Tektronix 485, Instruction Manual

Page: 25 / 197

Circuit Description—4 8 5 /R 4 8 5 Service
more negative than —200 mV, the output will generate an
error to stop the Inverter. Under normal operating condi­
tions, current from positive supplies to the Balance Node
will equal current to the negative supplies, allowing pin 2 to
operate near ground, typically within 50 mV. If any supply
is shorted the currents will be unbalanced, causing the
voltage at pin 2 to shift into the error region. Shorting a
positive supply moves the Balance Node negative, and vice
versa.
TP1958 (HV) is the high voltage current sense point.
Normally the signal at TP1958 does not have sufficient
amplitude to cause conduction in CR1958 and therefore
has no effect on the Balance Node. If excessive current is
drawn from the cathode multiplier or the H.V. winding in
T1960, the amplitude at TP1958 will increase causing
CR1958 to conduct, producing an error which stops the
Inverter. The DC level at TP1953 (LIM ) will normally be at
+15 V when the intensity controls are off. As the intensity
is increased, CRT beam current w ill increase, causing
TP1959 to move toward ground. If TP1959 goes more
negative than —.7 V, CR1959 will conduct, causing an error
at the Balance Node, which stops the Inverter. This will
only occur when the normal beam lim it circuit is in­
operative.
Overcurrent Protection
Pin 13 of U1910 is the input to the Inverter Current
Regulator circuit. If a semi-regulated supply is shorted, the
regulator circuit allows the inverter current to increase to
the lim it value and remain there throughout the sampling
period. A t the end of the sampling period, pin 8 of U1910
will go positive, stopping the Inverter.
Low Line Voltage Protection
If the line voltage drops significantly below the mini­
mum specified value, the Inverter will not maintain the
correct voltages at the semi-regulated supply outputs. When
this occurs, an error w ill be generated by the voltage
regulator, which will cause pin 8 of U1910 to go positive,
stopping the Inverter. The Line Stop Circuit will also stop
the Inverter if the line voltage is low (see Line Stop Circuit
Description).
Sampling Period Timer and Overvoltage Protection
When an error is present at the input to the OR circuit,
(see Fig. 3-5) a current is generated which charges C l 912
thru pin 1 of U1910. The time it takes pin 1 to reach
+0.7 V determines the sampling period. When pin 1 reaches
+0.7 V, pin 8 of U1910 will go positive, stopping the
Inverter. Over-voltage on the +120 V supply will cause
VR1912 to conduct, charging C1912 which stops the
Inverter.
CR T C IR C U IT
General
The CRT Circuit produces the high voltage potentials
and provides the control circuits necessary for the opera­
tion of the cathode-ray tube (CRT). This circuit also
includes the Z-Axis amplifier and the Auto-Focus ampli­
fier. Fig. 3-7 shows a detailed block diagram of the CRT
circuit. A schematic of this circuit is shown on diagram 13
in the rear of this manual.
Filament Voltage
Filament voltage for the CRT heaters is provided by a
separate winding on T1960. The filament voltage is elevated
to cathode potential through R1687 and decoupled by
Cl 687. Short circuit protection is provided by DS1687.
High Voltage Supplies
A semi-regulated voltage for operation of the high
voltage supplies is provided by the high voltage winding of
T1960. One end of T1960 is connected to ground through
the high voltage current sensing resistor R2094. A 3 kV
peak-to-peak square wave is generated and provides the
power necessary to operate the Anode Supply, Cathode
Supply, and DC Restorer Circuits.
Anode Supply. The Anode Supply consists of 6X
multiplier assembly U1600.
Cathode Supply. The —3000 V CRT cathode voltage
(marked —2950 in some instruments) is generated by a
2X m ultiplier consisting of CR1601, CR1602, C1601, and
C1603. R1611 and C l611 provide high frequency filtering.
R1612 and C1612 provide high frequency filtering and an
AC coupling path for the cathode regulator.
Cathode Regulator. The cathode regulator maintains the
cathode at —3000 V and reduces AC ripple. U1624 is a
non-inverting preamplifier and Q1614 and Q1618 form an
inverting output amplifier. A DC change at U1624 input
sensed by R1642B and R1642C (thick film resistors) starts
the regulator action. If the voltage at U1624 input goes
positive, the output at TP1614 goes negative. This causes
the voltage on C l 601 to increase during the positive voltage
cycle of T1960. Note that the voltage on C l 601 is the
difference between the positive voltage on T1960 and the
voltage at TP1614.
During the negative half of T1960 voltage cycle, the
increased voltage on C l601 increases the voltage at the
output of the cathode m ultiplier, thus correcting the
original error. R1633 and Cl 633 provide a low impedance
3-12
REV. B, MAR. 1978