solder – AB1DQ

Elecraft K2 Build – Part 1

Control Board Assembly

As the proverb goes, a journey of a thousand miles begins with a single step. So, after procrastinating for two plus years after my XYL and soulmate Ellen gifted me the Elecraft K2 HF Transceiver Kit for our tenth anniversary, I finally got my nerve up to start what will easily be my most ambitious kit build ever.

It is my intent to blog about my experience as I proceed, circuit board by circuit board, sharing my experience and inviting others who have built the K2 to share as well.

The Control Board

Having set up and outfitted a new protected work bench using a folding banquet table in the shack where I will work only on the K2, I started work on the first circuit board, the Control Board on January 9, 2022.

As with many kits I’ve built, the Elecraft instructions call for the builder to start by inserting and soldering all of the fixed resistors first. As others have reported, I found the instructions to be, for the most part, very well written. The instructions for the fixed resistors provided each resistor’s value in ohms as well as the color code and the builder is encouraged to install the resistors with the first color band towards the top or the right of the circuit board to facilitate verifying the correct resistor is in the correct space when reviewing or troubleshooting your work.

As electronic components have gotten increasingly smaller, my vision has gotten progressively worse as I’ve gotten older. Whenever kit building, I always verify values with my VOM before inserting any resistors into a circuit board. After installing the 13 resistors, the instructions called for the installation of seven resistor arrays and one trimmer pot, all easily identified.

*The Control Board after installing all resistors.*

Next the builder needs to install an 82 mH inductor, which I confirmed I had the correct part with my L/C meter, followed by a pair of silicon diodes. I then encountered the first variation in my kit. Where the original PCB had a screened space for D3, the instructions called for an 82K ohm resistor to be installed here.

The instructions next call for the builder to install and solder in place 36 fixed capacitors. Identifying and verifying the value of each capacitor was a notable challenge. Not only were the stamped or printed values on the caps miniscule, the capacitors also varied in type, shape, and manufacturer and it was clear that some of Elecraft’s suppliers changed over the twenty-five years they have been offering the K2 kit, as some of the caps did not match in appearance to the identifier pictures in the otherwise excellent parts list in the appendix.

To guarantee I placed the correct capacitor in the correct space, I took the time to identify every capacitor and laid them out neatly on my workbench in the same order the instructions called for their installation.

To identify the values, I used the camera on my iPhone and zoom in on the part. Sometimes when the value is etched on the capacitor, I needed to shift it so my bench light catches the labeling just right to read. I took the time and used my L/C meter to verify the value of every capacitor.

*LEFT: using my iPhone as a digital magnifier to read capacitor labels*
*CENTER: checking capacitor values with an L/C meter*
*RIGHT: laying out verified capacitors in the order of installation*

Once all of the capacitors were laid out in the order of installation, I carefully installed each capacitor into its space, having taken the time again to double-check values with the L/C meter. It was a slow process, but in the end, I felt very confident all of the capacitors were soldered in the correct space giving me peace of mind. Trouble shooting for mis-placed capacitors would be a very tedious process if necessary.

The next several steps went along easily as I installed the electrolytic capacitors, a trimmer cap, a dozen bipolar junction transistors, a pair of crystals, two voltage regulators, one IC socket and various connectors. All of these parts were easily identified, and when working with the transistors, I proceeded in the same manner as I did with the fixed caps, identifying and verifying value, arranging them in the order of installation and double-checking values as I installed each.

*The bipolar transistors, values checked and laid out in order of installation.*

Now it was time to install the ICs on the control board, and this is where I first ran into trouble.

The very first chip was an NE602, the AGC mixer. I mentioned that the K2 has been on the market for twenty-four years and over the course of a quarter century, technology moves on and the availability of parts change. By the time Elecraft had kitted my specific K2, the NE602 was no longer widely available in through-the-hole DIP casing. The industry had since begun moving on away from through-the-hole components in favor of tiny, less expensive surface-mount versions.

Instead of the DIP version of the NE602, my kit came with an equivalent SMD NE612 which was pre-soldered to a small square ‘carrier board.’ The carrier board is a PC board cut to the same footprint of an 8 pin DIP case and the builder is instructed to cut eight 1-inch pieces of wire, insert them into the eight holes on the carrier board, solder them in place, then insert the bottom ends of these wires through the DIP-spaced holes and solder to the control board.

I damaged the carrier board while attaching the wires by working sloppily with a too-hot iron. The carrier board was not of the same high quality as the control board which featured double plated holes. The carrier board had plating only on the top of the holes, and I managed to lift the plating off of the number 2 and number 3 hole, breaking connectivity. I tried to bridge the contacts to the chip contacts with a solder blob, but that only made a bigger mess of things.

