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Replacing the LCD backlight tube in an original M400 console
This document applies to 1997 - 2002 M400 controls with 10.4"
Some of the earliest M400 consoles (from their
introduction ca. September 1996 through approximately April 1997)
used a 9.4" LCD panel instead. This procedure does not apply
to those units.
The average life expectancy of the LCD backlight fluorescent tube is
approximately 10,000 hours. There may be considerable individual
variation. Over time, the LCD will become progressively dimmer.
Eventually the tube may go out entirely. In this case the screen
will appear to be blank, but all control functions will operate
The backlight tube runs across the top of the LCD screen, inside
the front panel of the console. It slides in and out from the
right (towards the jog panel). It is plugged into a small
inverter (power supply) that is mounted on the back of the
motherboard mounting plate (the side towards the back of
the LCD panel).
The inverter generates high voltage, high frequency
AC power for the fluorescent tube, using a 12VDC input supply.
The replacement tube is NEC type 104LHS35.
Remove the nine 6-32 socket head cap screws from the top and sides
of the console front panel.
Loosen the three screws across the bottom edge about two full turns
each (leaving them in their holes).
If available, turn some 1" long 6-32 machine screws a few turns
into the middle holes on each side, to limit how far out the front
panel can lean.
Gently separate the front panel from the surrounding sheet metal.
It will usually be stuck to its gasket material, and may require
gentle prying. Try to minimize damage to the gasket.
Lean the panel forward, hinging on the loosened bottom screws,
so you have an inch or two gap at the top.
Locate the end of the backlight tube. It will have three wires coming
out of it: two white and one gray. They lead to a white plastic
connector on the backlight inverter.
Unplug the connector from the inverter. Needlenose pliers may
be helpful here.
Locate the small black plastic latch on the LCD panel near the end
of the backlight tube.
Press it down and pull gently on the wires
to start sliding the tube out of its sleeve at the top of the panel.
Slide the tube the rest of the way out and set it aside.
Slide the new unit into the panel until it reaches the latch.
It can take a little effort to get it started. Don't force it.
Newer replacement units may have a narrower frame, so they don't
actually click over the latch. That is okay. You may need to
remove the green tape which holds the gray wire in place along
the side of the unit in order for it to slide in freely. Clean
any tape residue off with alcohol before installing the unit.
Plug the new unit into the inverter. The gray wire goes to your right
(towards the jog panel).
Replace a couple of the screws to secure the top of the front panel.
Power up and test.
If all looks good, then reinstall the remainder of the screws.
If the screen is still dark after replacing the backlight tube,
re-check your plug connection to the inverter.
If that is not the problem, then you may have a failed inverter,
or a problem with the 12VDC supply to the inverter.