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488 lines
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<TITLE> Text-Terminal-HOWTO: Repair & Diagnose </TITLE>
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<LINK HREF="Text-Terminal-HOWTO-21.html" REL=next>
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<LINK HREF="Text-Terminal-HOWTO-19.html" REL=previous>
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<LINK HREF="Text-Terminal-HOWTO.html#toc20" REL=contents>
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<A HREF="Text-Terminal-HOWTO-21.html">Next</A>
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<A HREF="Text-Terminal-HOWTO-19.html">Previous</A>
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<A HREF="Text-Terminal-HOWTO.html#toc20">Contents</A>
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<HR>
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<H2><A NAME="repair_"></A> <A NAME="s20">20.</A> <A HREF="Text-Terminal-HOWTO.html#toc20">Repair & Diagnose </A></H2>
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<P> Repairing a terminal has much in common with repairing a monitor
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and/or keyboard. Sometimes the built-in diagnostics of the terminal
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will display on the screen. By the symptoms, one may often isolate
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the trouble to one of the following: bad keyboard, CRT dead, power
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electronics failure (distorted display), or digital electronics
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failure. It's best to have a service manual, but even if you don't
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have one, you can often still repair it.</P>
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<H2><A NAME="repair_info"></A> <A NAME="ss20.1">20.1</A> <A HREF="Text-Terminal-HOWTO.html#toc20.1">Repair Books & Websites </A>
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</H2>
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<H3>Books</H3>
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<P> Bigelow, Stephen J.: Troubleshooting & Repairing Computer
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Monitors, 2nd edition, McGraw-Hill, 1997. Doesn't cover the character
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generation electronics nor the keyboard.</P>
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<H3>Websites</H3>
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<P> The FAQ
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<A HREF="http://www.repairfaq.org">http://www.repairfaq.org</A> for the newsgroup:
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sci.electronics.repair is long and comprehensive, although it doesn't
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cover terminals per se. See the section "Computer and Video
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Monitors". Much of this information is applicable to terminals as are
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the sections: "Testing Capacitors", "Testing Flyback Transformers",
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etc. Perhaps in the future, the "info" on repair in this HOWTO will
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consist mainly of links to the above FAQ (or the like).</P>
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<H2><A NAME="ss20.2">20.2</A> <A HREF="Text-Terminal-HOWTO.html#toc20.2">Safety</A>
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</H2>
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<P> CRT's use high voltage of up to 30,000 volts for color (less for
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monochrome). Be careful not to touch this voltage if the set is on
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and the cover off. It probably won't kill you even if you do since
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the amount of current it can supply is limited. But it is likely to
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badly burn and shock you, etc. High voltage can jump across air gaps
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and go thru cracked insulation so keep your hands a safe distance from
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it. You should notice the well-insulated high voltage cable connected
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to one side of the picture tube. Even when the set is off, there is
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still enough residual voltage on the picture tube cable connection to
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give you quite a shock. To discharge this voltage when the set is
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unplugged use a screwdriver (insulated handle) with the metal blade
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grounded to the picture tube ground cable with a jumper wire. Don't
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use chassis ground.</P>
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<P>The lower voltages (of hundreds of volts) can be even more dangerous
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since they are not current limited. It is even more dangerous if your
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hands are wet or if you are wearing a metal watchband, ring or the
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like. In rare cases people have been killed by it so be careful. The
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lowest voltages of only several volts on digital circuitry are fairly
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safe but don't touch anything (except with a well insulated tool)
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unless you know for sure.</P>
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<H2><A NAME="ss20.3">20.3</A> <A HREF="Text-Terminal-HOWTO.html#toc20.3">Appearance of Display</A>
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</H2>
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<P> If the display is too dim, turn up the brightness and/or contrast.
