Repairing a terminal has much in common with repairing a monitor
and/or keyboard. Sometimes the built-in diagnostics of the terminal
will tell you what is wrong on the screen. If not, then by the
symptoms, one may often isolate the trouble to one of the following:
bad keyboard, CRT dead, terminal digital electronics failure. It's
best to have a service manual, but even if you don't have one, many
terminals may still be repaired.
Bigelow, Stephen J.: Troubleshooting & Repairing Computer
Monitors, 2nd edition, McGraw-Hill, 1997. Doesn't cover the character
generation electronics nor the keyboard.
Websites
The FAQ
http://www.repairfaq.org for the newsgroup:
sci.electronics.repair is long and comprehensive, although it doesn't
cover terminals per se. See the section "Computer and Video
Monitors". Much of this information is applicable to terminals as are
the sections: "Testing Capacitors", "Testing Flyback Transformers",
etc. Perhaps in the future, the "info" on repair in this HOWTO will
consist mainly of links to the above FAQ (or the like).
Shuford's repair archive of newsgroup postings on terminal
repair is another source of info.
CRT's use high voltage of up to 30,000 volts for color (less for
monochrome). Be careful not to touch this voltage if the set is on
and the cover off. It probably won't kill you even if you do since
the amount of current it can supply is limited. But it is likely to
badly burn and shock you, etc. High voltage can jump across air gaps
and go thru cracked insulation so keep your hands a safe distance from
it. You should notice the well-insulated high voltage cable connected
to one side of the picture tube. Even when the set is off, there is
still enough residual voltage on the picture tube cable connection to
give you quite a shock. To discharge this voltage when the set is
unplugged use a screwdriver (insulated handle) with the metal blade
grounded to the picture tube ground cable with a jumper wire. Don't
use chassis ground.
The lower voltages (of hundreds of volts) can be even more dangerous
since they are not current limited. It is even more dangerous if your
hands are wet or if you are wearing a metal watchband, ring or the
like. In rare cases people have been killed by it so be careful. The
lowest voltages of only several volts on digital circuitry are fairly
safe but don't touch anything (except with a well insulated tool)
unless you know for sure.
If the display is too dim, turn up the brightness and/or contrast.
using knobs on the exterior of the unit (if they exist). If the
width, height or centering is incorrect, there are often control knobs
for these. For some older terminals one must press an arrow key
(or the like) in set-up mode.
You may need to remove the cover to make adjustments, especially on
older models. You could arrange things so that a large mirror is in
front of the terminal so as to view the display in the mirror while
making adjustments. The adjustments to turn may be on a printed
circuit board. While a screwdriver (possibly Phillips-head) may be
all that's needed, inductors may require special TV alignment tools
(plastic hex wrenches, etc.). The abbreviated name of the adjustment
should be printed on the circuit board. For example, here are some
such names:
V-Size adjusts the Vertical height (Size)
H-Size adjusts the Horizontal width (Size). It may be an inductor.
V-Pos adjusts the Vertical Position
H-Pos adjusts the Horizontal Position
V-Lin adjusts Vertical Linearity (Use if width of scan lines
differs at the top and bottom of the screen)
V-Hold adjusts Vertical Hold (Use if screen is uncontrollable
scrolling)
Bright adjusts brightness (an external knob may also exist)
Sub-Bright adjusts brightness of subdued intensity mode (often
the normal mode: dimmer than bold or bright mode).
Changing linearity may change the size so that it will need to be
readjusted. A terminal that has been stored for some time may have a
small display rectangle on the screen surrounded by a large black
border. If it's difficult to adjust, wait a while before adjusting it
since it will likely recover some with use (the black borders will
shrink).
If the terminal made some noise just before it failed (or when you
turn it on after it failed) that noise is a clue to what is wrong. If
you hear a sparking noise or see/smell smoke, immediately turn the
terminal off to prevent further damage. The problem is likely in the
high voltage power supply of several thousand volts. Remove the cover
and if the bad spot is not evident, turn it on again for a short time
in a dimly lit room to look for arcing. The high voltage
cable (runs between the flyback transformer and the side of the
picture tube) may have broken insulation that arcs to ground. Fix it
with high-voltage insulating dope, or special electrical tape designed
say for 10,000 volts.
The flyback transformer (high voltage) may make only a faint clicking
or sparking noise if it fails. You may not hear it until you turn the
terminal off for a while to rest and then turn it back on again. To
track down the noise you may use a piece of small rubber tubing (such
as used in automobiles) as a stethoscope to listen to it. But while
you are listening for the noise, the terminal is suffering more damage
so try find it fast (but not so fast as to risk getting shocked).
