T-Kit 1380 Kit Build: Part 2

Today, I’ll be continuing my 80m transceiver build that I started in T-Kit 1380 Kit Build: Part 1.

At the end of the last post, the board looked like this:

The full board

The full board

Today I’ll be moving on to the VFO section of the board.  A VFO, or variable frequency oscillator, is the circuit that allows you to tune a radio.  This particular VFO is based on a Collpits oscillator, and can tune over a 50-70 kHz range centered on a frequency determined by the component values.  The frequency range shown in the image may seem a bit strange.  This transceiver can be built to cover that 50-70 kHz range somewhere near 3.5 MHz to about 3.75 MHz.  The short explanation is that the frequency we’re interested in is shifted by the frequency of the VFO to an intermediate frequency of 8 MHz, where we can do filtering and amplification at a single fixed frequency.  Since a lot of circuit characteristics are frequency-dependent, performance is much better if the components can be selected for just one frequency.

schematic

The majority of the components are supplied with the kit, so their values are fixed.  One of them, an inductor, I had to wind myself.  Since this phase required quite a few components, I decided I’d lay them out before I started.

components

Rather than start building immediately, I decided to wind the inductor first, so I could get that out of the way.  The instructions specified 28 turns of the green #28 enameled wire on the red toroid core.  I had to count the turns several times to be sure.

inductor

The inductance of the coil is dependent on a lot of things, including the material the core is made of, the diameter of the core, the number of windings, and the spacing between the windings.  Later on in the build, I tweak the range covered by the transceiver by adjusting the coil spacing.

phase-2-complete

From this point on, it was simply a matter of stuffing the board and soldering, as per the instructions.  The one thing I would have changed was the process for doing initial testing of the inductor.  They have you tack a couple leads to the pads you’re going to use, and then tack the inductor to those.  Unless your inductor is wildly off, you’re not going to be rewinding it, so I would have skipped that step and just soldered it in directly at the beginning.

The testing of phase 2 was relatively simple, because I’m using a frequency counter.  I just hooked up the frequency counter, and adjusted the spacing of the turns on the coil I mentioned before until the VFO covered the range between 4.470 MHz and 4.391 MHz.

I’ll talk more about it in the next post about the transmit mixer and filter, but that provides an actual range of 3.530 MHz to 3.609 MHz.  This includes the QRP CW calling frequency at 3.560 as well as W1AW’s code practice sessions transmitted on 3.5815.  It does not include the main CW DX window between 3.500 and 3.525 MHz, but I’m still working on getting my Amateur Extra license, so I’m not authorized for that part of the band anyway.

 

T-Kit 1380 Kit Build: Part 1

I won a T-Kit 1380 80m 3 watt CW transceiver kit at the WCRA Hamfest back in 2014, and it’s been sitting on my bench unopened since then.  I didn’t have my license at the time, but I got my General license about a week later. I decided that this summer was a good time to start building it. Here’s a link to one you can pick up if you’re interested : http://www.rkrdesignsllc.com/-13/

I have quite a lot of kit-building experience, but most of it is digital electronics, so this is probably the most complex kit I’ve ever built, both in number of components and circuit complexity.

If you’re not familiar with amateur radio, this kit will let you transmit and receive on the 80m band (between 3.5 and 3.75 MHz) using CW (morse code).

1380 Manual

The schematics in the manual are a bit low-res, but the instructions for assembly are very good.  My biggest complaint with the manual so far is that errata are supplied as a stack of papers inside the manual.  Some of them referenced parts this kit doesn’t use, so it was a bit of a chore to go through and update the instructions and update the steps by hand.

The assembly process is documented in phases, with testing procedures at the end of each phase.

Phase 1 is construction of the DC input circuitry as well as the keying circuit.  The keying circuit is connected to the code key, and disables the receive circuitry while transmitting.  Here’s the diagram for phase 1.

Phase 1 schematic

Phase 1 schematic

Here’s the board as assembled:

Phase 1 assembled

Phase 1 assembled

This is a pretty densely packed board, and the silkscreen suffers for it.  The manual gives pretty decent drawings of the section of the board each phase is concerned with, and this helps quite a lot.  You can usually locate a component by finding a nearby component you’ve already installed, or one whose silkscreen isn’t broken up by a pad.

Once this phase was assembled, there was a short test procedure to verify that it is operating correctly.  Essentially, I had to apply 12v to the 12v input, and then verify that R13 (the resistor in the center of the board, just between the two beige ceramic capacitors) read 0v while the key wires were disconnected (the white and black wires just under ‘J1’), and 12v while they were touched together.

I misread the directions and it took me a while to figure out what I was doing wrong (I was measuring voltage drop across the resistor, not between the resistor terminal and ground), but in the end, everything checked out.

The full board

The full board

As you can see, there’s quite a lot of work still to do, so come back next time, when I move on to assembly and testing of the VFO section!