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using knobs on the exterior of the unit (if they exist). If the
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width, height or centering is incorrect, there are often control knobs
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for these. For some older terminals one must press an arrow key
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(or the like) in set-up mode.</P>
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<P>You may need to remove the cover to make adjustments, especially on
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older models. You could arrange things so that a large mirror is in
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front of the terminal so as to view the display in the mirror while
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making adjustments. The adjustments to turn may be on a printed
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circuit board. While a screwdriver (possibly Phillips-head) may be
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all that's needed, inductors may require special TV alignment tools
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(plastic hex wrenches, etc.). The abbreviated name of the adjustment
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should be printed on the circuit board. For example, here are some
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such names:</P>
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<P>
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<UL>
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<LI> V-Size adjusts the Vertical height (Size)</LI>
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<LI> H-Size adjusts the Horizontal width (Size). It may be an inductor.</LI>
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<LI> V-Pos adjusts the Vertical Position</LI>
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<LI> H-Pos adjusts the Horizontal Position</LI>
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<LI> V-Lin adjusts Vertical Linearity (Use if width of scan lines
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differs at the top and bottom of the screen)</LI>
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<LI> V-Hold adjusts Vertical Hold (Use if screen is uncontrollable
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scrolling)</LI>
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<LI> Bright adjusts brightness (an external knob may also exist)</LI>
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<LI> Sub-Bright adjusts brightness of subdued intensity mode (often
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the normal mode: dimmer than bold or bright mode).</LI>
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</UL>
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</P>
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<P>Changing linearity may change the size so that it will need to be
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readjusted. A terminal that has been stored for some time may have a
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small display rectangle on the screen surrounded by a large black
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Before adjusting it, leave the terminal on for a while since it will
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likely recover some with use (the black borders will shrink).</P>
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<H2><A NAME="ss20.4">20.4</A> <A HREF="Text-Terminal-HOWTO.html#toc20.4">Diagnose</A>
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</H2>
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<H3>Terminal Made a Noise or Smoked</H3>
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<P> If the terminal made some noise just before it failed (or when you
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turn it on after it failed) that noise is a clue to what is wrong. If
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you hear a noise or see/smell smoke, immediately turn the terminal off
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to prevent further damage. A pop noise may be a capacitor exploding
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or a fuse blowing. A buzzing noise is likely due to arcing. The
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problem may be in the high voltage power supply of several thousand
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volts.</P>
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<P>Remove the cover. Look for discoloration and bulging/cracked
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capacitors. If the bad spot is not evident, turn it on again for a
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short time and look for smoking/arcing. For arcing, a dimly lit room
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will help find it. The high voltage cable (runs between the flyback
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transformer and the side of the picture tube) may have broken
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insulation that arcs to ground. Fix it with high-voltage insulating
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dope, or special electrical tape designed say for 10,000 volts.</P>
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<P>The flyback transformer (high voltage) may make only a faint clicking
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or sparking noise if it fails. You may not hear it until you turn the
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terminal off for a while and then turn it back on again. To track
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down the noise you may use a piece of small rubber tubing (such as
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used in automobiles) as a stethoscope to listen to it. But while you
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are listening for the noise, the terminal is suffering more damage so
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try find it fast (but not so fast as to risk getting shocked).</P>
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<P>A shorted power supply may cause a fuse to blow. Replacing a blown
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fuse may not solve the problem as the same short may blow the fuse
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again. Inspect for any darkened spots due to high heat and test those
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components. Shorted power transistors may cause the fuse to blow.
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They may be tested with a transistor checker or even with an
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ohm-meter. Use the low ohm scale on an ohm-meter so that the voltage
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applied by the meter is low. This will reduce the possible damage to
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good components caused by this test voltage.</P>
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<P>If the terminal has been exposed to dampness such as being stored in a
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damp place or near a kitchen with steam from cooking, a fix may be to
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dry out the unit. Heating a "failed" flyback transformer with a blow
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dryer for several minutes may restore it.</P>
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<H3>Terminal Made No Noise</H3>
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<P> A blank screen may be due to someone turning the brightness
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control to the lowest level or to aging. The next thing to do is to
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check the cables for loose or broken connections. If there is no sign
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of power, substitute a new power cord after making sure that the power
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outlet on the wall is "hot".</P>
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<P>If the keyboard is suspected, try it on another terminal of the same
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type or substitute a good keyboard. Wiggle the keyboard cable ends
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and the plug. Wires inside cables may break, especially near their
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ends. If the break is verified by wiggling it (having the problem go
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on and off in synchronization with the wiggles), then one may either
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get a new cable or cut into the cable and re-solder the breaks, etc.</P>
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<P>One of the first things to do if the keyboard works is to put the
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terminal into
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<A HREF="Text-Terminal-HOWTO-19.html#local_mode">Local Mode</A>. If it works OK
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in local, then the problem is likely in the connection to the host
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computer (or incorrect interface) or in the UART chips of the
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terminal.</P>
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<H2><A NAME="ss20.5">20.5</A> <A HREF="Text-Terminal-HOWTO.html#toc20.5">Detective work</A>
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</H2>
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<P>By carefully inspecting the circuitry, one may often find the cause
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of the problem. Look for discoloration, cracks, etc. An intermittent
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problem may sometimes be found by tapping on components with a
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ball-point pen (not the metal tip of course). A break in the
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conductor of a printed circuit board may sometimes be revealed by
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flexing the board. Solder that looks like it formed a drop or a
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solder joint with little solder may need re-soldering. Soldering may
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heat up transistors (and other components) and damage them so use a
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heat sink if feasible. One failure may cause others, so unless you
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find the original cause, the failure may reoccur.</P>
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<P>If you have a common brand of terminal, you may be able to search the
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Internet (including newsgroup postings) to find out what the most
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frequent types of problems are for your terminal and perhaps
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information on how to fix it. If you find that a certain component
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is bad you may search for this component (for example R214 wyse) and
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hopefully find a report by someone else who had the same problem.
