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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" LCD panels.

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 normally.

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.


  1. Remove the nine 6-32 socket head cap screws from the top and sides of the console front panel.
  2. Loosen the three screws across the bottom edge about two full turns each (leaving them in their holes).
  3. 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.
  4. 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.
  5. Lean the panel forward, hinging on the loosened bottom screws, so you have an inch or two gap at the top.
  6. 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.

  7. Unplug the connector from the inverter. Needlenose pliers may be helpful here.
  8. Locate the small black plastic latch on the LCD panel near the end of the backlight tube.

  9. Press it down and pull gently on the wires to start sliding the tube out of its sleeve at the top of the panel.

  10. Slide the tube the rest of the way out and set it aside.
  11. 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.

  12. Plug the new unit into the inverter. The gray wire goes to your right (towards the jog panel).

  13. Replace a couple of the screws to secure the top of the front panel.
  14. Power up and test.
  15. 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.

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Copyright © 2015 Marc Leonard
Last updated 18-Feb-2015 MBL