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Theory of Operation
6–6
760A / 760D / 760N
U818 provides approximately 30 kHz clocking for the counter. Its output is a
square wave whose rising edge defines clock phase 1. An inverter U825
provides clock phase 2, whose rising edge is delayed from phase 1 by one–half
the clock period.
U828 and U623 form a two–digit BCD down–counter. The counter output is
delayed half a clock period by latch U424 which in turn drives the bar decoders.
Monostable circuit U715 provides intersegment blanking by disabling the latch
outputs for a few microseconds after each clock pulse.
ROMs U127 and U137 form the look–up table, which determines the shape and
range of the scale. Twelve data bits provide the needed resolution. In addition
to the normal scale information, the ROM also contains the magnified scale
actuated by the EXPAND SCALE button. Other scales for calibration and
service are selected by a special service switch S340. Refer to SECTION 4,
MAINTENANCE, for information on settings and uses. The ROM data outputs are
latched by U132 and U142, to synchronize the output data, and applied to the
DAC.
U146 is a 12–bit DAC with adjustments provided for gain (R252) and offset
(R253). The DAC output range is from 0 V (all LED segments lit) when all
DAC inputs are high, to +5 V (all LED segments off) with all DAC inputs low.
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, "15 V, and +5 V.
BAR SCANNER/DRIVER &
DAC <3>
Master Clock
Down Counter
Scale Shaper Lookup
Table
DAC
LOW VOLTAGE POWER
SUPPLY <6>