MFJ MFJ-123 User Manual

MFJ Enterprises, Inc.

300 Industrial Park Rd

Starkville, MS 39759 USA

MFJ-123 Atomic Clock
Instruction Manual

The MFJ-123 works by receiving the time radio signal generated by the US Atomic Clock
located in Colorado. Any place that can receive the signal can use the MFJ-123. Any place
that cannot receive the signal will have to set the time manually. The Atomic Clock in
Colorado is operated by the US Department of Commerce’s National Institute Standards and
Technology. To enhance good reception of the signal, avoid placing your MFJ-123 near
television sets, metal surfaces, computers, or any other electrical device that may interfere
with the reception of the radio signal.

The upper LCD display show the time and radio signal mark. The lower LCD display shows
the current relative humidity and temperature readings. The unit can be placed on a desktop
by pulling out the stand at the back or mounted on a wall (mounting hardware not included).

Battery Replacement and Initialization
The MFJ-123 uses two AAA batteries (not included). Before inserting the batteries select
the 12/24HR display format by placing the 12/24 HR SELECTION SWITCH (on the back of
the unit) to the desired position (Note: the 12/24 HR SELECTION SWITCH will have no
affect after the batteries are inserted). Now open the battery cover and insert the batteries. If
the LCD Display becomes dim, replace the batteries with new ones.

After installing the batteries, the relative humidity and temperature readings might have all
segments lit. If this is this case, the MFJ-123 must be re-initialized by pressing the MIN
button on the back of the clock. If the relative humidity and temperature readings appear
normal re-initialization is not needed. The hygrometer and thermometer sensors must be
initialized before locating the unit to receive the radio signal. To initialize the MFJ-123 push
the MIN button on the back of the clock and the readings will be displayed after a few
seconds.

Selecting the Appropriate Time Zone
With the MFJ-123 in the normal time display mode, press the SET button (on the back of the
unit) three times to enter the Time Zone Selection Mode. The upper LCD will show “H-0”,
“H-1”, etc. (It will be “H-0” if no time zone was previously selected). Select the appropriate
time zone by pressing the HOUR button to increase the number or the MIN button to
decrease the number. In most cases the time zone will need to be changed twice a year: when
changing from standard time to daylight saving time in the Spring and when changing from
daylight saving time to standard time in the Fall. Please refer to the table below for setting the
time zone of your area in the USA. After setting the appropriate time zone, press the SET
button again and the unit will display the current time.

Time Zone Standard Time Setting Daylight Savings Time

Setting

Pacific “H-8” “H-7”
Mountain “H-7” “H-6”
Central “H-6” “H-5”
Eastern “H-5” “H-4”
GMT “H-0” “H-0”

Note: The number following the “H-” indicates the number of hours difference from GMT.

Receiving the Time Radio Signal
The unit will search for the time signal automatically after battery installation. The radio
signal mark on the right on the Time Display will pulse to show that the clock is searching for
the radio signal. If the time signal is received, synchronization will take 3 - 10 minutes. When
the radio signal is received and synchronization has taken place, the radio signal mark will
stop pulsing and display three solid curves above the mark. If no signal is received, go to the
next section on SETTING THE TIME MANUALLY. Note: If you do not set the correct
time zone, the clock will display the incorrect time for your area.

To receive the radio signal at any time after initialization, press the SET button and hold it for
a few seconds. The process of receiving and synchronizing stated above will again take place.
The clock will automatically search for the radio signal once a day and synchronize itself to
the radio signal. This will occur at 2:00 AM.

Setting the Time Manually
Since the unit works on receiving the time radio signal, there may be locations that the signal
is weak and cannot be properly received. Under such conditions, the radio signal mark will
not display any curves. Try to rotate the unit or place it in another location to achieve better
reception. Re-install the batteries to initialize the reception again. If the radio signal still
cannot be received, the time should be set manually.

In the normal time display, press the SET button (on the back of the unit) once to bring the
unit to time setting mode. The time display will flash. Press the HR and MIN buttons to