Dave Richards AA7EE

December 3, 2011

The 3rd and Final Stage Of K2 Assembly

The K2 has been finished for about 3 weeks and it’s time I made the post concerning the final stage of assembly. After completing the second stage and gaining a working receiver on 40M, the majority of the work was done. At this point it felt as if I was about 3/4 of the way through, and I think I probably was. The final basic  K2 consists of 3 boards – the front panel board which you can’t see in this photo, as it’s obscured by the control board which sits behind it, and the RF board. All boards are now fully populated, with the exception of the occasional jumper or connector here and there which are reserved for extra options. The on-board frequency counter is shown plugged into the test point that reads the BFO frequency – useful in regular operation if you want to adjust the filter settings:

Although I know my own toroids don’t look as pretty as the pre-wound ones from Mychael AA3WF would have,  I’m getting better at winding them, and they don’t look too bad at all.  There are many examples of attention to detail  in this kit. One of them is that when you pull the toroid leads tight and then solder them, the toroids are actually straight on the board. Often in kits, the holes for toroid leads are placed such that when you pull the leads through and solder them, they are skewed just a little. It’s a small point, but it makes boards look a bit messy. Hats off to Elecraft for helping to line my toroids up in nice neat rows:

A view from above:

Here’s the view from underneath. Note the nuts securing the 2 PA transistors to the case:

The 2 chassis pieces that I received for the back of the transceiver had already had the green masking tape removed. That was a little disappointing, as I wanted to leave the tape in place over the holes that are currently unused.  I’m sure Elecraft would have replaced them for me but it didn’t seem worth contacting them about. I’ll use some regular masking tape:

What a beaut!  –

Everything about the K2 seems to be working the way it should. Sensitivity and handling of strong signals seem fine.  I plan to measure the minimum discernible signal at some point when I get a calibrated signal source. There are only 2 things that are not quite as perfect as I’d like them (though neither are deal-breakers):

Firstly, due to the way that the transceiver handles the D-A conversion, when stepping through the filters, the BFO is not on precisely the same frequency for each setting of the filter.  My filter settings are (nominally) 1.5K, 1K, 700Hz and 350Hz (though I understand the real bandwidths are narrower in practice). When receiving a signal and stepping through the bandwidths, the signal is at the same pitch except when the 350Hz filter is selected, when it lowers in pitch about 15 – 20 Hz. It’s not a lot, but to someone like me who is sensitive to pitch, it’s a bit annoying.  No matter how hard I try to adjust the BFO frequency, I cannot get it any closer. I understand this is due to the number of bits in the D-A convertor and was one of those compromise decisions that often have to be made during product development.

The other thing is not really an issue. It’s more of an indicator of how used I get to certain receivers and how I often am very picky about how I want my radios to operate. I suspect I’m not the only operator who wants his rig to operate exactly the way he wants it to so that it “fits like a glove”.  On comparing the K2 receiver to the Norcal 2N2 receiver, I notice 2 things. Firstly, the rushing background band noise seems to be pushed to a lower level in the background by the 2N2’s filtering.  It’s difficult to make a direct comparison though, as I don’t know the exact bandwidth of the crystal filters in the 2N2 and the K2  – this would be something worth measuring.  The K2 has more audio output power, so part of it may also be that I simply have the AF gain turned higher on the K2, creating the illusion of more rushing background noise.  On some very weak signals, I find that the 2N2 has a very slight edge.  I don’t think this is a sensitivity issue; I think it has more to do with the audio filtering – the audio on the 2N2 seems to peak more sharply at a fairly specific frequency (I set up all my rigs for a 500Hz sidetone), while the K2 doesn’t have this extra peak. My guess is that the audio circuitry on the 2N2 is tailored more specifically to CW, as it is a CW-only rig. The K2’s audio chain needs to accommodate wider SSB signals so needs to be fairly flat with a passband of a few KHz.  So I think the next step is to figure out some kind of extra audio filtering for the K2 – perhaps a KAF2 or the DSP option?

