Dave Richards AA7EE

November 2, 2016

An Improved Knob for the K2 – plus the KAF2 and KNB2 Options

Lately, I’ve been thinking more and more about how the K2 is not going to be available for ever. As a literal statement, it is obviously true. I suppose it would be more accurate to say that with the way that a few of the original leaded components in both the K2 and some of the options, have been replaced by their SMT counterparts, and the fact that the audio DSP option, the KDSP2, has already been discontinued due to unavailability of one of the main parts, I wonder how long it will be before this happens with the K2 itself. Who knows? It could be imminent, or it could still be some years off. I am so attached to my K2 that I would be dismayed if any of the options I wanted were to become unavailable before I had a chance to assemble and install them, so I have been working towards acquiring all the options I might possibly need, which is basically everything except the KPA100 100W PA and KAT100 ATU, and the KDSP2 (already discontinued). I decided that I cannot justify the extra cost of adding the 100W option, especially as having 100W is just not that important to me. As far as the KDSP2 goes, although I was initially a little miffed when it was discontinued, I later realized that I am not so keen on the audio artifacts that this type of audio DSP produces, and that the KAF2 was most likely a better option for me. I purchased the KAF2, KNB2, and the KBT2-X options a couple of months ago and, uncharacteristically for me, let them sit for a while – after a thorough inventory to ensure all parts were present. At the time, I was scratch-building an SST for 20M, and modding a WBR for 30M. With those 2 projects behind me, I began work on 2 of these options.

First though, a quick word about knobs – or to be more precise, the main tuning knob on the K2. Many find the stock tuning knob to be perfectly serviceable the way it is but others (myself included) find it just a little less than ideal. Compared to tuning knobs on most commercial rigs, the one on the K2 has a rather sharp edge, the effects of which can become obvious if you tune a lot by resting one finger against it. Unless your knob has the (now unavailable) finger dimple, this is probably the way you tune.

Reading the Elecraft reflector archives on Nabble, I found a rather useful tip that, at least partially, solves this problem at no cost. A recommendation was to use one of those small, thick rubber bands that are used in the produce section of the market to hold bunches of broccoli together. I found a yellow one at my local Whole Foods that looked rather nifty on the K2. It could have been a slghtly tighter fit on the knob, but it worked, and it did improve the ease and comfort of tuning. Considering that it cost nothing (the gentleman in produce said I could have it), it was well worth trying –


Another solution I’ve been thinking of was purchasing one of the heavy weighted knobs manufactured by Fred N8BX’s company, 73CNC.com. From the various reports, and the video on the product page, it looks as if it provides a very smooth and silky feel to the tuning. I was sorely tempted. The only possible downside I could find to this heavy knob was a comment from Don W3FPR, on the Elecraft reflector archives. He was wondering if the extra weight would put more stress on the encoder shaft, leading to early failure. At the time, there were no reports, and no data, so it was purely speculation. The knob appears to be very well balanced. It is extra weight on the shaft though. In the meantime, another solution presented itself, in the form of more posts in the aforementioned archives, about the suitability of the FT100 knob, which some have used in place of the stock K2 knob. Someone commented that the rubber ring that fits around the FT100 knob also fits around the K2 stock knob and makes the act of tuning much more comfortable.

The FT100 knob and rubber ring are not available from Yaesu parts any more, but I did find an eBay seller in Taiwan who still has some for sale. It’s a bit more money than it was when Yaesu still supplied it but at $20 inc shipping, I thought it was worth a shot. As of this writing, the seller still has some. This rubber ring is Yaesu part # RA0068200. It is a tight fit over the K2 knob and at first, I didn’t think it was going to fit. It does require some stretching, but once you’ve got the ring stretched over the edge of the knob, it’s a fairly simple matter to push and snug it all the way down. Perhaps the color isn’t quite as eye catching as the yellow brocolli band, but it provides a superior tuning experience –

After a week or so of using this rubber ring, I think it’s going to be my long term solution.

