Power Supply And Probe Adjust - Tektronix 2205 Service Manual

Theory of Operation—2205 Service
The DC Restorer is referenced to the -1 .8 kV crt
cathode voltage through R858 and CR854. Initially,
both 0855 and 0854 charge up to a level de­
termined by the difference between the Z-Axis out­
put voltage and the crt cathode voltage. Capacitor
0855 charges from the Z-Axis output through R858,
CR854, and CR855, to the crt cathode. Capacitor
C854 charges through R858, CR854, R854, and
CR853 to the crt cathode.
During the positive transitions of the ac drive,
from the lower ciamped level toward the higher
damped level, the charge on C854 increases. The
voltage increase across 0854 is equal to the ampli­
tude of the positive transition. The negative tran­
sition is coupled through C854 to reverse bias CR854
and to forward bias CR855. The increased charge of
C854 is then transferred to 0855 as 0854 dis­
charges toward the Z-Axis output level. Successive
cycles of the ac input to the DC Restorer charge
C855 to a voltage equal to the initial level plus the
amplitude of the clamped square-wave input.
The added charge held by 0855 sets the control-
grid bias voltage. If more charge is added to that
already present on C855, the control grid becomes
more negative, and less crt writing-beam current
flows. Conversely, if less charge is added, the
control-grid voltage level becomes closer to the
cathode-voltage level, and more crt writihg-beam
current flows.
During periods that 0854 is charging, the crt
control-grid voltage is held constant by the long
time-constant discharge path of 0855 through R860.
Fast-rise and fast-fall transitions of the Z-Axis
output signal are coupled to the crt control grid
through 0855 to start the crt writing-beam current
toward the new intensity level. The DC Restorer out­
put level then follows the Z-Axis output-voltage level
to set the new bias voltage for the crt control grid.
Neon lamps DS858 and DS856 protect the crt
from excessive grid-to-cathode voltage if the po­
tential on either the control grid or the cathode is
lost for any reason.
CRT Supply
The voltage doubler of CR975, CR976, and C980
produces -1.8 kV for the crt cathode. The— 1.8 kV
supply is filtered by a low-pass filter formed by
R976, R978, C976, and 0979.
3 -1 4
Focus Circuit
Focus voltage is developed from the -1 .8 kV
supply by a voltage divider formed by R894 and
R892 and FOCUS potentiometer R893.

POWER SUPPLY AND PROBE ADJUST

The Power Supply circuitry converts the ac power
line voltage into the voltages needed for instrument
operation. It consists of Ac Power Input. Start-Up,
Current Control, Regulator, and Inverter circuits on
the primary side of the high frequency power
transformer. The secondary side of the high fre­
quency power transformer provides the necessary
supply voltages for the instrument. See Figure 3-6
for the block diagram of the Power Supply.
Ac Power Input
LINE SWITCHING. Ac tine voltage of either 115 V
or 230 V may provide the primary power for the
instrument, depending on the setting of the LINE
VOLTAGE SELECTOR switch S902. POWER switch
S901 connects the selected line voltage to the
primary winding of the transformer T901 via fuse
F901. The ac line voltage is filtered by L901, L902,
0903, 0904, and 0905.
LINE RECTIFIER. The secondary of T901 is recti­
fied by CR901. CR902, CR903, and CR904 and
filtered by C900. The rectified voltage is about 60 V
and is applied to the power supply Start-Up circuit
and Current Source.
Start-UP
The control circuits for the power supply require
, a separate power supply to operate. The inde­
pendent power supply consists of Q982, Q985,
Q988, and associated components.
Initially, when instrument power is applied, the
positive plate of capacitor C982 is charged toward
the 60 V supply through the resistor string R984,
R983. and R970.
When the voltage across G982 reaches about
45 V, a 3 V drop occurs across R987 causing Q985
and Q988 to conduct. The collector current of Q985
passes through VR982. placing 16 V at the base of
the emitter follower Q982. Emitter of Q982 drives the
15 V supply. This allows current flow through R989 to
the base of Q988, keeping it on. Transistor Q988 is
now being kept on by the 15 V supply instead of the
45 V across 0982. As long as the 15 V supply
remains above the 12 volt level, the positive feed­
back through R989 will keep Q988 saturated.