FT817 RF Clipper Microphone Review

DF4ZS RF Clipper  

A couple of months ago I purchased an audio RF clipper module, designed by Joachim Münch DF4ZS (SK), to fit inside the microphone of my new FT817ND. I based my purchase on some enthusiastic reviews posted on eham (www.eham.net/reviews/detail/2732).

I had previously built and tested an audio compressor based around the SSM2216 chip which was trialed on a fellow amateurs FT817 (thanks to Mark 2E0VOV).  It suffered terribly with RF feedback problems, even though standard EMC design practices were adhered to, with respect to layout and earthing. Needless to say it did leave me with the impression that any audio circuit left out side the transceiver chassis was going to suffer a similar fate.

When the RF clipper arrived I eagerly unpacked it and inspected the hand crafted PCB with its mix of conventional active devices and surface mount discrete components - all looked to be in order however, my heart sank when I saw that there were no installation instructions included - just a QSL card! I need not have worried, as all the data I required was available on Joachim's website , I also found it useful to download the MH-31 microphone data provided by Roy Frettsome G4WPW web pages.
 

Circuit diagram of Yaesu MH-31 mic.

I sat and pondered all the information for a while and once I'd  got my head around taking spare components out of the mic.  PCB and adding a 1.2k resistor, the installation of the clipper went easily. I even re-wired the tone switch to enable the processor to be switched in and out of circuit.
 
Note: There is a service offered by the supplier of the module to install the clipper for you, although  once you've factor in the postage both ways for the microphone it looses its appeal. However, for those not gifted with the necessary skills it is a nice option.

The newly modified microphone uses an electret type insert mounted of the rear of the clipper PCB, which I though was a neat idea (more of that later). Once the Clipper PCB was wired in place I button up the microphone assembly and it all worked first time, I found it necessary to reduce the transceivers microphone gain down to approximately 15 (with the clipper out of circuit) and adjust the clippers output to match the normal microphone setting using a small preset on the clipper PCB (I was a bit premature completely reassembling the mic!).

The mic. gain and ALC adjustments were made by ear, aided by the use of a second receiver fitted with a decent pair of over-the-ear headphones, to prevent audio feedback while monitoring the FT817 transmission.  Setting the FT817  ALC is very "hit and miss" using the LCD bar graph provided, however, I did managed to eventually adjust the gain settings to produce  a few meter bars on speech peaks, both with and without the clipper. 

After satisfying myself that I was not overdriving the transmitter I finally re-assembled the microphone and then did a couple of videos showing the modulation envelope of the transmitted RF signal, with and without the clipper in circuit. I'm sure once you've viewed both videos you'll be able to instantly see the increase in peak envelope power made by the clipper. 

Un-processed 

Processed

I decided to perform a couple  on-air tests with another amateur, fortunately Mark 2E0VOV lives within 2 meter QRP range so I set a schedule using both FM and SSB. Mark can always be relied upon for an unbiased audio report, he's spent some time with me fine tuning his FT817 audio, which now uses an electret microphone insert, his logbook bares whiteness to its clout in chasing QRP DX.

It's fare to say the results on both FM and SSB were disappointing, the overall increase  in average signal level was good but the audio quality was very poor - this was using the supplied electret mic. Mark commented that both microphone setting were like listening to me through a wet sock, at this point I got frustrated and a little confused, as the audio was poor with and without the clipper in circuit! After some thought I decided to try mounting the  supplied mic. insert in a baffle plate (as shown), after all this is the way Mark and so many other amateur have modified their standard Yaesu microphones to take an electret insert.

The upshot of my experiment has finally produced audio  quality that now matches the increase in talk power provided by the clipper. :0)

However, I can only recommend this mod. to those with a steady hand, a good set of eyes and decent fine tipped soldering iron. 

73 to Ivonne Glaser (DO2AL) and Harry Gödde (DO5AL). Thanks to Harry and his partner for all their help in perfecting this modification, they have now taken over the production and development of the DF4ZS RF clipper.

Note: Having discussed the above modification with Harry, he has promised to investigate my findings once he has finished re-designing other products in their range. The clipper sells well as it is and my modification just adds icing to the cake. 

I hope you have found the review useful and interesting, I have yet to do any serious DX chasing with the RF Clipper Microphone, because as soon as the project was finished HF propagation took a very rapid downturn.  I live in hope of better times with the onset of the  sporadic-E season. 

73 Rick DE G6AKG