Tuesday, September 15, 2009

HF Lunchbox 1 lives again

This morning I dropped in to see Barry VK5ZBQ. We had talked about catching up to sort out one of the two Codan 6924 HF radios that I purchased at an AHARS members meeting auction.

So this morning was "the day". I had a minor ordeal to get there, including losing my bus ticket, missing the bus, then catching a lift after the car would not start. I arrived about 9am and was undoing the case screws to get down to business shortly afterwards.

Barry has a manual for the Codan 6924 Mark 1, so we looked up various sections that I was curious about. Barry set the rig up on the test bench, powered it up, connected it to the signal generator, selected the frequency for channel 1 and we heard the dual tone from the speaker on the front panel. Win number 1. Then we set about transmitting from the rig. The power meter on the signal generator shows out going power when hitting the antenna test position on the mode selector. So out with the multimeter, leading to the detection of a broken microphone lead.

The chassis can be separated by removing some screws and nuts, so you need tiny fingers or a little tool to hold the nut so you don't loose it. So Barry opted to replace the microphone lead with something a little more robust than the original lintz wire type. Apparently you can't solder it very well. So the new lead went in. Took a little while as its quite a fiddley job. We reassembled the chassis and microphone and connected up the output to the oscilliscope. Fired up the HF rig, tuned to channel 1 and tested it. Win 2. We have a nice upper side band trace on the scope and after a bit of fiddling with the HF radio antenna and dummy loads, we hear the transmitted voice from the HF rig.

At this point I'm very happy, as the radio works properly and we work out that it puts out 8-11 watts PEP. The antenna tuner varies the output power a lot. Need to sit down with it and work out how exactly the tuner works, it has a loverly wound toroid with wiper for continuously variable inductor. Now I'm ready to attack the idea of a VCO in stead of a fixed crystal oscillator.

We break for a cup of tea and Barry explains to me how the upper side band (USB) and lower side band (LSB) filters work in the 6924 and what I'll need if I change the oscillator to a 80 or 40 meter band. Apparently there were kits for this, but I've found no documentation. So I will draw everything up and go back to Barry and run it by him. I think I'll work out what crystals I need and populate the remaining empty crystal oscillator board first to get the hang of it first =)

Now that the weather is warming up, I can spend more time in the radio shed/shack. I've put together a dipole for 40 meters along with a 4:1 balun and suitable cable. Should be fun.


No comments:

Post a Comment