30 meter Transceiver for Straight Key Night 2005

"Kitbashing" a Vectronics transmitter to make a useful rig

The SKN Special

This project is a 30 meter transceiver built for Straight Key Night 2005-2006. With only three weeks to build the SKN transceiver, I could hardly come up with something original, so I decided to convert an existing 20 meter Vectronics VXO transmitter to 30 meters, and I also did some modifications to the rig. In the model railroading hobby, they call this "kitbashing". I pieced together a receiver design based in part on at least three of Dave Benson, K1SWL's designs (note the credit etched into the receiver's circuit board), adding a highly-selective triple-tuned receiver bandpass filter (design compliments of Jim Kortge, K8IQY, who provided me with a design he had homebrewed) and a homebrew 4-pole Butterworth crystal ladder filter for the IF of 7.68 MHz. Wes Hayward, W7ZOI gave me some pointers on the crystal filter, especially the L-match networks to interface the filter with the SA-612 mixers' 1500 ohm impedance.

Vectronics Transmitter Mods

The transmitter was modified to share the DC supply with the receiver, and the T/R and muting circuitry was modified to use the capabilities of the receiver. As in all of the similar Dave Benson designs, a sample of the transmitter RF is sent through the receive audio chain as a "sidetone", gated by an in-line FET via the keying circuit. This is far superior to the normal modus operandi of the Vectronics transmitter with its direct conversion twin receiver...no sidetone, and loud popping in the headphones when the rig is keyed, an unpleasant experience. I removed the push-button power switch, and power is supplied now to all modules via a front panel toggle switch. I also removed the crystal select pushbutton, and I mounted a DPDT switch for that on the front panel as well. In accordance with good engineering practice, I also added a 33V 0.5 Watt Zener diode across the collector-to-emitter of the 2N3053 PA to limit any collector voltage swings that might damage the final. Recent changes in FCC spurious emission regulations require QRP rigs to meet the same requirements as QRO rigs. Previously, with 5 Watts output or less, your spurious emissions had to be only 30 dB below the carrier frequency output. Now, the the requirement is 43 dB. I could have probably gotten leniency under the "grandfathering" provisions of existing designs, but I added a Chebychev 7-element low-pass filter which modeling showed that the second harmonic to be well below the FCC requirements for spurious emissions. This was later confirmed by testing on a spectrum analyzer. The "trans-receiver" in one box would have to do until I could eventually build a proper transmitter, sharing the receiver's VFO. The two crystals, 10.106 and 10.116 provide good VXO coverage of most of the CW activity. My original plans were, if I had time, to add some circuitry to the Vectronics transmitter before SKN. It would have entailed adding an SA612 transmit mixer, the associated 7.68 mHz local oscillator, and a bandpass filter. That didn't happen, so I instead have made a transmitter board to someday perhaps replace the Vectronics board. It uses an SA612 mixer, receiving input from an auxiliary output from the VFO, and the circuitry beyond the post-mixer filter is similar to the Sierra from the ARRL Handbook, modified for 5 watts output using a different configuration for the PA stage.

Some photos from the project in progress are shown below.

Click on the photos for a higher resolution image.


Main PC Board

The receiver main board

This is a shot of the main board for the receiver. The VFO is a separate shielded module in the case. The receiver board was created in Eagle Lite. I get chided sometimes for building everything using CAD tools rather than ugly construction, but my projects tend to get a bit ugly anyway. So far, each board I've designed has been a prototype. I have yet to actually build something that didn't need changing when I assembled and tested. To me, CADding the boards is part of the fun. I've been using CAD programs of one type or another since 1983.


Transmitter Low Pass Filter

The low pass filter, before final enclosure

See, I do ocassionally build things ugly-style. Strictly speaking, the transmitter could be grandfathered under the old requirements for spurious emissions, since I built it several years ago. But, in the proper spirit of things, I added an extra low pass filter. I came up with a Chebychev design (with a bit of ripple down around 7 MHz) using standard value capacitors that knocks the second harmonic down to -54 dB below the fundamental.

As installed, the filter is completely enclosed in PC board scraps and tin sheeting and mounted on the inside back wall of the case. I used RCA fittings internally and BNC's externally. Much cheaper that way. We have a local surplus store that has the solderable jacks and plugs, and the plugs have a nice long neck on them, perfect for complete shielding when coax is soldered on. If you can't get that type, Wes Hayward, W7ZOI shows a method on his web site for attaching brass sleeves to the back of the plug.


