First I was satisfied with my Yaesu FT-817 (s/n 2J56..) with its original dynamic microphone. The sound was very good for a non-high-fidelity dynamic capsule. But as I decided to build in the DF4ZS rf speech processor into the stock microphone I wanted to be able to use the FT-817 on FM too. A rf speech processor isn't designed for that, so now I use the backward switch to switch between the electret capsule (output via a 4.7mF tantal) or the whole rf speech processor.
The tone switch capacitor of 0.33mF was removed, but the 680 ohms resistor was put in place. If you remove the 680 ohms resistor too you would overdrive the FT-817 mic amp input and distortion would result.
But with the lonely electret capsule I noticed a shrill, bright FM modulation with really no basses in the sound characteristic !! The modulation sound characteristic of course was real good with the dynamic microphone, but with a even better electret capsule FM isn't really usable. So what happens ?
I have the MH-36 DTMF microphone on my FT-90 and people who use this mic on the FT-817 should expire the same shrill sound in my opinion too, cause there are nearly the same few external components in the mic line like on my modificated stock microphone. No problem on SSB (AM I've not tested). So the problem only can be in the FM modulator part of the FT-817.
After looking into the circuit diagram I found the fault very fast. The input capacitor at the beginning of the amp/limiter/lowpass line of the FM modulator has a too low value. It builds up a af highpass with the input impedance of IC Q1096, but I can't say where the lower frequency corner is. But depending on the resulting sound it must be over 600 Hz or more !
So here my mod:
Replace C1380 on the down side of the main unit with 0.033mF (original value is only 0.0047mF !!!)
On mine FT-817 I soldered a 0.022mF SMD capacitor over C1380. So I get a total value of nearly 0.027mF.
Now the sound is absolutely great. I reduced menu #29 (FM mic) from original 50, and 70 with the dynamic capsule, down to only 15 ! But the modulation sounds even better than original, is much louder than original, has basses and heights and is nearly the clipping level on the peaks. A real FM DX modulation.
For sure I even can't use the DF4ZS rd speech processor on FM in addition, that would be too much stuff, hi. But with this mod you don't need it on FM.
This mod is only for the real specialists !!! You'll understand that when you see the real tiny size of the C1380 capacitor on the board. Do it on your own risk !