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Such a report may indicate other components that failed at the same
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time. If a component is damaged so badly that its value can't be
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read, then you might find it on the Internet. The manufacturer may
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have on-line data that search engines don't index.</P>
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<P>To see if the digital electronics work, try (using a good keyboard)
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typing at the bad terminal. Try to read this typing at a
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good terminal (or the console) using the copy command or with a
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terminal communication program such as picocom. You may need to hit
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the return key at the terminal in order to send a line. One may
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ask the bad terminal for its identity etc. from another terminal.
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This will show if two-way communication works.</P>
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<H2><A NAME="ss20.6">20.6</A> <A HREF="Text-Terminal-HOWTO.html#toc20.6">Error Messages on the Screen</A>
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</H2>
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<P> You are in luck if you see an error message on the screen. This
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usually happens when you first turn the terminal on.</P>
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<H3>Keyboard Error</H3>
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<P> This usually means that the keyboard is not plugged in, or that
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the connection is loose. For more serious problems see
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<A HREF="#keyboards_">Keyboards</A></P>
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<H3>Checksum Error in NVR</H3>
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<P> NVR is "Non-Volatile RAM". This means that the NVR where the
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set-up information is stored has become corrupted. The terminal will
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likely still work but the configuration that was last saved when
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someone last configured the terminal has likely been lost. Try
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configuring again and then save it. It might work. On very old
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terminals (early 1980's) there was a battery-powered CMOS to save the
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configuration so in this case the problem could be just a dead
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battery. Sometimes the EEPROM chip (no battery needed) goes bad after
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too many saves. It may be hard to find. If you can't fix it you are
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either stuck with the default configuration or you may have escape
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sequences sent to the terminal when you start it up to try to
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configure it.</P>
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<H2><A NAME="ss20.7">20.7</A> <A HREF="Text-Terminal-HOWTO.html#toc20.7">Capacitors</A>
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</H2>
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<P> Electrolytic capacitors have a metal shell and are may become weak
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or fail if they set for years without being used. Sometimes just
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leaving the terminal on for a while will help partially restore them.