ELSIE Plot of LP Filter

ELSIE Plot of the LP Filter

This is a screen capture of the filter design from Jim Tonne's fine freeware ELSIE program.


VFO

VFO prior to having its roof put on

One ugly-style VFO. It's a varactor-tuned Colpitts, using an MV1662 and a 100k pot at 8 volts regulated. I had the wrong polystyrene capacitors for the VFO range, due to looking at the wrong schematic when I placed the order. I made an adjustment by changing the toroid core type to T50-6, and winding for 2.7 uH. When I built the VFO, the frequency range was about 135 kHz too low, so I went back to my calculation spreadsheet and the toroid chart and saw that I could hit the range dead center by removing one turn from the toroid. The VFO is now set to tune from 2.421 to 2.458 MHz, which equates to 10.101 to 10.138 MHz. I have a trimmer capacitor with an access hole in the VFO box that can swing the frequency about 10 kHz. The wires penetrating the VFO compartment are reinforced with heat shrink tubing, and I added hot glue to the holes to keep them from moving around and possibly abrading the insulation.

I've added some photos of the completed rig to this page, and the final photo shows the tuning scale for the VFO. It is non-linear, which is characteristic of this type of VFO. My SW20+ had the same extreme compression of the top end of the range, and the expanded scale of the lower end of the tuning range. I modified the VFO for the SW20+ for not only better linearity, but a greater tuning range, and a method to switch in a resistor at the cold end of the potentiometer that provides a nearly completely linear range of 12 kHz, from one end of the pot to the other, centered around 14.060, the QRP calling frequency. I based my experiments on information from N7XJ in his ARS Sojourner article on improving the SW40+. I don't need the additional range for this rig, but I will improve the linearity.


Receiver Test Lash-up

Receiver main board and VFO set up for testing

After some intense troubleshooting of various stages, the receiver began receiving over-the-air signals on 3 feet of hookup wire from my daylight basement. One definite snafu was in my schematic. I had a series resistor feeding the 47 pF electrolytic cap going to the headphones jack designated the wrong value. Audio, while still on the RF signal generator at the receiver front end, was extremely low, although I had a nice signal on the scope from the output of the crystal filter. Turns out that instead of a 10 ohm resistor, I had a 1 Megohm one in its place! That will definitely reduce the audio to the headphones!

All in all, however, I am pretty pleased. This project was generally lacking in my usual comedy-of-errors quotient. The way I designed the components to go together using RCA plugs and jacks internal to the case, it was pretty much plug and play from that point on. The flying green wire in the photo connects to the keying circuit on the transmitter board, and is connected to the W7EL-style gated FET audio mute circuit on the receiver board.


Finished rig pics:

Here is an assortment of photos of the finished (except for labeling and beautification) 30 meter transceiver. I took a cue from Mike, KO4WX and annotated some of the photos to identify different boards, etc.

I was re-doing my basement hamshack/electronics build and test area, and I had everything cleared out of this area prior to getting this project going. I'm now set up in front of the window, which will be the permanent operating area, because I can feed the ladder line from the 80m doublet in through a Lexan replacement window pane. After 25 years of nothing but a 20 meter vertical, this is really going to be interesting. In order to keep a sked Straight Key Night, I had to drag out of the closet my only 40 meter-capable rig, my HW-101.

I would like to thank the following people for their support and assistance on this project: Jim Kortge, K8IQY for his design of the input double-tuned filter, and Wes Hayward, W7ZOI for his patience answering some of my questions regarding impedance matching the crystal filter using L-networks, and for reviewing my filter design. I also would like to thank Dave Benson, K1SWL for his designs of the SW-30+, the 40-40 and his NN1G rigs, from which I borrowed heavily.


SKN Setup VFO, Low Pass Filter and Transmitter Modules Custom Receiver Input Bandpass Filter (K8IQY-assisted)and Crystal Filter (W7ZOI-Assisted) Transmitter Module (Vectronics 20 meter transmitter, converted to 10 m Cabling Connecting Various Modules SKN Transceiver Front Panel Temporary setup for operation The NoGa Challenge SKN Special