Tektronix 585 - Page 45

Circuit Description — Type 585
The negative lead of the + 350-volt rectifiers is connected
to the + 180-volt unregulated bus of the +100-volt power
supply. Thus both the + 225-volt and + 350-volt regulated
supplies are elevated on the + 180-volt unregulated bus.
Good engineering practice does not permit using a semi 
conductor two diode full-wave rectifier system because line
voltage transients can exceed the peak inverse voltage rating
of the diodes. Thus an advantage of the two voltage system,
as used with the + 225-volt and + 350-volt regulated sup 
plies, is that capacitor C730 protects all diodes from high
peak inverse voltage transients.
Reference for the 4-350-volt supply is a voltage located
at a point near ground obtained from the divider R710-
R711. V724 essentially compares the reference voltage to
ground. The operation of the regulated circuit is the same
as that described for the 4-100-volt supply.
+5 00-Volt Supply
Rectified voltage from terminals 20 and 21 of T601 is
added to the regulated 4-350-volt supply to furnish power
for the + 500-volt regulator. Reference for the + 500-volt
supply is a voltage located at a point near + 350 volts
obtained from the divider R740 R741 . V754 essentially com 
pares this voltage to the + 350-volt supply. The regulator
action of this circuit is the same as that described for the
+ 100-volt supply.
+ 12.6-Volt Supply
A transistorized 4-12.6-volt d c regulator supplies voltage
for use within plug-in units of the Type 585 Oscilloscope.
Rectified voltage from terminals 33 and 34 of T601 is
used both by the Time Delay Relay K600 with K601, and
by the + 12.6-volt Regulator.
Reference voltage for the + 1 2.6-volt supply is at the
center point of four resistors in series-parallel between the
+ 100-volt supply and ground, R781 A and B, and R782-R783.
The reference voltage is applied directly to the base of
amplifier Q774 where it is essentially compared to the
+ 12.6-volt bus. Emitter follower Q793 provides current
gain to correction signals amplified by Q774, controlling the
Collector-to-Emitter resistance of Q797 to maintain the proper
voltage of the + 1 2.6-volt supply lead.
The + 12.6-Volt Supply voltage tolerance is not as rigid
as the other regulated supplies, and its output may go as
high as + 13.25 volts under small load conditions.
In order for the + 12.6-volt supply to warm up tube
heaters before the Time-Delay Relay closes, a special turn-on
voltage is applied to the base of Q774 via R780 from the
+ 180-volt unregulated supply.
To protect the power transistors in the event Q774 is
removed from its socket, diode D792 will clamp the base
of Q793 to the + 20-volt unregulated supply. Otherwise
the base of Q793 would rise toward the + 180-volt unregu 
lated bus and damage both itself and Q797.
Time-Delay
A Time-Delay relay K600, in conjunction with relay K601,
delays the application of power supply voltages to the
oscilloscope tubes for about 15 to 45 seconds. This delay
allows the tubes to warm up before operating potentials are
applied.
CRT CIRCUIT
Cathode-Ray Tube Control Circuits
The INTENSITY control R826 varies the voltage at the grid
of the c rt to control the beam current. The FOCUS control
R856 varies the voltage at the focusing anode to focus the
trace. The ASTIGMATISM control R864 varies the voltage at
the astigmatism anode to adjust principally the spot vertical
dimension. The GEOM. ADJ. R861 varies the field the beam
encounters as it passes from the vertical plates to the hori 
zontal plates to control the linearity at the extremes of de 
flection. The VERT. SHIELD VOLTS ADJ. control R860 permits
minor changes of c rt deflection sensitivities and linearity.
High-Voltage Supply
A 60-kc modified Hartley oscillator furnishes power for the
three power supplies that provide accelerating potentials
for the c rt. The main components in the oscillator circuit
are the pentode V800 and the primary of T801 tuned by
C808.
A half-wave rectifier V862 provides — 1350 volts for the
ert cathode. V862 supply is the reference supply, and is the
only one of the three that is firmly regulated. A half-wave
voltage tripler circuit, V832, V842 and V852, provides + 8650
volts for the post deflection accelerator anode. The two
supplies provide a total accelerating voltage of 10,000
volts. Both supplies are referenced to the + 100-volt regu 
lated supply through the decoupling filter R801-C801.
A floating half-wave rectifier V822 furnishes bias voltage
of — 1450 volts for the c rt grid. This floating grid supply,
independent of the cathode supply, allows dc-coupled un 
blanking to the c rt grid. All three supplies employ capacitor 
input filters.
Reference for the — 1350-volt cathode supply is a voltage
near — 150 volts obtained from a tap on the divider net 
work R840 through R844. The resistance ratio of the divider
is determined by the setting of R840 labeled HV. ADJ. The
reference voltage is essentially compared to — 150 volts by
V814B. When R840 is properly adjusted, the voltage at the
HV ADJ. Test Point will be — 1350 volts.
If variations in load change the voltage on the — 1350-
volt bus, an error signal will exist between the grid and
cathode of V814B. The error signal will be amplified by
V814B and V814A; the plate of V814A varies the screen
voltage of the oscillator tube V800, thereby controlling its
output.
The +8 650-volt supply and the negative bias supply are
regulated indirectly, as the output voltage of all three sup 
plies is proportional to the output of the oscillator circuit.
4-15