A short in the power supply may cause a fuse to blow with a pop.
Replacing a blown fuse may not solve the problem as the same short may
blow the fuse again. Inspect for any darkened spots due to high heat
and test those components. Shorted power transistors may cause the
fuse to blow. They may be tested with a transistor checker or even
with an ohm-meter. Use the low ohm scale on an ohm-meter so that the
voltage applied by the meter is low. This will reduce the possible
damage to good components caused by this test voltage.
If the terminal has been exposed to dampness such as being stored in a
damp place or near a kitchen with steam from cooking, a fix may be to
dry out the unit. Heating a "failed" flyback transformer with a blow
dryer for several minutes may restore it.
Terminal Made No Noise
A blank screen may be due to someone turning the brightness
control to the lowest level or to aging. The next thing to do is to
check the cables for loose or broken connections. If there is no sign
of power, substitute a new power cord after making sure that the power
outlet on the wall is "hot".
If the keyboard is suspected, try it on another terminal of the same
type or substitute a good keyboard. Wiggle the keyboard cable ends
and the plug. Wires inside cables may break, especially near their
ends. If the break is verified by wiggling it (having the problem go
on and off in synchronization with the wiggles), then one may either
get a new cable or cut into the cable and re-solder the breaks, etc.
One of the first things to do if the keyboard works is to put the
terminal into
Local Mode. If it works OK
in local, then the problem is likely in the connection to the host
computer (or incorrect interface) or in the UART chips of the
terminal.
By carefully inspecting the circuitry, one may often find the cause of
the problem. Look for discoloration, cracks, etc. An intermittent
problem may sometimes be found by tapping on components with a
ball-point pen (not the metal tip of course). A break in the
conductor of a printed circuit board may sometimes be revealed by
flexing the board. Solder that looks like it formed a drop or a
solder joint with little solder may need re-soldering. Soldering may
heat up transistors (and other components) and damage them so use a
heat sink if feasible.
If you have a common brand of terminal, you may be able to search
newsgroup postings on the Internet to find out what the most frequent
types of problems are for your terminal and perhaps information on how
to fix them.
To see if the digital electronics work, try (using a good keyboard)
typing at the bad terminal. Try to read this typing at a
good terminal (or the console) using the copy command or with a
terminal communication program such as minicom. You may need to hit
the return key at the terminal in order to send a line. One may
ask the bad terminal for its identity etc. from another terminal.
This will show if two-way communication works.
You are in luck if you see an error message on the screen. This
usually happens when you first turn the terminal on.
Keyboard Error
This usually means that the keyboard is not plugged in, or that
the connection is loose. For more serious problems see
Keyboards
Checksum Error in NVR
NVR is "Non-Volatile RAM". This means that the NVR where the
set-up information is stored has become corrupted. The terminal will
likely still work but the configuration that was last saved when
someone last configured the terminal has likely been lost. Try
configuring again and then save it. It might work. On very old
terminals (early 1980's) there was a battery-powered CMOS to save the
configuration so in this case the problem could be just a dead
battery. Sometimes the EEPROM chip (no battery needed) goes bad after
too many saves. It may be hard to find. If you can't fix it you are
either stuck with the default configuration or you may have escape
sequences sent to the terminal when you start it up to try to
configure it.
Electrolytic capacitors have a metal shell and are may become weak
or fail if they set for years without being used. Sometimes just
leaving the terminal on for a while will help partially restore them.
If you can, exercise any terminals you have in storage by turning them
on for a while every year or two.
The keyboards for terminals are not the same as keyboards for
PC's. The difference is not only in the key layout but in the codes
generated when a key is pressed. Also, keyboards for various brands
and models of terminals are not always interchangeable with each
other. Sometimes one get an "incompatible" keyboard to partially work
on a terminal. All the ASCII keys will work OK, but special keys such
as set-up and break will not work correctly.
How They Work
Most keyboards just make a simple contact between two conductors
when you press a key. Electronics inside a chip in the keyboard
converts this contact closure into a code sent over the keyboard's
external cable. Instead of having a separate wire (or conductor)
going from each key to the chip, the following type scheme is used.
Number the conductors say from 1-10 and A-J. For example: conductor 3
goes to several keys and conductor B goes to several keys, but only
one key has both conductors 3 and B going to it. When that key is
pressed, a short circuit is established between 3 and B. The chip
senses this short and knows what key has been pressed. Such a scheme
reduces the number of conductors needed (and reduces the number of
pins needed on the chip). It's a similar scheme to what is called a
"crossbar switch".
