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19. Repair & Diagnose

Repairing a terminal has much in common with repairing a monitor and/or keyboard. Sometimes the built-in diagnostics of the terminal will display on the screen. By the symptoms, one may often isolate the trouble to one of the following: bad keyboard, CRT dead, power electronics failure, or digital electronics failure. It's best to have a service manual, but even if you don't have one, you can often still repair it.

19.1 Repair Books & Websites


Bigelow, Stephen J.: Troubleshooting & Repairing Computer Monitors, 2nd edition, McGraw-Hill, 1997. Doesn't cover the character generation electronics nor the keyboard.


The FAQ for the newsgroup: 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.

19.2 Safety

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.

19.3 Appearance of Display

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:

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 Before adjusting it, leave the terminal on for a while since it will likely recover some with use (the black borders will shrink).

19.4 Diagnose

Terminal Made a Noise or Smoked

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 noise or see/smell smoke, immediately turn the terminal off to prevent further damage. A pop noise may be a capacitor exploding or a fuse blowing. A buzzing noise is likely due to arcing. The problem may be in the high voltage power supply of several thousand volts.

Remove the cover. Look for discoloration and bulging/cracked capacitors. If the bad spot is not evident, turn it on again for a short time and look for smoking/arcing. For arcing, a dimly lit room will help find it. 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 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 shorted power supply may cause a fuse to blow. 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.

19.5 Detective work

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. One failure may cause others, so unless you find the original cause, the failure may reoccur.

If you have a common brand of terminal, you may be able to search the Internet (including newsgroup postings) to find out what the most frequent types of problems are for your terminal and perhaps information on how to fix it. If you find that a certain component is bad you may search for this component (for example R214 wyse) and hopefully find a report by someone else who had the same problem. Such a report may indicate other components that failed at the same time. If a component is damaged so badly that its value can't be read, then you might find it on the Internet. The manufacturer may have on-line data that search engines don't index.

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.

19.6 Error Messages on the Screen

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.

19.7 Capacitors

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.

Note that cheap electrolytic capacitors designed for use in audio circuits may fail if used in high frequency horizontal circuitry. For this, you need low resistance (low ESR) capacitors. Replace non-polarized capacitors (NP) with the same (or with "bi-polar").

19.8 Keyboards


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".

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.

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.

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. One 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. A keyboard that has gotten wet may not work at all until it's dry.

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

Row upon row of the same character appears

This may happen when auto-repeat is enabled and a key is held pressed down (or the like). It may be a key that sticks down when typed or it could be an electrical short that has the same effect.

Key sticks in down position (individual switches)

If it's a stuck key on a keyboard with individual switches, a good way to fix it is to remove the keycap (if it's removable). See id="kbd_sw" name="Keyboards with individual switches">. Use a small amount of cleaner on the push rod. Press repeatedly on the key until it works OK and displays its character on the screen. At first, the cleaner may cause the key to fail to display its character. Some keys stick due to stickiness on the keycap bottom surface.. If the key sticks in the fully down position this could be the problem. So you might need to clean this this area too.

While the best method is to use cleaner as per above, just hitting the key a lot to exercise it may help, but the problem is likely to return. If you suspect the push rod is sticking you might try to type it while pushing sideways on it with a small screwdriver. You should push it sideways in one of the four directions and try different directions. What you are doing by this is attempting to force out a foreign particle that is rubbing on the side of the key's push-rod and making it stick. Again, the problem may return later.

To test the key, push it down very slowly and see if it sticks. Also push it sideways a little as you're pushing it down. If you hit it fast or push it straight down, then you may not observe the stickiness.

Key electrically shorted

If you suspect that a key is shorted out, fix it by cleaning the contacts per Cleaning Keyboard Contacts. If this problem happens at the login prompt see Key shorted.

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 (or all) 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 continue to 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.

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