K3WWP's QRP Rigs

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Reviews of QRP Rigs

This review of the VEC 1240K transmitter was contributed by Carlo Strozzi, IZ4KBS.

I built this little transmitter kit out of curiosity, after having read the positive review by Bill Mabry N4QA. I needed a small QRP TX to use together with my newly built SS-40 Receiver, by the Four State QRP Group, and the VEC-1240 seemed to be the perfect choice. What I found most attractive was the built-in solid-state T/R switch, which would save me some of the cable salad which is usually needed when operating separate transmitters and receivers through an external T/R switchbox. And I was not disappointed in that respect. Back to Bill's review, he was mentioning a "spot" modification but he did not provide any further details. After enquiring and having received more info I decided to do it differently, by using a momentary switch and a diode (D4), so that when the main power switch is off only the oscillator stage can be activated, thus making the rig emit a very weak signal that is perfect for tuning the receiver on spot. The details can be seen in the modified schematic. The modified schematic shows also an additional diode D5, which I added along the keying input, and which is only needed to have the rig work with the NoGa Guppy external T/R switch, but that's another story, and that diode is not normally needed.

Then, as Bill had already pointed out in his review, the amount of TX signal leaking to the receiver at key-down in the original design is way too much, and albeit it cannot damage the receiver it can certainly blow one's ears off if the receiver itself does not sport a *really* fast AGC and a good dynamic range. So I decided to add also a miniature relay to the built-in T/R circuitry, with which the amount of TX signal going to the receiver drops from an insane +7dBm to a more sensible -23 dBm. This mod is also visible in the modified schematic at the above link, and an actual picture.

On a side note, I find the limited frequency range offered by the VEC-1240 to be just right for the job. Too often I see people trying to expand the narrow range covered by most VXO-based QRP rigs, and I used to be among them. Eventually I realized that moving too far from the usual QRP "watering holes" isn't really advisable, because a weak signal is more likely to be either ignored or stamped on when found say in the DX window of a band, where it is not expected to be. On the contrary, it's best for QRP'ers to keep where others expect them to be found, which increases the chance of making contacts rather than not. So, a few KHz of flexibility around those frequencies are all that is needed to get the most from QRP, and the VEC-1240 holds to that tenet.

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