Keyfix - mylar keyboard replacement

We don’t sell the Keyfix board preinstalled in a keyboard. I’d like to, maybe some kind of core exchange. But I need cores! If you have an electrically bad but cosmetically good keyboard you’ve already replaced, let’s talk.

Anyway, CoNect offers a repair kit, or a bare board for experimenting with different switches. You might wonder what kind of work is involved installing the kit, so here we go. 

The obvious first step is to split the CoCo case. Six screws to cast <<<<- set aside, flip it right side up and the top lifts off. Same for the keyboard. There is no lock on the mylar ribbon – just pull straight up. Set the CoCo bottom half aside for now.

Your install jig is the case top. Place it in typing position, then flip your keyboard left to right and drop the keys into the cutout, so none of the keys are being pressed. Next remove 18 tiny Phillips screws, cast them aside for reals after finding the envelope of 2x6mm replacement screws.

Note besides the screws two locating posts stick through the metal plate. They are a tight fit. Lift backplate from the top edge and don’t break these off. Also, don’t spill the 56 individual springs! If you do find them all, the green spring is for the spacebar. 

Assembling the keyboard

Grab the Keyfix board, align the two locating posts and push down a bit. But don’t screw it down just yet. To accommodate different types of physical switch, the board is designed to float on all the key springs, and the clickity height is set by how far you screw it down.

Best to hold it down a bit and install the zigzag of screws in the middle by about 3 turns each. That will hold the board down while the outside screws get their 3 turns. Repeat the pattern, tightening every screw 3 turns more. You’ll still be able to push the board in a little.

Flip the keyboard over and check key travel in a couple places. Push a key slowly until it clicks, continue until it bottoms out and note how much travel was left. You can adjust the mounting screws in or out a bit to change where the key clicks.
Before putting everything back together, push each and every key in succession. They should feel and sound about the same.

Putting CoCo back together

Grab the CoCo base, plug in keyboard connector by pushing down evenly top of the plastic shell either end. This is a fragile bit of PCB,aim! Then sit keyboard in place. Does it touch (or very close) the rubber cushions on both side posts? If not, remove the rubber cap from the center post under the keyboard.

Finally, fire up the CoCo and check all the keys. Don’t worry about the keys that usually ‘do nothing’, if the regular printable keys all work we are gold. If not, there will probably be a half dozen out, meaning one line isn't connected. Remove and reinsert the keyboard connector, pulling on the cable, not the housing, to remove. Yes, that is weird.

Assemble the case - note the screws for the back 2 holes are longer than the other four.

Bare board notes.

If you want to play with your own keyboard ideas, the bare board accepts thru hole or SMT switches. The pads are actually 3/8 x ¼ inch thru hole, but large enough that 4x6mm and 6x6mm SMT switches will stick along with the thru hole packages. A height of 0-3mm deck to button top is workable, but here is the gotcha – 60gf or less. Your garden variety ‘tactile switch’ wants 100-120gf and ainagonna switch.

Soldering SMT. You’ll want to center each switch in the silkscreen box. My approach is to presolder one corner of each switch position, then hold each switch in position as I reheat that corner to fix the part in place. When all the switches are properly aligned and double checked, flip the board over and solder the remaining three corners from the backside, going hot and fast but using enough solder it doesn’t suck into the via when you go, at least flat. When you inspect the top there should be a visible meniscus of solder attaching switch terminals to board.

And that’s about it. I’ll have a CoCo2 version soon.