*LEFT: the Control Board space for U1, an NE602 DIP*
*RIGHT: The SMD carrier board for U1 which I damaged with a heavy hand and hot iron*

Elecraft has a form on their website to order replacement parts and I reached out on January 1qth to inquire about purchasing a replacement NE612 and carrier board. I did receive an acknowledgement that they received my inquiry from a mail bot, but as of this writing, four days later I have not received an actual reply.

In the meantime, I began wondering whether I could still find DIP cased versions of the NE602 elsewhere online. I searched Mouser, Digi-key, eBay and Amazon and found that an Amazon vendor had some available, which I ordered. The vendor did not disclose the name of the manufacturer nor the source country, but I’m assuming the chips were manufactured in China. I did order five (there are three others elsewhere in the K2) and they arrived within a day or two.

I discussed my dilemma with one of my Elmers, Steve, KZ1S, who has built many a kit in his day and is a physics professor who works with electronic circuits and RF in his work.

Steve offered a couple of suggestions. The first was to use a proper SMD to DIP converter board with proper pins spaced correctly. I liked this idea very much, but the drawback is that it would require me soldering the SMD chip to the converter board and again, with my failing vision and dexterity, this would be a bit of an unpleasant challenge.

Steve also suggested looking for genuine NOS chips online, either on eBay, or from Radwell International. Steve mentioned you can source just about any obsolete part with Radwell, but they can be expensive. I did locate the NE602s on there with a retail price of about $5 – not a deal-breaker, but given I need four for the K2, that’s an additional $20 + shipping.

For the time being, I decided to place a DIP socket on the control board and once in place, I inserted the NE602 of dubious origin I purchased from Amazon in the socket. For the time being this would let me continue with the build and be able to test resistances. I could then swap out the AGC chip for a genuine NOS or SMD + converter at a later date.

I finished the build of the control board this morning, directly soldering in the remaining ICs and adding the two CW key-shaping capacitors on the solder side of the board.

After carefully double-checking all of my work, I used my VOM to perform the resistance checks. All of my test resistances were within range, excepting U6 pin 29 (DASH) and U6 pin 30 (DOT/PTT) which were marginally over spec. The acceptable range for both is 70K – 90K ohms and I measured 96.6K on pin 29 and 96.8K on pin 30. I will revisit these values later.

So that concludes the build of the first circuit board, the Control Board. I counted a total of 110 components soldered to the board and I completed the work in eight days working at my deliberately leisurely pace.

I welcome comments and suggestions from any and all, particularly from anyone who has built the K2 and had to deal with the SMD carrier board themselves. Drop me a line at james@ab1dq.com to share your thoughts and opinions.

Next up – the Front Panel Board – stay tuned!

The Murania “One Transistor” Boy’s Radio Kit

INTRODUCTION & CONSTRUCTION

The Four State QRP Group (Oklahoma – Kansas – Arkansas – Missouri, in case you were wondering), founded in 2003, is one of the best developers and retailers of high quality and reasonably priced QRP (low power) ham radio and other do it yourself electronics kits.

I have a few of their kits over the past few years, most recently including the Bayou Jumper Paraset transceiver last year. I presently have the NM0S 4S-Tuner/Antenna Coupler kit on order.

Tonight I tackled one of their popular new non-ham radio kits, the Murania, a one transistor Tuned Radio Frequency (TRF) AM broadcast band receiver kit. The kit was designed by NM0S, David Cripe, who has engineered several of the 4SQRP kits.

The documentation for the Murania tells of the advent of transistor radios in the 1950s and how radios with 1 or 2 transistors were considered toys and therefore not taxed like radios containing more transistors. These 2 or less transistor “toy” radios became known as “Boy’s Radios” and are highly collectible today.

The designers of Boy’s Radios employed some creative design techniques to maximize the performance of these minimalist circuits, with sometimes amazing results. The Murania kit was inspired the design of those simple high performing transistor radios.

My Murania kit arrived quickly within 2 days of placing my order online….WOW!

The Murania features a unique construction technique called “Pittsburgh Construction” developed by W0MQY , Joe Porter, in which components are soldered to the surface of pads on a silk screened double sided PCB.

Like other 4SQRP kits, the assembly manual needs to be downloaded from their website. Documentation is very good with clearly expressed step by step directions, but lacks pictures which might be helpful in illustrating potentially confusing steps for the newbie builder, such as the correct orientation of a polarized component such as an LED, diode or electrolytic cap.

The 4SQRP website suggest the kit can be built in about 2 hours time, and that was my experience. The radio is built in five stages… (1) wind the coil, (2) build the voltage regulator, (3) build the audio amp, (4) build the RF circuit, (5) final assembly.

1. Winding the Coil

The first task is to wind the coil which consists of 37 turns of No. 22 AWG enamel wire around a ferrite core. The instructions call for covering the core with a layer of masking tape first and using masking tape to hold the first and last winding in place.

