Power ~Upp~Y; Mains Input; Preregulat~R - Tektronix 2212 Service Manual

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.
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