Tektronix WFM601, Service Manual

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Theory of Operation
3–18
WFM 601 - Service Manual
controlled by two control lines and the chip selects. If neither chip select
(REFCH SEL 2 or 3) is asserted, there will be no output. This condition exist
when external reference is selected.
When the INT EXT control line is asserted, it goes low and U70D and U70A
close to drive Q19 from the rear panel EXT REF input. Q19 is an emitter
follower providing a high impedance output that drives Ref Video sync separator
on Diagram 15. Q17 clamps the input side of U70A to ground when Internal
Reference is selected.
Diagram 12 Vertical Output
The input CAL LEVEL is from a DAC, shown on Diagram 17. It drives the
inverting input of U47A, an operational amplifier. Q20 is a saturating switch
driven by a 100 kHz output from the Line Rate Controller on Diagram 15. The
squarewave output is 1.096V.
The Filter MUX is an 8-to-1 multiplexer controlling the input selection for the
Vertical Amplifier. In order to accommodate an additional three inputs U66
selects the signal to be applied to the AUX input of U67. Switches are closed
when their control lines are asserted low by synchronous outputs from the Line
Rate Controller shown on Diagram 15. The Y Audio and Time Code (not
implemented) inputs are differential. U84A (Audio) and U88B (Time Code) are
converters, for the differential inputs, that output a single ended signal to drive
the Filter MUX.
The Vertical Amplifier, U55, contains two independent amplifiers. The external
gain controlled Auxiliary amplifier is used as differential output chrominance
amplifier. The output of the Chroma Filter Amplifier (Diagram 3) is input to the
+ input of the Aux amplifier. Its gain is controlled by the CHROM GAIN level
from a DAC shown on Diagram 17. The differential output is converted to a
single ended output by U73 to drive the Vector Chroma Amplifier shown on
Diagram 15
.
The main Vertical Amplifier has inputs for the filtered video signal and the
differential readout signal. The single ended output from the Filter MUX is
converted to the differential output required to drive the Vertical Deflection
Amplifier. Output is switched between video signal and the readout by the V
RO SEL signal from the Line Rate Controller.
Vertical control levels, such as Gain, and Position from the DACs (Diagram 17)
and the Magnifier control signal from the Line Rate Controller (Diagram 15)
control the output gain and positioning of the displayed signal. Gain and
Square wave Calibrator
Filter MUX
Vertical Amplifier