Tektronix WFM 601A, Service Manual

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Block Diagram Descriptions
WFM 601A, WFM 601E, & WFM 601M Service Manual
3–7
The following discussion covers the main blocks on the A1 Power Supply
schematics.
Line power enters through an RF filter on the rear panel. The Line Rectifier
circuit correctly converts 90 to 250 VAC. The line voltage or primary side of the
power supply is fully isolated from the secondary or DC side by transformers and
optical signal isolators.
Initial power conversion is controlled by a current-mode pulse width modulator
circuit centered on U6. The switcher operates at 100 kHz. U6 senses the output
voltage level from U4 and a ramp voltage representing the current in the
transformer switcher, Q14. On power up, C62 accumulates a trickle charge to
16 V, which powers U6.
When the power switch is set to on, the 170 kHz oscillation on the Power Switch
Oscillator (Q13 and T2) stops, causing the Mag Latch to switch state. The Mag
Latch remembers its state mechanically. The output of the Mag Latch is summed
with the output of the Overload Time-out to enable or disable the Controller. A
shutdown occurs within a few milliseconds and lasts 1 second. The Snubber
circuit clamps any voltage that exceeds twice the rectified power line input.
The secondary circuits include the low voltage output filters and error detection.
The Error Amplifier signals the Controller to help regulate the +5 V supply. The
5 V adjustment is in this circuit. The Overvoltage Protection circuit monitors the
+5 V supply and grounds the 5 V supply if it rises above 5.5 V. The –5.2 V
supply can signal an over-voltage condition through Q21 and ground the 5 V
supply. The power supply goes through its start up sequence after this grounding.
LED DS6 indicates the 5 V supply is on.
The Output Filters rectify the T1 secondary windings and filters the low voltage
outputs. The exception here is the –5.2 V Buck circuit. It provides regulated
–5.2 V from the –11 V supply. U10 is the pulse width modulator that operates
similar to U6 on the primary side. The 100 kHz out of U10 drives Q18 with a
varying duty cycle that produces a DC average out of L10. An input from the
+5 V secondary helps synchronize the 100 kHz oscillator to the Primary side.
The High Voltage Power Supply is generated by a sine-wave oscillator and
step-up transformer. The main components of the high-voltage oscillator and
error amplifier circuit are Q7 and T3, which form an Armstrong oscillator
running at about 22 kHz. Error amplifier U2 regulates the +100 V output and
keeps the High Voltage Power Supply constant under varying load conditions by
controlling the base current to Q7. C26 and Q8 delays the start up of the
high-voltage supply until the Low Voltage supply is running.
The CRT cathode supply consists of rectifier CR7 and filtering capacitors. The
supply provides –2530 V to the CRT cathode. U1 is a four-times multiplier
Primary Side
Secondary Side
High Voltage Supply