Wednesday, 30 October 2013


One of the items I was pleased to obtain at Rishworth, were a pair of QRPme SBSS clamps. Getting them involved ensuring the cost had been covered by my junk sales, and then being roundly questioned by the proprietor of QRPme over my choice of T-shirt and lack of hat. Now, these clamps are not cheap, at £20 for the pair, but they are remarkably good at what they are for, namely building PCB material boxes. Having bought them, I was very impressed to be shown the original Tuna Tin transmitter built by Doug DeMaw, and kept safe in a wooden box. I was also happy to receive a few free MeSquares and MePads!

The clamps themselves of course are no help if you do a shoddy job of cutting the board to start with, so I was very, very careful to mark and scribe the board prior to cutting. I did make the mistake of not correcting for the board thickness, but that was soon rectified. Using one of the SBSS clamps, I soon had a little 1" x 1.5" box built, in which to rebuild my little VLF to soundcard interface

The picture shows it with the sockets installed. The one big mistake I did make, is that after marking around the washer of the earth terminal, I then forgot to measure the diameter hole needed for the body, and drilled it out the full width of the washer! Luckily, the blue terminal post just stayed in place in that hole (well tightened!) so it wasnt too bad a disaster.

Much effort went into trying to solder nuts in to attach the lid, but they just wouldnt hold. Eventually a pair of hex posts did solder in, but its a loose fit as I dont have any screws of the right thread.

The VLF interface is about the simplest receiver that can be made, simpler really than even a crystal set. It really consists of little more than an antenna wire connection to a soundcard input via a blocking capacitor. My version is a tad more complex, as it also includes three levels of protection - there is a 100k resistor to ground to bleed off static buildup on the antenna; a pair of back-back Germanium diodes that will clip the incoming signal above about 0.3v; and now a small wire ended Neon lamp acting as a gas discharge protector

Next job is to complete the RT-remote clone, which just requires the internal wiring loom completing, so I can then test, and indeed control, the LDG RT-11 Remote Auto-ATU.

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