Tektronix WFM601, Service Manual

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Theory of Operation
WFM 601 - Service Manual
3–25
The VPP1 supply is a +12V source used to program the Flash EPROM. For
normal operation P4 is in the 2-3 position. U20 is a voltage sensing regulator
whose output voltage is established by R46 and R47.
Diagram 20 Front Panel
There are 28 momentary contact switches arranged in a matrix. When U7, a
serial in/parallel out shift register, is loaded, shifted, and read, PB8 – PB10 are
pulled low along with the CS (chip select and SH/LD (shift/load) for U6 and U8.
When one or more of the push-button switches is closed a low state is loaded
into one of the U6 parallel inputs. The levels on the inputs are clocked into and
through the serial output by the BSCK (buffered serial clock). The serial output
is put onto the MOSI (serial interface bus) to be read by the Microprocessor.
Low levels to complete the LED circuits are loaded into the serial in/parallel out
shift registers (U2, U3, U4, and U5) from BMOSI (buffered serial interface bus).
Levels are then shifted into the parallel register by the LED CS and clocked out
by the LED EN, which is delayed by U10A, a D-type flip-flop.
Diagram 21 Low Voltage Power Supply
The Low Voltage Power Supply converts the mains line voltage (90–250 Vac) to
supply the power requirements of the instrument. The voltages supplied by the
Low Voltage Power Supply are +40 V,
11 V, and +5 V.
The Low Voltage Power Supply is a fly back switcher. When the switcher
MOSFET (Q14) is turned on, its drain voltage drops to approximately 0 V. The
current through the 350
H primary winding of T3 begins ramping up. The
voltages present at all secondaries is such that the rectifier diodes are reverse
biased. Energy is being stored in the magnetic field of T3. When Q14 turns off,
the drain voltage “flies back” in a positive direction. Current now flows in all of
the secondary windings and supplies power.
The input line voltage is filtered by the rear-panel connector to reduce the
electrical noise conducted into or out of the instrument. R123 limits the initial
charging current through the rectifier diodes and C71.
CR32, CR33, CR34, and CR35 form a bridge rectifier. C71 filters the 110 – 350
Vdc rectifier output. L8 filters the switching noise produced by the switcher.
R116 reduces the circulating current in the parallel circuit consisting of L8 and
C52. DS7, R116, and R118 form a line voltage indicator. R120 and R122
Switching
LED Drivers
Line Rectifier and Filter