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D-70 / Apr 2000
M E T E R B R I D G E
Console Clock
The Wheatstone digital clock is a six-digit time-of-day clock with LED
display. The clock is designed with CMOS circuits and an on-board
crystal-controlled time base oscillator. There are two basic parts to the
clock: a main PCB containing the clock circuits and clock set controls (also
may include capacitor backup) and a second PCB containing displays.
Clock set controls may be accessed by removing the meterbridge cover.
Controls
The clock is controlled by a trimmer and two switches; the trimmer and
switches are mounted on the main clock PCB assembly.
The trimmer alters the frequency of the quartz-controlled oscillator,
which in turn causes the clock to run slightly slower or faster. In order to
keep accurate time, the oscillator must run at 4.096 MHz. The oscillator is
set to this frequency at the factory. However, due to the nature of quartz/
crystal-controlled oscillators, there may be a slight change in the fre-
quency of the oscillator during the first few months of operation due to the
aging effect of quartz crystals. A minor readjustment of the trimmer will
compensate for this effect.
A divided by 4 buffered output of the oscillator is available at pin #7 of IC
#U1 to assist in adjusting the oscillator.
Setting the Time
The setting controls consists of two switches: MODE and SET. To set
the clock, open the meterbridge cover and stand behind the console:
1) The control switches (mounted inside the meterbridge bottom) are
labelled "M" and "S". "M" (Mode) is used to scroll from seconds to
minutes to hours; "S" is used to Set the time. The procedure is to set
the clock slightly ahead of the current time, hold the second count at
"00" until the current time catches up, and then release the count.
2) Press the MODE button until the hour digits blink. Depress the SET
button until the desired hour is displayed.
3) Press the MODE button until the minute digits blink. Depress the
SET button until the desired minute count is displayed.
4) Press the MODE button until the second digits blink. Depress and
hold the SET button; the seconds display will hold at "00". When the
current time catches up to the display, release the SET button. The
clock will start counting. Hit the MODE button three more times to
place the clock into working mode.
Capacitor Backup
With the meterbridge open note the super capacitor at C25. This super
capacitor is self charging. Note that the super capacitor does NOT light up
the clock display; it powers the clock crystal to keep it from losing count
( it will do this about one hour).
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