I chose to use black electrical tape, and that was definitely a mistake – the electrical tape made it difficult to compress each winding snug against the previous winding and it didn’t do a very good job of holding the first and last winding in place.

I believe this may have also affected performance of my radio (see below). I am planning on modding the set and rewinding the coil with 61 turns (also, see below) and will use the recommended masking tape at that time.

2. Voltage Regulator

The first circuit constructed is the power supply/voltage regulator which consists of installing the volume control pot and attached power switch, one electrolytic capacitor, the battery connector, another capacitor and a resistor and the LED which serves three functions – power on lamp, signal strength indicator, and voltage regulator delivering 1.6 – 1.8 +VDC to power the radio.

I appreciated that the instructions called for testing the voltage regulator circuit before proceeding on to the audio amp stage. My Murania was putting out 1.792 VDC+ within the acceptable range of 1.6 – 1.8 volts.

3. Audio Amplifier

The Murania has a single stage of audio amplification based on the 2N3904 NPN transistor that drives the speaker through a matching transformer.

Other components in the stage included a pair of capacitors, a single resistor and of course, the transformer and speaker.

4. RF Stage

The bulk of the RF work is handled by a single IC, the TA7642, which has its origins in the late 1960s. Equivalent to the ZN914 and MK484, the TA7642 contains ten transistors and performs the task of RF amplification, audio detection, and automatic gain control. The documentation points out that with the TA7642, it is possible to construct a Tuned Radio Frequency receiver with useful sensitivity and selectivity, using only a handful of components and that this device served as the basis of many radio receivers that were the successors to the Boy’s Radios.

The 10 transistor equivalent circuit of the TA7642 per the datasheet.

Like the voltage regulator and audio amplification stages the RF stage went together without a hitch. All parts in the kit were properly identified and clearly referenced in the assembly manual. The etching on the circuit board made mis-installation pretty much an impossibility if you’re paying attention to what you’re doing.

The assembled Murania TRF radio ready for testing.

5. Final Assembly

After testing the radio to make sure it works (it did), the last step was to assemble the rest of the cabinet which is comprised of five additional pieces of yellow PCB material with pads strategically placed to match up for soldering to connect.

The pieces fit together perfectly, although I should have taken time as recommended in the directions to file off burs and rough spots so the pieces fit together more perfectly. Overall this is a pretty ingenious way to build a radio cabinet.

THE FINISHED PROJECT – IT WORKS!

PERFORMANCE AND MODS

I was very pleased that the radio worked right away. I was able to pick up several AM stations with ease. Stations received were clear and the audio, while not as loud as I would have liked, was not distorted.

One problem I did encounter that is worth mentioning is that after I tested the radio on my bench I attached the back to the radio and brought it to my wife to show off my handiwork.

She was impressed, however when she turned the radio on, the LED lit up but there was no sound coming from the speaker – absolute silence – UGH!

I took the back off and quickly diagnosed the problem – the top of the 9V battery was shorting the speaker terminals – a problem easily fixed with a piece of electrical tape across the speaker terminals.

I did expect the radio to be a little more sensitive than it was initially and I realized that the radio’s performance might have been inhibited by my sloppy coil winding.

The unmodified Murania Schematic ©NM0S, 4SQRP Group

Online I found a list of three simple mods for the radio published by Jim Marco, WB2LHP in MI, the third of which that involves additional windings on the coil so I thought I’d give them a try.

Here are Jim’s mods…

1. Detector Gain Control…

FLOAT the wiper lug of R3 and place a jumper between the PADS for the R3 wiper and the high side of R3.

Lift the leg of R1 that intersects with R2 and R3 and connect a jumper between the floated leg of R1 and the wiper of R3.

According to Jim, this allows R3 to control the gain of the detector stage in the TA7642 acting similar to a regen control where there is both volume and gain reaction. The audio amp runs wide open and R3 should be adjusted for the best sounding audio.

2. Reduced audio distortion…

Changing R2 from 1K to 2.7K biases the output stage of the TA7642 for linear operation.

3.Frequency coverage and dial mapping…

Increasing L1 from 37 turns to 61 turns and removing C8 centers the frequency coverage and makes the dial tracking spot on…

The revised schematic based on WB2LHB’s mods

I am pleased to say that the mods were easy to accomplish and I had no difficulty with any of them. I did not have a 2.7K ohm resistor on hand so I tied a 2.2k and a 470 ohm resistor in series for R2. Using the recommended masking tape instead of rubbery electrical tape on the ferrite rod made a world of difference too – winding the 61 turns was a snap.

My modified Murania – notice the new ferrite coil, the replacement of R2, the removal of C8 on the right, and the jumper going from R1 over the speaker to the VR R3.

And how did it work? Even better than before – the radio seems to be more sensitive and is picking up more stations and the audio is definitely more crisp as promised. If you’re looking for a fun one-evening project that will take you back to your earlier days of melting solder – the Murania TRF receiver is worth building.