Tektronix 543B Instruction Manual page 13

Circuit Description — Type 543B /R M 543B
terrelated supplies that operate together as a system. This
system delivers filtered and regulated voltages of —150,
+ 100, +225, +350, and +500 volts as well as an un­
regulated dc voltage of +325 volts. A common power trans­
former, T601, supplies the input power to each of the sup­
plies, as well as heater power to thermal time-delay relay
K600 and the tubes in the oscilloscope. Unless otherwise
specified, the Type 543B is shipped with T601 wired for
115-volt ac input. A connection diagram on the side of the
transformer shows alternative connections for other input
voltages. An optional ac converter is available to provide
60-cyclc power for the fan motor if it is desired to operate
the oscilloscope on line frequencies from 50 to 60 cps and
400 cps.
The 115-volt ac input power is applied to T601 through
POWER ON switch SW601.
vided by fuse F601. Thermal cutout TK601 in the primary
circuit of T601 is a protective device that opens the trans­
former primary circuit if the temperature inside the oscillo­
scope rises above a safe level. TK601 resets automatically
when temperatures return to normal; and to shorten the
cooling time, the fan continues to run while TK601 is open
(except when T601 is connected for 216-, 230-, or 244-volts
operation). Thermal time-delay relay K60G provides a fila­
ment warmup time of approximately 30 seconds before
the dc power supplies are activated. The heater of K600
is rated at 6 volts and is connected to 6.3 volts on the T601
secondary winding. During heater warmup time, conra+s
4 and 9 of K600 remain open. At the end of heater warm­
up time, contacts 4 and 9 close and apply power to mag­
netic relay K601. Contacts K601-1 of K601 remove the
heater power from K600, but before K600 can open, con­
tacts K601-1 lock the holding circuit to the coil of K601.
K601 now remains energized until the power to the oscillo­
scope is switched off or otherwise interrupted. When K601
is energized, contacts K601-2, K601-3, and K601-4 are also
closed and thus activate their respective dc supplies.
— 150-Volt Supply. The -150-volt supply in the Type
543B is the reference voltage source for the other supplies
and must be very stable. The —150-volt supply includes a
high-gain electronic voltage regulator designed to give good
regulation under extreme operating conditions. This regu­
lator circuit contains a series regulator, a glow-discharge
tube reference source, an error detector, and an amplifier.
In operation, the input power to the -150-volt supply
is supplied by one secondary winding (pins 6-11) of T601.
The ac output of the secondary winding is rectified by
silicon-diode rectifier bridge D642 and filtered by C640. In
series with the positive side of the supply and ground are
series regulators V627, V637 and V647, paralleled by shunt­
ing resistor R647. The output of the —150-volt supply is
taken from the negative side.
Error sensing in the volfage-regu'ator circuit is accom­
plished by comparator V624. Current flow through V624
is established by the setting of the tap on R616 in the volt­
age divider R615, R616, and R617. The voltage on the grid
of V624A is held at approximately —85 volts by reference
stage V609. Assuming that the output voltage of the -1 5 0-
volt supply increases due to increased line voltage or some
other cause, the voltage increase appears on the cathodes
of V624 and, through the top on R616, on the grid of V624B.
Due to the voltage divider, only a part of the voltage in­
crease appears between the grid and cathode of V624B,
3-2
Overload protection is pro­
but the full change appears on the grid and cathode of
V624A. The increase is in the negative direction, therefore.
V624A increases its conduction to maintain the proper bias
between grid and cathode, and this holds both cathodes
more or less fixed while the grid of V6248 is pulled negative
by the increasing negative voltage across the voltage
divider. The increasing negative voltage on the grid of
V624B causes a decrease in current; thus the plate voltage
goes positive.
The positive change in plate voltage is amplified and in­
verted to a negative change by amplifier V634. The am­
plified error signal from V634 is applied to the grids of
series regulators V627, V637 and V647. The negative-going
error signal on the grids of V627, V637 and V647 decreases
the current through the tubes, effectively increasing their
resistance and the voltage drop across them. The voltage
necessary to provide the increased drop across the series
regulator tubes and shunt resistor can only be obtained
by subtracting it from the negative side of the supply, so
the undesired increase in negative voltage is absorbed in
the series regulators and shunt resistor. If the output of the
—150-volt supply has decreased instead of increased, then
the error voltage applied to the grids of the series regula­
tors would have been positive-going.
error voltage on the grids of the series regulators would
lower the resistance of the series regulator tubes, and the
voltage drop across them would decrease, leaving more
voltage for the negative side of the supply. Since the out­
put voltage of the —150-volt supply depends upon the re­
lationship of the voltage on the tap of R616 and fhc ref­
erence voltage from V609, accurate adjustment of the out­
put voltage is provided by making R616 variable.
Filter capccitor C640 does not remove all the ripple from
the output of the bridge rectifier, and the series regulator
circuit functions a:so to reduce this output ripple voltage.
Any ripple between the —150-volt output point and ground
reaches the grid {pin 7) of V624B via capacitor C617. This
input ripple voltage is amplified by V624 acting as a cath­
ode-coupled amplifier. The ripple output voltage at the
plate (pin 6) of V62^B has the same polarity as the ripple
voltage at the —150-volt output. C628 couples this ripple
output voltage to the grid of V634. The ripple voltage is
further amplified by V634 and applied to the grids of the
series regulator tubes with a polarity that opposes the
original ripple voltage. Ripple in the positive side of the
— 150-volt supply is coupled into a degenerative feedback
loop through R637 to the screen of V634.
Some of the components in the —150-volf supply are not
necessary in normal operation bur are included to insure
proper operation of the circuit under adverse conditions.
R640 and R641 protect against large surge currents, while
C649 suppresses sudden !oad changes that fall outside the
bandwidth of the regulator circuit.
+ 100-Volt Supply. The input to the +100-volt supply is
the output of fhc secondary winding (pins 8-15) of trans­
former T601 and silicon-diodc bridge D672. In addition to
its other loads, the + 100-volt supply is required to supply
current to a series string of filaments at all times. When
the Type 543B is first turned on, relay K601 contacts are
open and all the regulated supplies are inoperative. During
this time, the series-string filaments arc supplied by the
unregulated side of the —100-voif supply through relay
contacts K601-3 and R675. By the time thermal relay K600
The positive-going