Dave Richards AA7EE

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 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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6 Comments »

  1. I know you took your time carefully considering buying a K2…is it safe to say you’re glad to have one now?

    Comment by John AE5X — December 1, 2011 @ 11:54 pm | Reply

    • This is definitely a journey that I needed to go on John, so I’m glad that I did it. I’m not completely satisfied with the filtering yet, but I think it might be the audio pass-band that I need to modify a little. So far, no regrets though.

      Comment by AA7EE — December 2, 2011 @ 8:24 pm | Reply

  2. Beautiful. I was sorely tempted by the K2, but I’ve decided to hold out for the KX-3 instead, especially since I have no way to operate from home and must operate portable only.

    I did just have a fun time putting together KD1JV’s ATS-4b kit, though. Not nearly as complex as a K2, but I certainly worked out my SMT hand soldering skills!

    Comment by Twylo NF6Q — December 2, 2011 @ 12:49 am | Reply

  3. Beautiful job, Dave!

    I wish you many years of fun and satisfaction with it. I’ve had mine for 7 years now; and it’s hard to believe that I’ve had it that long. It was immense fun building it and it’s immensely fun to use.

    Larry W2LJ

    Comment by Larry — December 2, 2011 @ 7:47 pm | Reply

    • Larry – I know that the K2 has been the mainstay of your very active QRP station for a long time now, so if it’s good enough for you, well, it must be a pretty fine rig. Thank you for your good wishes!

      Comment by AA7EE — December 2, 2011 @ 11:50 pm | Reply

  4. I really enjoy your radio build posts complete with excellent pictures. Thank you.

    Comment by Joe KJ6QBA — December 2, 2011 @ 11:04 pm | Reply


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