Explain the NeXTStation "dim monitor" problem, etc...

From: r. 'bear' stricklin <red_at_bears.org>
Date: Tue Apr 16 13:05:59 2002

On Tue, 16 Apr 2002, Douglas H. Quebbeman wrote:

> > First off -- can somebody explain the common problem with the
> > monitors going dim? Can I fix it? (How?) Is there an internal
> > "intensity" pot that I can adjust to get more life out of the
> > monitor?
>
> I have always assumed that the electrons just kick the sh*t
> out of the phosphor, and the the phosphor just dies... but I
> hope that's wrong, and that something can indeed be adjusted
> or replaced (other than the daggone tube itself).

>From the comp.sys.next FAQ:

5.23 How to adjust MegaPixel Display brightness and focus?
   brightness, MegaPixel focus, MegaPixel

   Adjust it using the following information.

   From: Charles William Swiger

   I have adjusted several monitors with no problems, but make sure you
   know what you are doing before opening anything. I expressly disclaim
   responsibility for any ill results that may occur.

   In order to adjust NeXT's MegaPixel display (called 'the monitor'
   hereafter), you'll need (a) the NeXTtool (or a 3mm Allen wrench), (b)
   a plastic adjustment tool (preferred) or a thin bladed screwdriver,
   and possibly (c) a Phillips-head screwdriver.

   (NB: A similar procedure will work for color monitors, but you should
   either know what you're doing or you'll probably be better off letting
   a pro deal with it.)

   Turn off the computer. Disconnect all cables to the monitor. Look at
   the back of the monitor. There will be 4 screws there; use the
   NeXTtool (or Allen wrench) to remove them. Remove the plastic back of
   the monitor and put it out of your way.

   Reconnect the cables and turn the computer back on. As the machine
   powers up, examine the back of the monitor. You'll see a metallic box
   (usually silver, though some are black) surrounding the monitor's
   vitals. This protects you against the dangerous voltages inside, and
   also insulates the monitor from electromagnetic noise. On the back of
   this box are several holes for performing adjustments. There are two
   focus controls (labeled 'focus' and 'dynamic focus'), a brightness
   control (labeled 'brightness' or possibly 'black level') and several
   others that adjust various things like screen size and position.

   Depending on the exact placement of the controls on the circuit board
   of your specific monitor, some of these controls may be difficult (or
   impossible) to adjust from the back. If this is the case, I will
   describe what's necessary below. Otherwise, adjust the appropriate
   controls using either an adjustment tool or a screwdriver. Be warned
   that a screwdriver probably will cause some interesting video effects
   when it enters the case. Ignore this the best you can, or find a
   plastic adjustment tool, which is what you *really* should be using
   anyway. Using a flashlight will help you see into the hole so that you
   can align the business end of the tool correctly.

   Focus and position controls are fairly obvious. Adjust them slowly
   until you're happy with the results. Don't muck with anything you
   don't need to; the factory settings are usually pretty decent.

   To correctly adjust the brightness, follow this procedure: Turn the
   brightness of the monitor all the way down using the keyboard. Adjust
   the brightness control on the back of the monitor until a barely
   noticeable picture forms. Then turn the brightness down a little so
   this picture disappears completely. Check that you can get adequate
   brightness by using the keyboard to brighten the screen. If the
   display isn't bright enough, adjust the brightness control on the rear
   of the monitor high enough so that the monitor display is adequate.
   Note that you won't be able to dim the screen completely from the
   keyboard...sorry.

   Once you're finished, shut down the computer, take off the cables,
   reattach the back of the monitor, and reconnect the cables. You're
   done.

   If the control you need to adjust proves to be difficult, you may need
   to enter the metal case. This happened on one monitor's focus control
   and another's brightness.

   WARNING: THE VOLTAGES INSIDE THE MONITOR'S CASE ARE VERY DANGEROUS,
   EVEN WHEN THE MONITOR IS OFF. BE VERY CAREFUL, OR YOU CAN SERIOUSLY
   INJURE OR EVEN KILL YOURSELF.

   DO NOT PERFORM THE NEXT INSTRUCTIONS UNLESS YOU ARE CONFIDENT THAT YOU
   KNOW WHAT YOU ARE DOING. You'll have to power off the computer again,
   and disconnect the cables.

   Looking at the monitor from the back, notice a section of metallic
   shielding on the right side of the metal box that extends to the
   picture tube. This is where the flyback transformer is connected. It
   shields a wire that is charged to about 25,000 V.

   WARNING: DO NOT TOUCH THIS WIRE, IT CAN SHOCK YOU THROUGH ITS
   INSULATION.

   Being very careful of this, remove the metal case by unscrewing the
   Philip's head screws that hold the case on. Don't touch the screws
   that hold the picture tube into the front of the monitor's case.

   Once you've gotten the metal box off, reconnect the cables. Figure out
   what control you're going to adjust, and make sure that you can do so
   without touching anything else inside. Again, *watch out* for the wire
   that connects to the picture tube on the right side.

   Power up the computer. I recommend that you use only one hand to make
   the adjustment, and that your other hand be placed in your pocket (or
   similar equivalent, if you're wearing clothes lacking pockets). This
   precaution reduces the chances that you'll make a short circuit
   between one hand, your heart, and the other hand --- a good idea.

   Perform the necessary adjustment(s), being very careful not to touch
   anything inside. Then shut down and reassemble the monitor, following
   the directions given above.

   Hopefully, these instructions will prove useful. Once again, please be
   very careful...I don't want your death and/or injury on my conscience
   (or a lawsuit, for that matter, either :-)

5.44 What makes aged NeXT monitors dim?
   monitor, dim

   The cause of the dimming monitors is the CRT cathode wearing out. The
   most common type of CRT (and the type used in most NeXT monochrome
   monitors and all of the NeXT color monitors) uses what is called an
   oxide cathode. A thin coating of oxide is deposited on the cathode to
   allow the electronics which form the picture to get off the cathode
   easily. The oxide gradually boils off the cathode itself, and when the
   oxide is gone, the CRT goes dim.

   Typically, the oxide will last from 10,000 to 20,000 power on hours
   (screen savers don't help the cathode, they only prevent phosphor
   aging). Unfortunately, the black monochrome monitors fall into the
   short end of the life range thanks to Toshiba who made the CRT's. The
   aging is more noticeable in Unix machines because they tend to be left
   on. Note that there are about 8,000 hours in a year. If you leave your
   monitor on all the time, all oxide type CRTs will be dim in three
   years.

   The other type of CRT cathode is the I-cathode or dispenser type. This
   type of cathode is porous and continually brings new activation
   material to the surface. Its lifetime is 40,000 hours or more. The
   last of the NeXT monochrome monitors (N4000B) used this type of CRT
   and they don't go dim. There aren't many of that type around because
   NeXT quit the hardware business after producing only a few thousand.
   If you can get an N4000B monitor, you won't ever have to worry about a
   dim monitor.

   Many manufacturers are going to dispenser cathode type CRTs in their
   monitors with Panasonic leading the way. The best advice is to turn
   off the monitor when not in use. If that is impractical, try to
   purchase one with the long life cathode.

   Spherical Solutions (smg_at_orb.com) has a supply of new N4000B long life
   monitors for sale in either ADB or non-ADB configurations. If you need
   to repair or replace a monochrome monitor, that is by far the best
   type to use.

   If you read this far, you probably know more than you ever wanted to
   about CRT aging, but I hope this helps.


HTH

ok
r.
Received on Tue Apr 16 2002 - 13:05:59 BST