HP 85662A, Troubleshooting And Repair Manual

Page: 89 / 594

When the loop is closed, the wiper of R103 will be at about V and is connected to the
negative input of The positive input of is tied, through R123, to the V
reference voltage generated by is an integrator below the frequency of the zero
formed by R105 and (112 Hz) and a gain of approximately three above that frequency.
is a dc gain stage of approximately four. R108 and provide offset to allow
the output of to be near the middle of its power supply rails (0 and V) while
the signal meets the input range requirement of pin 5 (MODULATOR). This input
compares the ramp to the pin 5 voltage and when the ramp reaches the same voltage as pin 5
the output pulse is terminated.
This is a resistive divider to sample the cathode voltage and compare it to a dc reference.
The difference in these voltages drives an integrator, which controls the pulse width of the
drive signal. This drive signal injects energy into the primary tank circuit of the high voltage
module that generates the cathode voltage. This is a second order loop, which has infinite
gain at dc, and the average dc cathode voltage is well-regulated. There is some voltage drop
during portions of the CRT refresh cycle that are outside the bandwidth of the control loop,
but it is only a few volts and has no effect on the display performance. The individual vectors
drawn on the CRT draw beam current for either 4 ms or 19 ms depending on the length of
the vector. The filter capacitors on the
High Voltage Regulator board provide filtering
of the cathode supply for the high frequency components of the beam current, while the
regulator provides the low frequency average.
This circuit regulates the cathode voltage. The life of the CRT is dependent on filament
voltage. It is very important that the filament have the correct voltage and that is held within
a narrow range. If the voltage is either too high or too low, the life span of the CRT will be
shortened. The filament voltage comes from the same transformer as the high voltage, and
there is good correlation between the two for any given high voltage module.
contains a
start
feature. The maximum length of the output drive pulse is
determined by the voltage at pin
6,
the
DUTY CYCLE
input. This voltage is derived from
the internal reference voltage from divided by and As charges on
power up, the drive pulse slowly increases in width and the high voltage ramps up slowly.
This prevents any large start up power surges. The start up power has a RC time constant of
about 400 ms and has a small (5-10 percent) overshoot as the integrator comes off the rail
and comes into regulation.
This power supply has a remote On/Off capability. This is activated by a TTL signal from
the IF Control called CRT DSBL (low = HV on). As a safety feature, holds the
supply of in the absence of the IF Control assembly. is used as an to get the
correct polarity signal to operate and attenuate the signal form to
an appropriate level for
In order to prevent an 20 noise from radiating to other circuits, a filter network isolates
the V dc that is used to power the control section of the HV power supply. and
provide a low pass filter and R125 damps the Q of the filter to ensure there will not be
any self resonance. The control section of the high voltage power supply does not use the
-15 V power supply because if it were to fail, the outputs would rise and turn the high
voltage regulator on harder than desired, perhaps causing an over voltage condition on the
CRT. If the V supply fails, the drive circuit stops working and the high voltage supply
will shut down.
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