A1/A3 Power Amplifier - HP 5501B Operating And Service Manual

8-117. The first counter (A3U6) in the divider section controls the liquid crystal and the sample and hold
circuits. A sample and hold is performed once per cycle of n.28 alternating between the vertical
and horizontal components [see Figure 8-8 (Optical Mode Timing Diagram)]. Thus each
component is sampled once every 2.56 seconds. The liquid crystal is allowed to stabilize for 0.64
seconds before the switch to the sample and hold is closed. The hold cycle begins when the
switch opens at the end of a cycle of n.28 and at the end of a half cycle of PASSIVE. The present
half cycle of PASSIVE/- ACTIVE determines which component is sampled. Op-amp A3U11A
computes the difference between the two sample and holds.
8-118. A1/A3 Power Amplifier Ⓘ
8-119. The output of A3U11A (from Subtracting Sample and Hold) drives the Power Amplifier section
which in turn, drives the laser tube heater. The frequency response of the Power Amplifier is
determined by both the input filter (A3R31, A3R27 and A3C18) and the A3U13 op-amp feedback
filter (A3R32, A3R33, A3R39 and A3C20). Feedback is through A3R65B and A3C37, which
integrates only the positive portion of the heater voltage. This integrated signal is proportional to
heater power. A3U13 drives the input of A3U20, a switching regulator, which divides the heater
power between the +15V and the -15V supplies. A3R36, A3CR7, and A3R61 form a current
limiter and prevents applying a negative voltage to A3U20 (Pin 9). A3R63 and A3C38 determine
the switching frequency (approximately 22 kHz or 46 microsecond period) and A3C36 controls
the deadtime (off) to insure that the +15V and -15V outputs are not on at the same time. A1Q8
and A1Q6 form the positive Darlington that drives the heater to +15 Volts. A1Q9 and A1Q7 form
the negative Darlington that drives the heater to -15 Volts. A1Q5 provides downshifting of
negative-drive voltage level. A3U20 is disabled at Pin 10 by A3U2C when the heater error voltage
is sampled during the warmup mode. The resistor networks (A3R54A, A3R54B, A3R54C,
A3R65A, A3R65C, A3R65D, and A3R66) drive the output Darlington's and limit the A3U21 input
voltage to +5 Volts.
Positive and Negative Darlington-configured transistors are located on
the A1 Connector board.
8-120. To maintain the +15V and -15V currents within maximum specified range and, because the High
Voltage Power Supply (HVPS) turns on at the start of optical mode (using only the -15 Volt
current supply), the maximum currents must be limited during each of the following three
conditions:
The current used by each of the two Darlington's is proportional to the pulse length. The pulse
begins at the fall of the positive rising sawtooth waveform (0.5V to 3.5V) at A3U20 (Pin 7) and,
during the maximum current condition, is terminated when the sawtooth reaches the voltage at
A3U20 (Pin 9), which is E REF at the base of Q7. A3CR8 provides temperature matching and
separation between A3U13 and A3Q7when current limiting is off. E REF is the output of a
multiple voltage divider network (A3R68, R69, R70, R71, R72, R73, R74) set by the open collector
logic network of A3U21 which monitors the Darlington "ON" states and the warmup signal line.
The voltage divider resistors are set sequentially. A3R73 and R74 are set first and limit the
maximum current of the -15 Volt Darlington's during the optical mode. Next, A3R68 and R69 set
the maximum current limit for the +15 Volt Darlington's during either warmup or optical mode.
And last, A3R70 and R71 set the -15 Volt Darlington's to the maximum current limit during the
warmup mode.
1. +15 Volt supply current during warmup and optical mode,
2. -15 Volt supply current during warmup mode, and
3. -15 Volt supply current during optical mode (-warmup mode).
NOTE
HP5501B
Service
8-35