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Gear Kits The Shack

The 4SQRP Hi-Per-Mite CW Audio Filter

The FT-817/818 has been in production for over 20 years and now that I own one, it is easy to see why. It is a fun radio to operate and fits a lot in a small package. There’s really just one thing that I don’t like about it: the wideness of its filtering can make operating difficult. The Hi-Per-Mite from the 4 States QRP Group offers an economical and easy to build solution to this radio’s shortcoming.

The receiver on the FT-818 hears about 1kHz to 1.5kHz of the band. This is probably fine for SSB, but it is less than ideal for operating CW on the weekend. I hear up to four stations at once while operating during peak times and conditions. I mentioned this to another operator while on the SKCC sked page and he recommended that I try out the Hi-Per-Mite kit offered from the 4 States QRP Group. Boasting a 200 Hz audio pass for only $28 shipped, it was a deal that I was eager to take advantage of.

The kit is small and easy enough for the beginner to complete as long as they can read a schematic and know how to solder. While the kit didn’t ship with instructions, there are some provided on the 4SQRP Hi-Per-Mite webpage.

The instructions were slightly confusing at times and differed from the schematic in a couple of ways. In fact, I had some trouble getting it to work once I had all the components installed. Once I connected power and my FT-818 to the circuit, I was disheartened when the audio was horrendously overdriven. Through some conversations with members of the QRP-ARCI and NAQCC organizations on groups.io, I managed to troubleshoot the issue.

All the problems went away once I disregarded the long-form instructions and followed the schematic as it was drawn. This meant that I eliminated a jumper across D1. The audio issues went away! If I read the schematic correctly, I was feeding 12VDC into places where it wasn’t meant to go which caused harsh audio.

Now that I have this audio filter working and in a nice enclosure, it permanently resides atop my FT-818. The performance is outstanding. I only hear one station at a time now even when the band is crowded. However, the audio adjustments are touchy at the radio now.

What I need now is a way to power it in a more seamless way. I’m considering adding a pigtail from my 12VDC supply cable to the Hi-Per-Mite so that there are fewer wires cluttering up the desk.

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