1. Unscrew the back of the radio, including the screws hiding in the battery compartments. Be careful of the wires running between the back and the main case.
2. Unscrew the main circuit board, including the screw hiding in one of the shielded compartments. Disconnect all of the ribbon connectors from the main board, and bend them out of the way, so that you may remove the board. An IC extracting tool is helpful here. Disconnect the 2 red wires from the transformer at the main board terminals. Lift out main board.
3. Unscrew the micro-processor board over the keypad, and bend it carefully back out of the way. You may want to tie it back out of the way. This should expose the plastic-welded-in-place keypad board.
4. Take a hot soldering iron with a chisel tip, and scrape off the plastic weld tips. Be like Clinton and don't inhale! Scrape the residue with a sharp knife, and lift out the keypad board and rubber keypad. Take notes of the jack numbers before you disconnect anything.
Now you can grab your owners manual, and start planning the new keypad. It is built using signal diodes, and micro-switches. The original resistive contacts take TWO micro-processor lines to common.
Using signal diodes allows you to run 2 micro-processor lines to one terminal of each push-button micro-switch. The other switch terminals are wired to the common line. Note the special case "sleep button" wiring. That wiring is similar to what we want to do to all the switches to create the 3 terminal to 2 terminal reduction.
See the schematic in DX-400 owner's manual for keypad wiring, or trace it out from keypad board itself. Wire the diode (+) terminals in a similar matrix. The diode (-) terminals form the schematic grid cross-point for each switch. Take notes of the wiring color-code, and jack numbers from micro-processor board to existing keys. You will be cutting the two sets of wire at the keypad board and connecting them to a new jack for the new external keypad. Each of the 12 micro-processor wires, including common, gets a current limiting resistor between 50 and 100 ohms. The one odd wire goes to the sleep circuit.
The new keypad has to go outside of the radio due to its size. A 13-wire cable is needed. I used a 16-pin ribbon cable which plugs into 16-pin IC sockets.
Keypad Layout: I left out the "fast" button. It really should be a slide switch, not a push-button. I moved the "step" button over where the "fast" button was, for easier use. I moved the "sleep" button to where the "step" button was originally.
The new keypad has faster response, and its table-top position makes it easier to use. Keypad clicks are heard in the audio on some bands at full gain, but they're not as loud as those radios with beeping keys.
Total cost for parts was around $30 including shipping. Labor amount was HUGE! Figure a minimum of 3 days.
Larry Russell .... in Flushing, Michigan