H/Z-100 Keyboard Key Repair

General Notes:

  • This article was published in the May-June 1994 issue of the "Z-100 LifeLine", issue #33.
  • WARNING:  The following fix requires some knowledge of electronics and soldering skills! Try this ONLY AT YOUR OWN RISK!
  • WARNING:  All-In-One users MUST KEEP CLEAR of the CRT power cables, the CRT second anode (the thick single cable going to the top of the CRT) and the high voltage transformer on the vertical video board during computer operation. Voltages in these locations are DANGEROUS.

 

Z-100 KEYBOARD KEY REPAIR

One of the most common and frustrating problems encountered with the Z-100 series computer is with the keyboard. It seems that all of them eventually have a key that no longer functions, or gives multiple characters when pressed. Cleaning is also a challenge and can cause problems if not done very carefully.

Fortunately, there is an inexpensive fix for malfunctioning keys that has a 90% or more success rate. The fix is easy, but does require soldering skills. Also, set aside a few hours of uninterrupted work as this involves disassembly and reassembly of the computer. A fancy desoldering gun is not required, as a squeeze bulb or desoldering braid works nearly as well.


Tools  (in addition to those needed to disassemble the computer):

  • Vacuum cleaner
  • Small, 1" or less, paint brush
  • Ohmmeter
  • Soldering gun (or iron) and solder
  • Desoldering gun, squeeze bulb, or desoldering braid
  • Two metal paper clips
  • Dental pick, if available
  • Small jeweler's screwdriver
  • Small needlenose pliers


Disassemble the computer in accordance with Chapter 1 of the Z-100 Users' Manual until the keyboard can be removed from the computer. Disconnect the two ribbon cables attaching the keyboard to the motherboard at the motherboard. They just pull straight up, but may require some gentle prying at each end. Remove the keyboard.

Though the keyboard need not be removed for cleaning, now is a good time to do that.

CAUTION:  DO NOT spray the interior of the keys with any kind of cleaner or lubricant such as WD-40. The cleaner or lubricant will leave a film that will damage the key's delicate interior.

Using the paint brush to loosen stubborn dust and the vacuum cleaner, vacuum the top of the keyboard, concentrating on the area under the key caps. Using a terry cloth or hand towel, soak a small section with a strong cleaner. I find 409 is excellent. Carefully, rub the top and around each key individually.

Noting the location of the malfunctioning key or keys, select the solder lugs that belong to those keys. Place the keyboard, inverted, on a towel on the work table. Remove the solder completely from these lugs using a desoldering gun, desoldering squeeze bulb, or desoldering braid, until the lugs move freely in their holes. Figure 1 shows the mounting of a couple of keys in the keyboard. Figure 1(a) shows the view of a fully installed key.

 

ZKey1


Flip the keyboard over. Straighten the two paper clips and form a 1/4" hook on one end of each using the needlenose pliers. Insert them under opposite sides of the malfunctioning key cap and remove the key cap by pulling straight up, gently - figure 1(b).

Study the revealed key switch. Figure 1(c) gives top and side views. On each side of the switch, and as shown by the arrows, is a broad plastic catch lever that retracts toward the center post to release the switch from the keyboard bracket. With the small jeweler's screwdriver, press one of these levers toward the center post. Release it and it snaps back into place. The object is to retract these levers while pulling the switch gently from its socket - no easy stunt. Pressing the solder lugs up from the bottom while pinching the levers together toward the switch's center post will usually work. An alternative method against a stubborn switch is to pry up on one wing of the switch by wedging a dental pick under it from the top, while pressing the lever of the switch on the same side to the center. Then do the same from the other side. Unless the key is on an edge, all work will have to be done from the top.

Once removed, the tough part is over. The switch is encased by a plastic cover on all sides except the top, which is separate. See figure 2. Clean off any remaining solder from the solder lugs as this case must be removed over these lugs.

 

ZKey2


Two sides of the switch case mate and interlock with the top cover. They can be carefully pryed away from the edge of the top cover, first one side, then the other, and the case then slid down the solder pins and removed. The remaining insides are simple - a spring, a centerpost slide and the wafer switch itself with the solder lugs attached.

Examining the insides, the centerpost slides down when pressed, placing pressure against the metal inverted "Y" leaf spring that then presses against the body of the wafer switch. In more than 90% of the cases, I have found that this inverted "Y" doesn't make firm enough pressure against the switch body to close the switch. To increase this pressure (unrelated to spring pressure) the bent angle of the inverted "Y" simply needs to be increased slightly.

Using a small pair of needlenose pliers, grasp the inverted "Y" at the free-end side of the existing bend and increase the bend slightly. Bending too far will make the key feel stiff when pressed and, if exaggerated further, may even interfere with the spring, jamming the key.

It is easiest to assemble the key case from the top down, imagining an inverted Figure 2.  Insert the white centerpost into the inverted top cover. Next insert the wafer switch itself, with the lugs sticking out and the inverted "Y" towards the centerpost. Pressing up on the centerpost should compress this inverted "Y" into the wafer switch. On the opposite side of the centerpost, insert the spring into the round recess. Finally, gently lower the key case over the solder lugs until the sides interlock with the edges of the top cover.

Before reinstalling the key into the keyboard, check the switch with an ohmmeter. Connect the leads of the ohmmeter across the solder lugs; the resistance should be infinity. Press the switch and the resistance should instantly drop to zero ohms. If it doesn't, recheck the assembly of the switch. The centerpost must compress the Y against the wafer switch.

If the switch is still inoperative, another can be ordered from me, or other remaining Z-100 hobbyists.

The switch is reinstalled in the keyboard by aligning the solder lugs with the solder holes on the circuit board and pressing down until it clicks into place. The lugs are offset so the switch can only be installed in one direction. The key cap is simply pressed on. Solder the solder lugs to the backboard and you're in business.

Before reassembling the computer, press the keyboard cables onto their motherboard connectors, connect an external monitor to the backboard or, in the case of the Z-120 All-In-One, gently place the monitor assembly onto the computer's base frame (Be very careful, but the monitor assembly will fit nicely on top of the metal sides), without the bottom cover installed, and connect the power cable to the vertical video driver board on the left side. Don't worry about the drives' data or power cables. You just want to try the keyboard. During the following test, All-In-One users MUST KEEP CLEAR of the CRT power cables, the CRT second anode (the thick single cable going to the top of the CRT) and the high voltage transformer on the vertical video board. Voltages in these locations are DANGEROUS.

Turn on the computer and at the hand prompt, press {T} for Test. Perform the keyboard test to ensure all keys operate normally. If not, fix these keys as above and retest.

Finally, reassemble the computer per the Users' Manual.

 

There are a few special keys, but the basic construction is the same.


The {CAPS LOCK} key is much more complex than a normal key. It is constructed with an additional leaf spring and a wire clip that, in combination with the special centerpost, keeps the key depressed until another tap releases it. The spring just beneath the key cap provides the key tension. The top of the spring fits in a hole about 1/2 way up the centerpost. See Figure 3.

 

ZKey3          ZKey4


The {RESET} key is also more complicated because it has a light emitting diode (LED) installed. See Figure 4. If this key fails, removal is the same as above, except there are four solder lugs - including two for the LED. The metal bracket fits over the top cover.  If the ohmmeter test confirms failure, it is probably best to just order a replacement.

Steven W. Vagts, Editor, "Z-100 LifeLine"

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