Build Instructions

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• 1

  Introduction

  
  

  Introduction

  
  

  This project is intended as an upgrade for an existing Mitsumi mylar "keyboard". As such, you will require one of these (probably non-functioning) "keyboards" to begin. We will be re-using all of the original keycaps, the metal PCB support strips and screws, and (optionally) the space bar support brackets and 15-pin ribbon connector.

  Don't feel bad for it. That brown abomination should have been relegated to landfill years ago. It's one of the lucky ones!

• 2

  Bill of Materials

  Here's the full list of items you'll need to complete this upgrade. Some of these items are optional, many are 3D printed, and some can be salvaged from your mylar monster.

  Qty.	Description	Thumbnail	Notes
  1	Mitsumi mylar TI-99/4A "keyboard"		To pillage keycaps and (optionally) other components
  1	KEYBOARD/4A-99 PCB		I recommend ordering it in Black. But, you do you. See /pcb
  47	Cherry MX compatible key switches		I have tested Akko V3 Cream Black (Amazon). Unfortunately, they only come in 45 packs. Such is life. Important: Check your switches have the two additional locating pins. Some of these only have the center locating pin. These won't be as stable.
  1	8mm latching button		For Alpha lock. GPBS-800L (Mouser)
  1	1N4148 Diode (Optional)		Can be jumpered if you don't want the ALPHA LOCK fix
  1	15-pin 0.1" pitch x ~75mm+ female IDC cable		Can re-use cable from original "keyboard". Can use new Dupont cable (optionally, with a header on the PCB). New parts: Can get two from a IDSS-15-D-06.00-G-R (Mouser) OR one from a IDSS-15-S-03.00-T-G-ST4 (Mouser)
  10	3x8mm self-tapping OR M3x5 machine screws		Either 3x8mm self tapping (Amazon) OR M3x5 machine screws (Amazon) to suit brass inserts.
  10	M3 x 5.7 x 4.6mm brass inserts (Optional)		M3 x 5.7 x 4.6 from Amazon
  1	PCB mount (left)		3D printed (self tapping or brass insert options available)
  1	PCB mount (right)		3D printed (self tapping or brass insert options available)
  2	PCB support backer		3D printed. The metal supports on the original keyboard will screw into these.
  2	Space bar support		3D printed (self tapping or brass insert options available) Two model options available: * One to re-use Mitsumi spacebar support (pictured) * Another two-piece model using all new 3D printed components
  45	Keycap adapter (standard)		3D printed. 7x7 array model is also available.
  1	Keycap adapter (space bar)		3D printed. I've found the slot in the space bar is slightly larger. It could just me mine?.
  1	Keycap adapter (right shift)		3D printed
  1	Keycap adapter (alpha lock)		3D printed

• 3

  3D Printing Notes

  All 3D printed models were designed using Onshape. If you need to make modifications, the project is available here: KEYBOARD/4A-99 on Onshape
  

  In developing this upgrade, all items were printed using:

  • Printer: Creality Ender-3 V3 KE
  • Nozzle: 0.4mm
  • Filament: 1.75mm Elegoo PLA+ (PETG would likely work very well too)
  • Layer height: 0.15mm
  • Supports: Yes, from bed only (except for standard keycap adapters - no supports)

  Note: The keycap adapters in particular require very fine tolerances. The models I've provided may or may not provide the correct tolerances with your printer/material/settings and also minor differences in keycaps and switches. Again, the Onshape project can be used to adjust these for your specific setup.

  Threaded inserts

  
  

  All of the models which accept screws have two versions. One which takes threaded inserts and another designed to take a self-tapping screw directly. e.g.

  keyboard4a99-pcb-mount-left-inserts.stl
  keyboard4a99-pcb-mount-left-selftap.stl
  

  Type	Hole diameter	Notes
  Brass inserts	4.5mm	M3 x 5.7 x 4.6 from Amazon
  Self tapping	2.8mm	3mm x 8mm self tapping screws from Amazon

  If you require a different hole size, the Onshape project includes a global variable #threadsize to control this.

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