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If you can, exercise any terminals you have in storage by turning them
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on for a while every year or so.</P>
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<P>Note that cheap electrolytic capacitors designed for use in audio
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circuits may fail if used in high frequency horizontal circuitry. For
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this, you need low resistance (low ESR) capacitors. Replace
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non-polarized capacitors (NP) with the same (or with "bi-polar").</P>
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<P>If the terminal display takes several minutes of warmup before it's
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OK then it's likely that you have one or more bad electrolytic
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capacitors. One trick to find the bad one is to parallel each
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suspected bad one with a good one (of at least the same voltage rating
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and capacitance of roughly the same order of magnitude). If the
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display improves a lot when you do this, then you've likely found the
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bad capacitor. Be careful not to get shocked when doing this. The
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actual voltage with respect to ground may be much higher than the
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voltage rating of the capacitor.</P>
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<H2><A NAME="keyboards_"></A> <A NAME="ss20.8">20.8</A> <A HREF="Text-Terminal-HOWTO.html#toc20.8">Keyboards </A>
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</H2>
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<H3>Interchangeability</H3>
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<P> The keyboards for terminals are not the same as keyboards for
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PC's. The difference is not only in the key layout but in the codes
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generated when a key is pressed. Also, keyboards for various brands
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and models of terminals are not always interchangeable with each
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other. Sometimes one get an "incompatible" keyboard to partially work
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on a terminal: All the ASCII keys will work OK, but special keys such
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as set-up and break will not work correctly.</P>
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<H3>How They Work</H3>
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<P> Most keyboards just make a simple contact between two conductors
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when you press a key. Electronics inside a chip in the keyboard
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converts this contact closure into a code sent over the keyboard's
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external cable. Instead of having a separate wire (or conductor)
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going from each key to the chip, the following scheme is used:
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Number the conductors say from 1-10 and A-J. For example: conductor 3
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goes to several keys and conductor B goes to several keys, but only
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one key has both conductors 3 and B going to it. When that key is
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pressed, a short circuit is established between 3 and B. The chip
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senses this short and knows what key has been pressed. Such a scheme
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reduces the number of conductors needed (and reduces the number of
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pins needed on the chip). It's a similar scheme to what is called a
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"crossbar switch".</P>
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<H3>Modern vs Old Keyboards</H3>
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<P> While the modern keyboard and the old fashioned type look about
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the same, the mechanics of operation are different. The old ones have
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individual key switches under the key-caps with each switch enclosed
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in a hard plastic case. The modern ones use large flexible plastic
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sheets (membrane) the size of the keyboard. A plastic sheet with
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holes in it is sandwiched between two other plastic sheets containing
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printed circuits (including contact points). When you press a key,
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the two "printed" sheets are pressed together at a certain point,
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closing the contacts printed on the sheets at that point.</P>
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<H3><A NAME="two_chars"></A> One Press Types 2 Different Characters </H3>
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<P> If, due to a defect, conductors 3 and 4 become shorted together
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then pressing the 3-B key will also short 4 and B and the chip will
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think that both keys 3-B and 4-B have been pressed. This is likely to
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type 2 different characters when all you wanted was one character.</P>
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<H3>Keyboard doesn't work at all</H3>
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<P> If none of the keys work try another keyboard (if you have one) to
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verify that the keyboard is the problem. One cause is a broken wire
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inside the cord (cable) that connects it to the terminal. The most
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likely location of the break is at either end of the cord. Try
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wigging the ends of the cord while tapping on a key to see if it works
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intermittently. If you find a bad spot, you may carefully cut into
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the cord with a knife at the bad spot and splice the broken conductor.
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Sometimes just a drop of solder will splice it. Seal up the cord with
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electrical tape, glue, or caulk. A keyboard that has gotten wet may
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not work at all until it's dry.</P>
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<H3>Typing b displays bb, etc. (doubled)</H3>
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<P> If all characters appear double there is likely nothing wrong with
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the keyboard. Instead, your terminal has likely been incorrectly set
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up for half-duplex (HDX or local echo=on) and every character you type
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is echoed back both from the electronics inside your terminal and from
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your host computer. If the two characters are not the same, there may
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be a short circuit inside your keyboard. See
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<A HREF="#two_chars">One Press Types 2 Different Characters</A></P>
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<H3>Row upon row of the same character appears</H3>
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<P> This may happen when auto-repeat is enabled and a key is held
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pressed down (or the like). It may be a key that sticks down when
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typed or it could be an electrical short that has the same
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effect.</P>
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<H3>Key sticks in down position (individual switches)</H3>
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<P> First try tapping on it hard several times but it's not likely to
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fix it. Next, your can either remove the keycap (if it is removable)
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and squirt a little cleaner on the push rod or work the key up and
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down while pushing on it sideways (or both). If this doesn`t work you
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may need to take the switch apart and clean the components.</P>
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<P>If you decide to remove the keycap see
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<A HREF="#kbd_sw">Keyboards with individual switches</A>. Press repeatedly on the
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push rod until it works OK and also displays its character on the
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screen. At first, the cleaner may cause the key to fail to display
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its character. Some keys stick due to stickiness on the keycap bottom
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surface.. If the key sticks in the fully down position this could be
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the problem. So you might need to clean this this area too.</P>
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<P>If you decide to push it sideways, use a small screwdriver to push
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sideways with while pushing the key up and down with both your finger
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and the screwdriver. You should push it sideways in one of the four
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directions and try different directions. What you are doing by this
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is attempting to force out a foreign particles that are rubbing on the
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side of the key's push-rod and making it stick. Again, the problem
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may return later.</P>
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<P>Always test the key just after fixing it and a short time later. To test
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the key, push it down very slowly and see if it sticks. Also push it
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sideways a little as you're pushing it down. If you hit it fast or
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push it straight down, then you may not observe the stickiness. This
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test will detect a key that seemingly works OK but is likely to cause
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trouble later on.</P>
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<H3>Key electrically shorted</H3>
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<P>If you suspect that a key is shorted out, fix it by cleaning the
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contacts per
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<A HREF="#clean_keys">Cleaning Keyboard Contacts</A>.
