Blanking Signal (8560E Only); Buck Regulator Control - HP 8560E Service Manual

Blanking Signal (8560E)
1. Connect an oscilloscope probe to A2J202 pin 3. Connect the
oscilloscope ground lead to TP3. Set the oscilloscope to the following
settings:
Sweep time ................................................................ 2 ms/div
Vertical scale ................................................................. 1 V/div
2. If a 4 Vp-p signal is not seen, the A2 controller assembly is faulty.
3. Repeat steps 1 and 2 with the oscilloscope probe on A2J202 pin 14.
4. Set the oscilloscope to the following settings:
Sweep time ................................................................. 1 µs/div
Vertical scale .................................................................. 2 V/div
5. Connect the positive probe lead to A2J202 pin 15. This is the
blanking output.
6. TTL-level pulses should be observed. If the signal is either always
high or always low, the display will be blanked; suspect the A2
controller assembly.
7. If the signals on A2J202 pins 3, 14, and 15 are correct, troubleshoot
the A17 CRT driver.

Buck Regulator Control

See function block H of the A6 power supply schematic diagram in the
component-level information binder.
The buck regulator control pulse-width modulates the buck regulator
and provides a synchronized signal to the DC-DC converter control
circuitry. The buck regulator control has two feedback paths. The first is
the output of the buck regulator, which provides coarse regulation. The
second is the feedback circuit which samples and compares the +5 Vdc
output of the output rectifier.
U202B and associated circuitry senses the output of the input rectifier
and will turn off U203 if the voltage at TP108 is less than
approximately +170 Vdc. Also, it will not allow U203 to start up until
this voltage exceeds +215 Vdc. A low on the output of U202B will also
clear the overcurrent latch in the DC-DC converter control circuitry.
Thermal shutdown occurs when RT201, mounted on the main heatsink,
reaches a temperature of 100 C. When this occurs, the voltage at U203
pin 13 exceeds 0.6 V and inhibits pulses to the buck regulator.
R203, R204, U211, and associated circuitry provide feedforward for
U203. This makes the loop gain independent of input line voltage and
cancels 120 Hz ripple by more than 10 dB.
U202C and its associated circuitry permit the power supply to start up
Chapter 12
Display/Power Supply Section
A6 Power Supply Assembly
647