2212
Service Manual
Power Supply
This section describes the power
supply
circuit (diagram 13
and
part of 12).
Diagrams are located
in
section 9 Diagrams.
The
Power Supply circuitry converts the ac-power-line voltage into all the
voltages required
by
the
instrument. It comprises:
Mains Input Transformer
Pre-regulator
Series Pass
Inverter
DC
Outputs and High Voltage
Mains Input
The
power switch (Jl 101) connects the ac-power line to the primary winding of
the
toroidal wound
input
transformer (T1301) via fuse F1301, filter components
L1301, L1302, C1301, C1302, C1303,
and
the
voltage selector connectors J1107
and
J1108. The secondary output is protected for overvoltage
by
varistor
RV1301, filtered by capacitor C1307 and rectified and smoothed
by
CR1301,
CR1302, CR1303, CR1304 and C1306. With
an
ac-input voltage of 220V the
voltage across C1306 is approximately 70V.
Additional components on the mains
input
circuit produce a line sync signal for
the
Trigger circuit. Transistor Q1301 is a floating differential amplifier
with
a
de-bias network comprising R1302, R1305 and R1303. Resistors R1301
and
R1304 apply a small line-frequency signal from the secondary
of
T1301 to the
base-emitter junction of Q1301.
The
resultant collector current of Q1301 is a
line-frequency, sine-wave signal that is fed via connector }1102-1 to
the
Main
Board (Al0).
Preregulator
The
70V power
supply
from the rectifier bridge is applied to
the
preregulator
circuit formed
by
U1310, Q1310 and associated components.
Zener diodes VR1310, VR1311 and R1326 reduce the incoming voltage for pre-
regulator U1310.
The
pre-regulator oscillates at a nominal 39kHz, as determined by timing com-
ponents R1320 and C1316. The square-wave output is level-shifted
by
Q1312,
and
fed to
the
Darlington-pair circuit formed by Q1311 and
power
transistor
Q1310. When Q1310 is conducting, current ramps
up
through L1380.
3-
33
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