Now onto the assembly and installation of the KAF2 audio filter and KNB2 noise blanker options. Here’s the obligatory photo of the packets as received from Elecraft –

The KAF2 audio filter has 3 elements to it – a clock (as in, one that tells the time, and displays it on command on the main display), a low-pass filter for greatly attenuating everything above 3KHz, such as hiss, and the high-frequency components of splatter, and a bandpass filter consisting of 2 cascaded sections that, on it’s narrowest setting, has a -3dB bandwidth of about 80Hz. If you operate exclusively phone, the KAF2 doesn’t have a lot to offer but for CW ops, it looks like it could be very useful. Thank you Erica, for packing the parts into the bag –

I don’t have much to say about the process of assembly. If you’re reasonably experienced at soldering and following instructions, putting these kinds of things  together is a snap. Here’s the finished board –

This is the underside of the board. At the far right-hand side, you can see the 33pF NPO capacitor which forms part of the frequency determining circuit for the clock, with the 32.768KHz crystal. The manual gives guidelines for adjusting the value of that capacitor if the clock gains or loses too much. Mine is only losing about 0.5 seconds/day, so I got lucky the first time –

Another view of that 33pF capacitor –

The CR2032 3V lithium battery will be inserted in the holder on the right edge of the board, so that the clock keeps time when the K2 is switched off –

At the lower left of the board in the next shot, you can see the switch S1, which can be used to switch the KAF2 in or out of circuit once installed. The two blue trimpots just to the left of the battery holder adjust the center frequency of the two cascaded bandpass filter stages –

Installation is fairly straightforward. There are a few components that have to be removed from the main K2 board as part of this process, as with installation of the KNB2. For removal of components, I recommend the use of quality solder braid, for which I use the Soder-Wick brand. Their size #2 seems to works best for most things. Radio Shack solder-braid doesn’t wick solder up very well unless you brush some liquid flux on it before use. The danger with that though, is that you can easily apply too much flux, and end up making your board look a bit messy. Later on in this post, you’ll see where the KAF2 installs in the K2, as well as a video of it in operation but first, let’s assemble the KNB2 noise blanker option. Elecraft employee Dylan did the honors with the packing of my KNB2 –

As with the KAF2, assembly is straightforward with the help of the detailed manual, so I won’t say much about it, other than to show you the completed board –



Here’s the inside of the K2, showing the KAF2 installed on the control board, behind the front panel. You can see the KAF2 board by looking for the microcontroller chip with the white “KAF2” label on it –

– and here’s another view of the inside of the K2, with the KNB2 noise blanker board installed right next to the KSB2 SSB option board. In this next shot, the KNB2 is at center left –

A wider shot of the internals of my K2, showing it’s current state. You can see the boards for the following options beginning at the bottom right, and progressing in a clockwise fashion – K60XV 60M option, K160RX 160M, and separate RX antenna option (with the blue toroids), KNB2 noise blanker option, KSB2 SSB option, and KAF2 audio filter option. To the left of the shot on the inside of the top cover, you can see the underside of the board for the KAT2 internal 20W ATU –

The big question is how well these options work. Here are two videos to show you. First is the video for the KAF2 option –

– and here’s the KNB2 noise blanker in action. Note that I made this video before the KAF2 video, when the yellow rubber band was still on the main tuning knob –

I still have the internal battery option to install but have not yet decided whether to go ahead with it, as I rarely operate portable. I purchased it in order to make sure that I have it, in case I ever change my mind. After using both the KAF2 and KNB2 options a little, I’m satisfied that they were worth the cost, time, and effort to install. The only option that I don’t have which I am still undecided as to whether I want, is the KIO2 serial interface. It’s tempting to order it, just in case, though I have never connected a rig to a computer, or felt the need to. If I did a lot of contest work, it would be useful.

PS – there was a KAF2 video which Jingles the blind cat crashed (again) but, sadly, it didn’t make the cut!





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