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Theory of Operation
3–66
AM700 Audio Measurement Set Service Manual
Focus Voltage and Dynamic Focus. The output of the focus voltage winding of T2
is rectified by CR6 and filtered by an RC π filter composed of C26, C27, and
R33. The main focus voltage of about +600 V is set by FOCUS pot R36 via a
DC restorer circuit composed of C28, CR8, and R38. Dynamic focusing, to
optimize edge focusing of the crt, is applied from a series resonant LC circuit
formed by L5 and C23 off the +55 V winding of T2. The dynamic focusing
waveform approximates a negative-going parabola between the positive pulses of
the winding output voltage. That waveform is AC coupled through C25 to the
focus voltage. The dynamic focusing waveform is shaped by the adjustment of
L5 to produce the best edge focus in conjunction with the setting of the FOCUS
pot for best overall focus.
The Trace Rotation circuit drives a separate deflection coil on the neck of the crt.
That winding produces a magnetic field that permits adjustment to horizontally
level the raster. Differential deflection voltage is produced by the circuit formed
by U5A, U5B, and U4. A voltage reference of +6 V is provided by U4, a
precision voltage divider. That reference is applied to the non-inverting input of
U5A. On U5B, the adjustable voltage from the Trace Rot potentiometer, R67, is
applied to the non-inverting input of U5B. When the amplifiers are balanced, no
current flows through the trace rotation coil.
When the Trace Rot pot is adjusted to unbalance the two amplifiers, the output of
U5B at pin 2, is driven to return the balance to that amplifier. That change is
coupled through R70 to the inverting input of U5A to produce an equal change
in the opposite direction at pin 10, thereby producing a current through the trace
rotation coil. Reversing the position R67 causes current in the opposite direction
in the trace rotation coil. The overall adjustment range for trace rotation is
approximately ±3.4_.
Trace Rotation