These 2 small points aside,  I’m happy with my K2 so far. Sure, it’s fun to compare figures and performance characteristics, but there is another very important factor that determines how useful a transceiver will be to the operator, and that is the feature-set and how easily accessible those functions are. No complaints in that regard yet.  Considering the relatively small size of the front panel and the fact that each button controls 2 different functions, Elecraft have made the most often-needed functions the easiest to access. I found it straightforward figuring out how to record and playback the keyer memories, as well as using the Fast-Play function, by which you can playback certain keyer memories with a single button push (great for contest operating).  Use of the dual VFO’s and operating split was intuitive – even for this guy who is fairly new to the world of operating split-frequency on HF.

In the few weeks since having it, I’ve worked T2T, YN7SU, HA3UU, JE4JPQ, JF1RWZ, PA0LEG, CO8WZ, 7N1PRD, BD4FM, CA2LQA, HL2DC, JE2UFF, DK1AX, CO6RD, PV8ADI, ZP6CW, ZP9MCE, GW4EVL, PJ2/W8WTS, C6ATA, JF1SQC, JR3NZ, JA6WFM, ZM1A, UA0ZAM, JF1NSD, JA7FTR, XE1CT, XE2B, JA5FDJ, PW7T, ZS4TX, PV0F, LS1D, C5A, HK1N, LW5EE, XE2AI, HK1R, JR1MQT, LU8YE, LT1F, VK4KW, JA1KGW and PY3ED. Oh – and a bunch of Stateside stations too 🙂

I think this rig is a keeper.  I’m already contemplating either the audio filter or DSP filter, the QRP internal tuner, the 160M option and (shock horror) perhaps also the SSB option 🙂

EXTRA NOTE:  It is now the end of July 2012 and I’ve owned this K2 for 8 months.  I’d like to add to the comments I made above about my comparison of the K2 audio response to that of the Norcal 2N2, which had been the main rig I was using for CW before. I had been used to the more peaked audio response of the 2N2 but after a few months with the K2, realized that the K2’s flatter response within it’s passband was preferable. I can now work a CW station slightly off-frequency without having to engage RIT.  With the 2N2, if I was working a particularly weak signal, adjusting the RIT was sometimes a necessity to maximize copyability of the signal.  My preference for a peaked response was based merely on what I was used to.

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December 1, 2011

Second Stage Of K2 Assembly Completed

I actually completed all 3 stages of K2 assembly 5 days after finishing the first stage, and about 9 or 10 days after beginning the assembly, which included a couple of days off in the middle. However, as soon as I’d finished building it I wanted some time to relax, play with my new radio, and generally recharge my batteries, hence the lack of a timely follow-up post.  Sometimes I temporarily lose my urge to communicate.  Sorry about that.

At the end of this post, I mentioned how just one part was missing from the kit – a 20-pin connector that connects the front panel board to the main RF board.  It only took a day to arrive so while waiting, I decided to make the best use of my time by pre-winding the toroids.  I had originally planned to buy the pre-wound toroid kit from Mychael AA3WF, reasoning that if I ever wanted to sell my K2, I’d want really great-looking toroids in it. When push came to shove however, budget started becoming very important, so I decided to wind my own and was glad I did. They don’t look quite as nice as Mychael’s, but I think they’re not half-bad. Here are 3 of them.  The one with the yellow core is the VCO coil and is shown with just one winding (I hadn’t wound the secondary at this point).  You can also see the 2.1mm crochet hook I used to pull the windings through the cores.  It’s a method I learned from Jim K8IQY and I find that it helps in keeping the windings fairly close to the core:

This second stage of assembly,  after which you have a working receiver on 40M,  is lengthy. There are a lot of resistors and capacitors to install and as many builders before me have commented, the only way to deal with it is to just get on with it.  For relatively inexperienced builders, this stage could be a bit of a nail-biter, as you have so many parts to install before getting any feedback on whether the circuits you’re building do indeed work. However, if you have a reasonable amount of experience, the manual is so detailed, and entire assembly so well thought-out and described that you just know you’ll either end up with a working rig, or Elecraft will help you correct any mistakes made during assembly.