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If this problem happens at the login prompt see
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<A HREF="Text-Terminal-HOWTO-19.html#key_shorted_getty">Key shorted</A>.</P>
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<H3>Liquid spilled on the keyboard</H3>
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<P> If water or watery liquid has been spilled on the keyboard (or if
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it was exposed to rain, heavy dew, or dampness) some (or all) keys may
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not work right. The dampness may cause a key to short out (like it
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was pressed down all the time) and you may see the screen fill up with
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that letter if auto-repeat is enabled. If it's gotten wet and then
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partially (or fully) dried out, certain keys may not work due to
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deposits on the contact surfaces. For the modern type of keyboard,
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one may readily take apart the plastic sheets inside and dry/clean
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them. For the old type one may let it dry out in the sun or oven (low
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temp.). When it's dry it may still need contact cleaner on some keys
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as explained below.</P>
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<H3><A NAME="clean_keys"></A> Cleaning keyboard contacts </H3>
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<H3>Keyboards with membranes</H3>
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<P> On some newer keyboards, the plastic sheets (membranes) are easy
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to remove for inspection and cleaning if needed. You only need to
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remove several screws to take apart the keyboard and get to the
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sheets. On some old IBM keyboards the sheets can't be removed without
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breaking off many plastic tabs which will need to be repaired with
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glue to put back (probably not worthwhile to repair). Such a keyboard
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may sometimes be made to work by flexing, twisting, and/or pounding
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the assembly containing the plastic sheets.</P>
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<H3><A NAME="kbd_sw"></A> Keyboards with individual switches </H3>
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<P> What follows is for older keyboards that have separate hard
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plastic switches for each key. Before going to all the work of
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cleaning electrical contacts first try turning the keyboard
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upside-down and working the bad keys. This may help dislodge dirt,
|
|
especially if you press the key hard and fast to set up vibration.
|
|
Pressing the key down and wiggling it from side to side, etc. often
|
|
helps.</P>
|
|
<P>If this doesn't work, you may try to clean the key switch with a
|
|
liquid contact cleaner (available at electronic supply stores) which
|
|
usually comes in a spay can. To get to the switch, you first need to
|
|
remove the key-cap (the square that you hit with your finger while
|
|
typing). Warning: Key-caps on modern keyboards can't be removed.
|
|
Often, the key-caps may be removed by prying them upward using a small
|
|
screwdriver with the tip placed under a key while preventing excessive
|
|
tilting of the key with a finger. There exists a special tool known
|
|
as keycap puller but you can get by without it. The key-cap may tilt
|
|
a bit and wobble as it comes loose. It may even fly up into the air
|
|
and onto the floor.</P>
|
|
<P>Then you may have two choices on how to clean the contacts: Use
|
|
contact cleaner spray directly on top of the key switch, or take the
|
|
key switch apart and clean it (the best way if it comes apart easily).
|
|
Still another choice is to replace the key switch with a new or used
|
|
one but this is often more work (and more cost if you have to go thru
|
|
the trouble of finding a replacement.</P>
|
|
<P>Directly spraying contact cleaner into the top of the key switch,
|
|
without taking the switch apart, is the fastest method but the cleaner
|
|
may not reach the contacts it's supposed to clean. Before spraying,
|
|
clean the area around it a little. With the keyboard live (or with
|
|
the key contacts connected to an ohm-meter) use the plastic tube which
|
|
came with the spray to squirt cleaner so it will get inside the key
|
|
switch. Try to move the key push rod up and down while spraying.
