This is a list of the modifications I perform on a DDF-1 before using it
vers. 2019.11.24.A

- disassemble - discard all but the 4 corner screws on both the shell and the circuit board
- unsolder and discard the RS232 conrtroller connector and 2.5mm audio inupt jack
- ream the holes in the upper shell for the three potentiometers 1/4" downward, to allow the bottom of the board to be tilted out first
- make holes in the top end of the upper shell for the controller XLR, 3.5" audio input jack, and 1K volume control pot
- install XLR, audio jack, and volume pot
- attach LM325 amplifier board to underside of upper shell near speaker using VHB tape
- create 3.6v source on underside of circuit board using 330 ohm resistor and two 1A diodes from 5v power, tied to RS232 pin 8
- on top side of circuit board, run 3.6v power through coil of 3.3v relay to XLR pin 1
- wire up the 3.5" audio jack to the 2.5 jack's pads 
- attach antenna switches 1-4 to XLR pins 2-5, on IC side of 470 ohm resistors
- connect a 5v piezo to +5 and to ground through the N.O. on the 3.3v relay
- wire up the speaker output on the board to the LM325 board's audio input
- wire up the volume pot to the LM325 board's gain adjust
- wire up the internal speaker to the LM325 board's audio output
- wire up +5/GND to the LM325 board's power

What this does:

- replaces the less convenient / less rugged RS232 antennna combiner connector with an XLR
- replaces the uncmmon 2.5" audio jack with a common 3.5" audio jack
- upgrades the antenna combiner switching signal to the improved frequency-independent method
- adds a speaker volume adjustment
- adds a self-resetting 3.6v antenna platform grounding alert
- improves ease of access by eliminating unnecessary screws
- reaming of pot holes allows board to be lifted out easily after installing pot/jack/XLR that would otherwise prevent board removal