An advantage to building such a late model K2 is that all the mods, such as the thermistor board to stabilize the VFO further,  the few extra components to shape the keying waveform, and the 2 diodes to improve the handling in the presence of very strong local signals on nearby frequencies, are included. There’s only one little thing that slightly bugged my detail-obsessed mind, and that was that some of the mods look a little messy to me.  It’s probably unrealistic to expect the board layout to be revised for every single mod after the K2 hit the market, but the board is of such high quality and looks so good, that a few components soldered at odd angles on the underside of the board offend my over-developed aesthetic sensibilities. Luckily, I got over it pretty fast. The components that are either horizontal or vertical are part of the original design while the ones that look patched on afterwards are, well, patched on afterwards.  In the real world, I’m sure that the economics wouldn’t allow having the board layout revised again for these small changes:

In one sign that through-hole components are becoming harder to find, to the left of the above picture in the middle, you can see D36, which is now an SMT part on a small board, modded to fit the main PCB. I feel a little sad at the slow passing of through-hole technology, as not everyone is comfortable with SMT construction and so the building of your own rig will be an experience available to fewer people in the future. Just under D36 is L33 which was originally a choke with axial leads. As I understand it, using a toroidally-wound component for this part and mounting it on the underside of the board gave greater suppression of the opposite sideband in SSB mode. It is wound from fine wire and there’s no need to worry – it comes already wound, though you do have to be very careful with the leads when installing it.

Another view of this part of the underside of the main RF board:

The entire underside of the main board after this second stage of assembly. Near the left side of the board about 2/3 of the way down, you can see the outlines where the 2 PA transistors will be installed in the 3rd and final stage of construction:

At this point, the majority of the circuitry has been built, and that is quite apparent in the following views. The length of mini co-ax connects the built-in frequency counter to one of 3 test points on the board. The K2, as you probably know, has a built-in DVM and frequency counter to assist in building and aligning the radio:

A close-up of the PLL upgrade board that increases the stability of the VFO. It’s the light-brown vertical board just to the left of the crystal:

Here’s the 5 pole variable bandwidth crystal filter. I did a slightly less-neat-than-normal job of soldering the ground leads to the crystal cases. The crystal nearest the front is the messiest-looking. These kind of things really bug me but it’s a perfectly serviceable joint, so I pulled myself together and got over it. In this picture, you can also see the crystals for the second, fixed bandwidth crystal filter and to the left of them (and slightly out of focus), the other SMT part that has been substituted for the original through-hole part. In this case, it’s U12, an MC1350 IC which you can see installed on a small PCB that solders in to the holes for the original DIP part:

This wider shot also shows the 2 BFO crystals. The manual (bottom left side of page 54 in revision H1 of the manual dated April 26, 2011) notes that the leads from one of the BFO crystals (X3) need to be folded over and soldered particularly close to the board in order to prevent fouling a rubber bumper that will be placed over it in a subsequent step.  I seem to remember that one (not both) of the leads from X3 did protrude directly underneath the rubber stem bumper.  I found that by pre-cutting the lead so that it didn’t quite poke out of the PCB underside and applying solder so that it filled the plated-through hole but didn’t spill over onto the pad, I ended up with a nice flat surface on which to lay the rubber bumper for L33 (the BFO inductor). If you’ve built a K2 before, this will make sense to you. If not – it will become clear when you get to that point in the instructions.  You are instructed later on (right-hand side of page 60 in my revision of the manual) to flush-trim all leads under or near L33 but if you were strictly following the manual, you would already have installed X3 at this point and folded over it’s leads which will allow the bumper to “exist” as it were, but will not allow it to sit completely flat.

Aww what the heck, here’s another very similar shot. In the background is the control board, and underneath the processor chip, you can see the crystal and associated trimcap that should be adjusted to exactly 4MHz so that the frequency display reads accurately. The manual tells you how to do it – you don’t need any extra test equipment:

This shot shows the VCO inductor. It’s the toroid wound on a yellow core to the right of the picture:

Oh – and the receiver? Well, it seemed to be working well on 40M.  I’ve read blogs from some builders who after a fairly lengthy period of assembly, were having such a blast listening to 40M on their K2’s that they gave themselves a break before continuing. I don’t remember exactly, but I think I had a short break of maybe a day, while entertaining company.  As I had been several months without any capability on the upper HF bands, I wanted to forge on so that I could listen to the higher bands and catch some sunspot action.

In the next post, I’ll show some pictures of the completed basic K2 and share my impressions so far.

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