Tektronix WFM601, Service Manual

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Theory of Operation
WFM 601 - Service Manual
3–23
for the sample-and-holds is identical and determined by a single RC circuit
(R376 and C87). The output of the sample-and-hold drives an integrator, U48C
or U48D, whose output is a negative-going saw tooth waveform. Charging
current is controlled by the Shape adjustment (R134 or R135). The output of the
Readout Stroke Generator drives the Vertical and Horizontal deflection circuits
when readout is enabled.
Diagram 17 DACs & Serial
U24 is a 4-line to 8-line decoder that outputs chip select and enable signals for
the non-synchronous switching control lines. U9 is an 8-bit parallel load serial
output shift register. Status of the A, B, C, and D input lines identify the Main
circuit board revision level. SW1 is included for troubleshooting purposes. R28
is a set of pull-up resistors for 8 parallel inputs and 3 of the Microprocessor
(Diagram 14) control lines.
U77 and U82 are 8-bit serial in/parallel out shift registers. Their outputs are
asynchronous switching control lines. The AC/DC switching controlled by Q24
is not used by the WFM
601.
U32 and U38 are 8-bit serial in/parallel out shift registers that generate DAC
load and chip enable signals. These signals are used by the DACs, Remote
interface parallel in/serial out shift registers (Diagram 18), and the chip selects
for the Bezel Controls A/D converters on Diagram 19.
U31, U35, and U86 are serial digital-to-analog converters with 16x12 static
RAM. VOUT (0–15) are the analog outputs, each of which has a sample-and-
hold for the output level. Serial data is loaded from the serial bus (BMOSI)
when the LD goes low. The clock signal (BSCK) is from the Microprocessor.
Output voltage levels provide the instrument’s operating levels.
U39C and U39D are buffer amplifiers. U45A and U45B are adders for
horizontal and vertical positioning voltages. The RC circuits across the adders
are low-pass filters.
Diagram 18 Remote & Digital Bus Connectors
U92 is an RS232 line driver receiver. C175 is part of the internal voltage
doubler circuit and C171 is part of a voltage inverter circuit. Input and output
signals are TTL. Chip output will be low with an input signal of +2.4 V or
more.
Serial Interface & Serial
Static Outputs
D/A Converters
RS232