I have a generic external speaker mic with the two-pin connection (1x 3.5mm, 1x 2.5mm mono jack), which doesn't work with my FT-470 (no real surprise, figured I'd just mod it.)
I've started the mod, using the packet interface for this rig as a starting point (2k resistor switched between mic tip and shield for PTT, 1uF cap between mic and mic plug tip for tx audio - this works fine on this specific radio)
Everything is fine on transmit.
On receive, when a carrier is received, audio from the mic is ending up in the rig's own AF amplifier, causing audio feedback (howling) through the speaker.
(I know this is feedback because the speaker mic has a headphone jack. Plug in headphones, tune into a signal, tap the speaker mic case and you can hear the sound of tapping amplified through the headphones.)
But only when there is a carrier, not just when the squelch is open.
Modifying the speaker mic with a DPST switch and using the second pole to only bring the mic in-circuit on PTT seems an obvious solution, but sourcing a DPST PCB mount micro switch is proving, well, impossible.
I have a 2k resistor in series with the PTT switch, mic plug tip and shield. I have tried a 1uF cap in series with the mic, mic tip and mic shield, like the required packet interface wiring for this rig. I have tried a 1uF cap in parallel with the mic, as appears (sans value) for the internal mic in the rig schematic.
I'm putting way too much effort into this, but I'm stuck. It's a shame to not use the old rig over something so small. Original accessories are long since unavailable. If I knew what was inside, e.g. MH-12A2B, I'd just mod this speaker mic to contain the exact same components.
Service manual with schematics is here. http://www.radiomanual.info/schemi/FT470_serv.pdf