers energy to
the secondary of T490. The amount of energy built up is
proportional to the on-time of 0488 in relation to its cycle
time which is controlled by the frequency of the astable
multivibrator. The frequency of the multivibrator is set by
the amount of current flow through the switching regulator,
which is controlled by the voltage at the +15 volt output.
Refer to the power supply schematic for the remainder of
the inverter description.
As battery voltage decreases, the operating voltages also
decrease. Any decrease in the +15 volt supply is reflected
at the base of 0465 reducing its current flow, which causes
a corresponding reduction of current flow in 0474. Reduc-
ed current flow in 0474 increases the discharge time of
C470 which increases the off-time of 0470 and 0480. This
increases the on time of 0488 and allows a higher energy
build up in T490. When this high energy build up collapses,
the transferred energy is higher, thus returning the operating
voltages to their proper level which is initially set during
calibration by R467.
The approximate operating frequency of the astable multi -
vibrator is 60 kilo hertz at the highest battery voltage and
20 kilohertz at the lowest battery voltage. The on-time of
0470 (or off-time of 0488) remains fairly constant at about
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