|
|
Don't let the cleaning liquid get under nearby keys where it may pick
|
|
up dust and then seep (with the dust) into adjacent key switches. If
|
|
you make this mistake you may fix one key but damage nearby keys. If
|
|
this should happen, immediately work (repeatedly press) the affected
|
|
nearby keys until they continue to work OK.</P>
|
|
<P>You might tilt the keyboard so that the cleaner flows better into the
|
|
contacts. For the CIT101e terminal with an Alps keyboard, this means
|
|
tilting the top row of numeric keys up toward the ceiling. While
|
|
moving the key switch up and down with a pen or small screwdriver
|
|
handle avoid getting the toxic cleaner liquid on your skin (or wear
|
|
gloves). You might try turning the keyboard upside-down while working
|
|
the key to drain off remaining cleaner. The more cleaner you squirt
|
|
in the more likely it will fix it but it is also more likely to do
|
|
more damage to the plastic or contaminate adjacent keys, so use what
|
|
you think is just enough to do the job. Once the key works OK, work
|
|
it up and down a little more and test it a half minute later, etc. to
|
|
make sure it will still work OK.</P>
|
|
<P>Sometimes a key works fine when the contacts inside are saturated with
|
|
contact cleaner liquid. But when the liquid dries a few minutes later
|
|
then the resulting scale deposit left from the evaporation of the
|
|
cleaning liquid on the contacts, prevents good contact. Then the key
|
|
may work erratically (if at all). Operating the key when the liquid
|
|
is drying inside may help. Some switches have the contacts nearly
|
|
sealed inside so little if any contact cleaner reaches the contacts.
|
|
The cleaner that does get to the contacts may carry contamination with
|
|
it (cleaning around the tops before spraying helps minimize this).</P>
|
|
<P>If you want to disassemble the key switch, first inspect it to see
|
|
if it comes apart (and if so, how). Sometimes one may remove the cover
|
|
of the switch without removing the switch from the keyboard. To do
|
|
this pry up (or pull up) the top of the key switch after prying apart
|
|
thin plastic tabs that retain it. You may be able to use two small
|
|
screwdrivers for this and be able to pry up the switch while prying
|
|
apart the plastic retaining tabs. Don't pry too hard or you may break
|
|
the plastic. If this can't be done, you may have to unsolder the
|
|
switch and remove it in order to take it apart (or replace it). Once
|
|
the switch has been taken apart you still may not be able to see the
|
|
contacts if the contact surfaces are sandwiched together (nearly
|
|
touching). You may put contact cleaner on the contacts by squirting
|
|
some cleaner on an edge so it can penetrate onto the contacts. Insert
|
|
a tiny screwdriver blade just a little so as to pry apart the edges as
|
|
you apply the cleaner. This will help the cleaner reach the contacts.
|
|
Work the contacts open and closed with a screwdriver to help clean
|
|
them and note if the key is working by looking at the terminal screen. </P>
|
|
<P>There may be some kind of clip holding the contact surfaces together
|
|
which needs to be removed so you can pry them apart. Take care not to
|
|
loose small parts as they may fly up into the air when taking apart a
|
|
key switch. As a last resort, you may try bending the moving part
|
|
that the push-rod pushes so as to make a stronger contact. In my
|
|
terminal, this part looks like the electrical contact but it just
|
|
pushes the real electrical contact thru a thin insulator.</P>
|
|
<P>When putting the key switch back together, make sure that the spring
|
|
is in the right place. If, after you assemble the switch, the key
|
|
pushes down too hard or too easy, the spring is likely not positioned
|
|
right. If the spring is supposed to be recessed into a hole on the
|
|
push rod, one way to temporarily "glue" the spring into the push-rod
|
|
is to use a half-drop of water on the end of the spring. Then insert
|
|
this end into the push rod and assemble quickly before the water
|
|
dries. This should keep the spring from falling out of the push rod
|
|
during assembly. Instead of using water, you may stand the keyboard on
|
|
end (or upside-down) to keep the spring from falling out during
|
|
assembly.</P>
|
|
|
|
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