Thursday, 30 January 2014

Soldering on!

With the sad demise of my long time trusty 18w Antex soldering iron, I had to quickly obtain another if I were to be able to carry on with my various radio projects. Since I mostly build 'dead bug' or 'Manhattan' style these days, often making the cabinet from the same PCB material soldered together, I opted to get the 25w Antex iron this time. Ordered on Amazon to take advantage of free postage, it came virtually next day!

I have to say, im not so keen on the new body shape of the iron, but its still ergonomic enough and works a treat. So, ive managed to complete the Two Tone test oscillator. It wasnt quite as simple as expected, as it turned out the salvaged phono socket i'd chosen didnt make contact with the plug pin! So, that was ripped out, and another salvaged connector, this time a BNC, installed in its place. Testing it on the now warmed up frequency counter (ive had the heater on for three hours as the workshop was freezing!), showed the low frequency oscillator running at 650Hz, and the high side at 1500Hz. Not exactly as i'd wanted, but not harmonically related, so ok for the proposed purpose.

Another bit of salvage, this time the knob, finished off the project apart from its aesthetics. So, since im painting a 1:72 model Vulcan at the moment, its getting a coat of Humbrol No. 164 Sea Grey acrylic!

I like to paint Altoids type tins when used for projects, as it makes it easier to put markings on them. I'd normally use enamel paint though, but the acrylic was out on the table already.

The oscillator has already had its first operational outing, driving the AF input of the 10m Wispy in its first test with a ground independent audio source. The results are promising - there seemed to be no RF output without modulation. A problem I have with this setup though is that my spectrum analyser cannot go below 1MHz bandwidth, which makes observing the DSB modulation with a pair of very narrow audio tones difficult. But from what I could make out, it seems to be ok.

The above shows the observed spectrum at the base of the driver transistor with the two-tone audio input. The picture below shows the 10m Wispy and the oscillator under test

As my test equipment and setup is not very sophisticated, I may still ship the Wispy to someone else with a better setup to have it tested and measured to see if all is really well. For instance, I cannot measure the very low Tx power at the moment, and am a little concerned that the 2nd harmonic isnt being attenuated sufficiently. In the meantime, I will start the build of the receiver section, presuming of course that the two MPF102 FETs I have in stock are working!

Another issue that I need to look into is the performance of my interface to the Alinco DX-70TH. This I built with a level control pot, but it seems to do some odd things! For instance, at certain settings for low Tx power, there is audio tone in the speaker? Odd. And at other settings, reducing the volume of the audio fed to it actually increases the Tx power! I suspect an odd ALC action going on here, so think I might remove the  level control and rely on setting the computers audio output level to set the Tx power.

Paul EI5KI, has sent me his 20m PSK-20 from Small Wonder Labs. This nice looking kit should produce a couple of watts on 20m and will be used for simple demonstrations, and maybe to provide a digital QRP station for the CHOTA event. I will give it a try on air either tonight or tomorrow.

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