Tektronix WFM601, Service Manual

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Theory of Operation
3–16
WFM 601 - Service Manual
either TOP or BOTTOM (from the Controller on Diagram 10) is asserted,
causing the output of U125A or U125B to go low and close either U124A or
U124D. The clamped amplifiers, U126A and U126B, have a dc level (V Offset
from the DAC on Diagram 10) at their noninverting inputs. This voltage is
adjusted to position the clamped backporch levels to the center of the lightning
graticule box vertically.
The Diamond Transcoder consists of resistance matrices, and an inverting
operational amplifier, U70. The resultant outputs, input to the Vertical and
Horizontal Amplifiers, are the four signals required to create the diamond display
(G+R, G+B, B–G, and R–G). Signal gain is approximately 0.5. C74 is adjusted
to match the phase of the inverted component (–G) to that of the B and R
components, which results in a straight, vertical black-to-white transition on the
diamond display.
The horizontal amplifier is driven by Q180, an emitter follower to provide a high
output impedance. Q151 and Q157 form an active low-pass filter that is used by
all of the component outputs. U162A and U162B are clamped amplifiers. Their
operation is very similar to that of the vertical amplifier clamps. The Horiz
Offset, from the DAC (Diagram 10), positions the clamped backporch levels to
the center of the lightning graticule box horizontally.
The selection of signals, driving Vertical and Horizontal Output Amplifiers to
display vectors, lightning, diamond, bowtie and audio are controlled by DIP
switches. LTNG Y is the vertical drive and LTNG X is the horizontal drive.
Switches U77B and U77A are closed for audio signals. U84A (LTNG X) is
closed for all other displays. U102A is closed for bowtie display (bypassing the
vertical amplifier low-pass filter), while U102D is closed for all displays except
bowtie and audio displays.
U74D is an inverter for the Z BRIGHT signal from the Coprocessor that
intensifies the CRT display.
Diagram 10 Control & Daculator
U206 is a First-In First-Out (FIFO) memory. It is written to asynchronously by
the Microprocessor and synchronously read out. U207 controls the readout of
the memory to synchronize the output to sync. U208 and U209 are shift
registers that are also clocked by an output from U207 to ensure that the
switching required to build the component displays remains synchronous. U202
is an address decoder enabling U206 so that the Microprocessor can write to it.
Diamond Transcoder
Horizontal Amplifier
Display Switching
Controller