One Press Types 2 Different Characters
If, due to a defect, conductors 3 and 4 become shorted together
then pressing the 3-B key will also short 4 and B and the chip will
think that both keys 3-B and 4-B have been pressed. This is likely to
type 2 different characters when all you wanted was one character.
Modern vs Old Keyboards
While the modern keyboard and the old fashioned type look about
the same, the mechanics of operation are different. The old ones have
individual key switches under the key-caps with each switch enclosed
in a hard plastic case. The modern ones use large flexible plastic
sheets (membrane) the size of the keyboard. A plastic sheet with
holes in it is sandwiched between two other plastic sheets containing
printed circuits (including contact points). When you press a key,
the two "printed" sheets are pressed together at a certain point,
closing the contacts printed on the sheets at that point.
Keyboard Doesn't Work At All
If none of the keys work try another keyboard (if you have one) to
verify that the keyboard is the problem. The most likely cause is a
broken wire inside the cord (cable) that connects it to the terminal.
The most likely location of the break is at either end of the cord.
Try wigging the ends of the cord while tapping on a key to see if it
works intermittently. If you find a bad spot, you may carefully cut
into the cord with a knife at the bad spot and splice the broken
conductor. Sometimes just a drop of solder will splice it. Seal up
the cord with electrical tape, glue, or caulk.
Typing b Displays bb, etc. (doubled)
If all characters appear double there is likely nothing wrong with
the keyboard. Instead, your terminal has likely been incorrectly set
up for half-duplex (HDX or local echo=on) and every character you type
is echoed back both from the electronics inside your terminal and from
your host computer. If the two characters are not the same, there may
be a short circuit inside your keyboard. See
One Press Types 2 Different Characters
The Keyboard Types By Itself
If a key is shorted out it is likely to type out a large number of
the same character if auto-repeat is enabled. If more than one key is
shorted out then repeating sequences of a few characters will be
typed. This may cause getty to respawn too fast if it happens at the
login prompt. See
Key Shorted. The fix
is to clean the contacts per
Cleaning Keyboard Contacts.
Liquid Spilled on the Keyboard
If water or watery liquid has been spilled on the keyboard (or if
it was exposed to rain, heavy dew, or dampness) some keys may not work
right. The dampness may cause a key to short out (like it was pressed
down all the time) and you may see the screen fill up with that letter
if auto-repeat is enabled. If it's gotten wet and then partially (or
fully) dried out, certain keys may not work due to deposits on the
contact surfaces. For the modern type of keyboard, one may readily
take apart the plastic sheets inside and dry/clean them. For the old
type one may let it dry out in the sun or oven (low temp.). When it's
dry it may still need contact cleaner on some keys as explained below.
Cleaning Keyboard Contacts
Keyboards with Membranes
On some newer keyboards, the plastic sheets (membranes) are easy
to remove for inspection and cleaning if needed. You only need to
remove several screws to take apart the keyboard and get to the
sheets. On some old IBM keyboards the sheets can't be removed without
breaking off many plastic tabs which will need to be repaired with
glue to put back (probably not worthwhile to repair). Such a keyboard
may sometimes be made to work by flexing, twisting, and/or pounding
the assembly containing the plastic sheets.
Keyboards with Individual Switches
What follows is for older keyboards that have separate hard
plastic switches for each key. Before going to all the work of
cleaning electrical contacts first try turning the keyboard
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.
Often the key-caps may be removed by prying them upward using a small
screwdriver as a lever while preventing excessive tilting with a
finger. There exists a special tool known as keycap puller but you
can get by without it. (Warning: Key-caps on modern keyboards don't
pry up.) The key-cap may tilt a bit and wobble as it comes loose. It
may even fly up and onto the floor. Then you 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. Still
another choice is to replace the key switch with a new or used one.
Directly spraying contact cleaner or the like (obtained at an
electronics store) into the top of the key switch is the fastest
method but it may not work. 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 tube which came with the spray to squirt
cleaner so it will get inside the key switch. 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 work OK.
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 digit row up toward the ceiling. Work the key switch up
and down with a pen or small screwdriver handle to 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. I don't usually do this. 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.
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).
If you need to disassemble the key switch, first inspect it to see how
it is installed and comes apart. 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. Don't pry too hard or you may break
the thin 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 get contact cleaner on the contacts by slightly
prying apart the conducting surfaces and squirting cleaner between
them. There may be some kind of clip holding the contact surfaces
together which needs to be removed before prying these surfaces apart.
With cleaner on the contacts, work them. Tilting the keyboard or
inverting it may help. Take care not to loose small parts as they may
fly up into the air when taking apart a key switch.
