Z-Axis Amplifier - Tektronix 2205 Service Manual

of Q829 rises to the blanking level. When blanked,
the output of the Z-Axis Amplifier drops to reduce
the crt beam current below viewing intensity.
When the BEAM FIND button is pressed, the
BEAM FIND line goes to about -8V sinking about
1 rfiA from the Z-Axis Amplifier, over-riding any
other current combinations, and therefore un­
blinking the trace.
DC Restorer
The DC Restorer circuit sets the crt control-grid
bias and couples the ac and dc components of the
Z-Axis Amplifier output to the crt control grid. Direct
coupling of the Z-Axis Amplifier output to the crt
control grid is not employed because of the high
potential differences involved. Refer to Figure 3-5
during the following discussion.
Theory of O peration—2205 Service
Ac drive to the DC Restorer circuit is obtained
from pin 3 of T902. The drive voltage has an ac peak
amplitude of about 100V, at a frequency of about
20 kHz and is coupled into the DC Restorer circuit
through C853 and R853. The cathode of CR851 is
biased by the wiper voltage of Grid Bias potenti­
ometer R851, and the ac drive voltage is clamped
whenever the positive peaks reach a level that for­
ward biases CR851.
The Z-Axis Amplifier output voltage, which varies
between +10 V and +75 V, is applied to the DC
Restorer at the anode of CR853. The ac drive
voltage holds CR853 reverse biased until the voltage
falls below the Z-Axis Amplifier output voltage level.
At that point, CR853 becomes forward biased and
clamps the junction of CR851, CR853, and R854 to
the Z-Axis output level. Thus, the ac-drive voltage
is clamped at two levels to produce a square-wave
signal with a positive dc-offset level.
3+-13