Tektronix 070-8462-03, Instruction Manual

Page: 83 / 271

Theory of Operation
4–4
1735 HD High Definition Waveform Monitor
The low voltage power supply is of the flyback type. It converts the mains line
voltage (between 90 and 250 volts) to supply the power requirements of the
instrument. The low voltages supplied from the central power supply are +40 V,
+15 V, +5 V, and –15 V.
The high voltage oscillator is an Armstrong oscillator operating at about 25 kHz.
The +100 V supply and the current in the HV secondary winding are monitored
to set the static voltage level and keep the high voltage constant under changing
load conditions.
The grid drive is supplied by capacitively coupling the Z-axis amplifier output
directly to the CRT control grid. Low frequency Z-Axis information is DC
restored to set the DC bias on the CRT control grid.
The focus amplifier is an operational amplifier to control the voltage at the
bottom of the focus voltage divider. Its output level is set by the front-panel
FOCUS control.
Circuit Descriptions
The following are component-level circuit descriptions which explain the
operation of the individual circuits in the 1735HD. The descriptions are
presented in a schematic-by-schematic order.
Inputs.
Each of the six video input channels of the 1735 HD has an identical
video input amplifier. These channels are arranged in two groups of three each
(1A-2A-3A and 1B-2B-3B). CH 1A circuit numbers are used here.
Each amplifier is a non-inverting feedback amplifier with two additional low
frequency feedback paths. One of these paths, the clamp feedback path,
implements the back-porch clamp function. The other path, the DC feedback
path, prevents DC offset and drift due to the bias current at the input of the
high-speed op-amp (U52 pin 3), and implements the DC block function.
Low Voltage Power Supply
High Voltage Power
Supply
Vertical Input
Diagrams
<1>, <2>, and <3>