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RF Speech Clipper into Yaesu's MH-31 Standard Microphone
Joachim Münch - Geniusbankstraße 7 - D 26388 Wilhelmshaven (+49)04421-3586055 email
A assembled tested modul, including Elektret-Mike, may be purchased in the webshop....
GENERAL DESCRIPTION: The general function of RF Clippers has been described on my web site; here I want to drop some words on the particularities of the RF Clipper for the FT-817 and the ideas behind it. The first question was whether to develop an external or an internal (built-in) module. In order not to change the overall character of the FT-817, I decided to develop an external unit. Moreover, it had to be easy, even for a less skilled OM, to build it up without permanent modifications of his equipment. The only feasible solution turned out to be a module fitting into the case of the hand microphone. To make room for the clipper module, the microphone capsule itself had to be exchanged by a less bulky one. Some time ago, I developped a „Mini Clipper“, which earned very favourable critics by Hams who integrated it into their hand microphone without any shielding; no problems, at all, were reported even at transmitter output powers of up to 400 watts. The new FT-817 RF Clipper, therefore, is based on that design. The transceiver supplies a regulated voltage of 5 volts (DC) through its microphone connecting cable, which can be used to supply the clipper module. The basic i.f. clipping circuit is identical to that of the PEP model. I, as well as several other hams, use it successfully (see also: HAM Reports); the effective gain, compared to an unclipped signal, is at least 10 decibels. Before you can build the clipper module into the microphone case, you have to exchange the microphone capsule. I recommend to read the instructions below very carefully. With some skills in soldering, you will certainly be successful. The clipper module may also be built into a desk microphone without much problems. Be aware that, if the available supply voltage exceeds 8 volts it has to be reduced by means of a stabilizer 78L06! Some of the pictures here still show the first version of the clipper module with the „old“ blue filters. In the meantime, these have been exchanged by better ones. The electret mike capsule now is an integral part of the module (soldered directly on top of the pc board). Optionally, the electret mike may be replaced by the Heil model HC-4; the model HC-5 of Heil would be too large to be integrated into the MH-31 without major mechanical redesign! The clipper module also has been prepared to be operated from a dynamic microphone (not part of the module!); you then have to mount an extra transistor (included) on the pc board. There is no need to realign the module, in this case.
The electret microphone capsule has been mounted on the soldering side of the board.
See here the installation of FT817 RF Speech Clipper (first version) into Yaesu's MH-31 Standard Microphone.
Position of the Electret Capsule. This picture shows the clipper module without the foam seal.
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