At a recent rally, I spied a rather beat-up old Yaesu FT-707. 'Selling for parts or repair' said the sticker. It was very cheap, so I asked about its condition. It turned out the PA was jiggered and there were a number of other problems. Hmm. At the price it was worth a punt, even if I just ended up stripping it for some useful bits. So it came home with me.
On the bench, it was obvious that the PA was indeed utterly boogered. However, the rest of the transceiver seemed to work and I briefly entertained the idea of fixing it up and adding Topband (The FT-707 does not include 160m coverage). It's a good thing I dithered for a while though, as one by one, other 'issues' bubbled to the surface. Once upon a time I would have doggedly worked my way through these problems, but these days I seem to have less staying power so I decided to remove the guts completely and use the case for ........ANOTHER TOPBAND RIG!
Why? No idea. The VFO was lovely and the case would look nice when done up, but beyond that I think I was just bored. Clearly some preparation was required so I started off by looking at the VFO, which had developed a nasty twitch. On un-canning the VFO, I saw the innards had been smothered with some kind of rubbery gunk which reminded me of my days in the TV repair trade. Some manufacturers used this stuff to anchor 'back of the board' components, but it was the source of many baffling faults as it aged and became slightly conductive. I decided it would have to come off. It's pointless trying to pull or pick it off, it sticks like crazy and over the years it will have oozed its way around the component leads etc, making it very difficult to remove. Below is the gunky VFO.
To cut a long story short, I ended up removing the PCB from the chassis and boiling it slowly in white spirit. This made the entire house a bit whiffy, but after about half an hour I was able to get the stuff off with minimal effort. Below is the cleaned PCB back in the chassis.
I also replaced a suspect mica cap while I was at it. The result was a clean, stable signal from 5.0 to 5.5 MHz. But that was the next problem...Not only was this VFO on the wrong frequency but it tuned backwards as well, a result of this rig's original conversion sceme.
With a 455KHz IF, I'd need a VFO covering 2.255 MHz to 2.455 MHz, but I didn't want to make any major changes to the VFO lest I ruin its stability. Only one thing for it - it would have to be mixed down. If I chose the mix-down oscillator correctly, I could correct both the tuning range and the direction in one go. A quick scrabble through my vast collection of xtals produced an 8 MHz rock, which when mixed with the otput from this VFO (and a small twiddle of its coverage) would produce the required range of frequencies and the direction of tuning would be cured as well. Blimey, a stroke of luck!
So the next job was to knock-up the crystal oscillator, mixer and filter on a piece of copper board. I chose (surprise!) an NE612 chip mixer for this task, as it is double-balanced and therefore produces only the sum and difference signals and cancels out the unwanted ones from the VFO and the xtal. The product I was after was the difference one (2.255 to 2.455 MHz), so this could be extracted with a pair of tuned circuits which should reject the 'sum' signal around 13 MHz. Good grief, it worked. Below is the VFO, mixer and display (a rally special from Les at AM Tools) on test.
Below is the the output spectrum of the VFO/mixer. This needs to be as clean as you can make it or you'll end up with lots of rubbish when you come to do the next mix in the TX and RX mixers. There are a couple of unwanted products visible, but at -60dBc they are small enough to ignore.
Next on the list was the PA, as I knew I had the bits to build it without having to nick the parts from my existing topband rig. I'd use the FT-707's heatsink and ferrites, a pair of beefy BUZ901p's out of the drawer and miscellaneous bits from my junk box. It produces up to 35 watts PEP from a 35 volt supply without much in the way of smoke or flames. And here it is below, almost completed. That thing dangling on the right is the output LPF.
I'll gloss over the RX and TX boards, as they are fairly standard and boring. Receive sigs come from the antenna c/o relay, through a pair of fixed BPF's and then amplified (not too much) by a nice 3N401 dual gate mosfet. Ancient but still perfectly good. So good that I used another as the mixer, which of course gets its local oscillator signal (2.255 to 2.455 MHz) from the VFO/mixer board. Output from the RX mixer is relay switched (no diode switching at G1HBE, it isn't natural) into a choice of AM (10 KHz wide) or SSB (2.5KHz wide) ceramic filters. An if amplifier strip consisting of a mixture of FETs and chips, diode (AM) and product (SSB) detectors, audio filters and some AGC produces audio which wends its way via some basic audio filtering and the volume control to the LM386 audio amplifier and the loudspeaker.
Transmit is the same but backwards - the mic signal is amplified and compressed (a novel circuit of my own using an NE612 as a gain-controlled amplifier) and then goes via level controls to the balanced modulator. On SSB this makes a double sideband suppressed carrier (DSBSC) signal with a carrier frequency of 453.5 KHz suppled by the 'carrier osc board'. With the rig switched to AM, the modulator is deliberately un-balanced so that a carrier (455 KHz from the 'carrier osc board') is produced with the usual pair of AM sidebands. These two signals (DSBSC or AM) are then filtered by suitable 2.5 KHz-wide or 10 KHz-wide ceramic filters to produce USB or AM signals ready for mixing up to Topband. This happens in the final TX mixer which gets its other signal (2.255 to 2.455 MHz) from the VFO/mixer board. The USB signal is inverted in this process, becoming a LSB one. The output from the TX mixer is filtered to get rid of the unwanted 'image' frequency (455 KHz + 2.455 MHz) and at this stage it is pretty small, so it is amplified by a two-stage bipolar amplifier (incorporating another image-rejecting BPF) before being sent down a piece of coax to the driver/PA assembly on the rear heatsink.
Below is a comparison of three different image filters I tried after the TX mixer. We don't want to be transmitting on 2. 9 MHz, so this filter has an important job to do.
Combined with the final LPF after the PA, this provides a nice clean transmission (below). The only unwanted showing up is the second harmonic, around 3.8 MHz. It's 52dB down on the main output which is fine, and the third harmonic is at noise level.
There is another board lurking. It's underneath the chassis and contains bits I'd forgotten about. The carrier oscillator sub-board is here - it provides the 455 KHz and 453 KHz signals for the TX modulators and the SSB product detector on RX. Also here are the voltage regulators and the main 8v change-over relay which applies the volts to the RX or TX boards as necessary. Lots of wires around here, as it acts as a kind of junction box for switched voltages and signals.
Here's the finished rig after a re-spray in blue/grey. Output is up to 35 watts PEP in both AM and LSB modes. I've added the Huff & Puff stabiliser to the VFO, as there was a slow drift on long QSO's. It now stays put.
On Friday and Sunday evenings at 8:30 local time, 1963 KHz is where you'll find several of us chatting. The net was started in the early 90's and many people have come and gone, but there's always a core of dependables who turn up regularly. Subjects range from trams and Real Ale to synthesizers and microwaves - it depends who is on. It's all very relaxed and you'll be made welcome. And it's AM only!
Below is my present antenna for Topband. The old 35-ft vertical which was supported by the back wall of the house had to go to make way for a 4 metre vertical. This new topband vertical is mounted away from the house and cannot be supported by wall brackets, so I've abandoned the old roach pole in favour of a more sturdy Spiderbeam 12m pole.
As before, the aerial is resonated just at the HF end of 160m with the main loading coil (you can just see the coil against the cloud), and the tuning box at the base contains a small adjustable inductor to bring the resonance to the required frequency.
The 'earth' is almost non-existent, consisting of about 100 sq ft (10x10) of B&Q 2-inch fencing mesh. Still, I suppose it works to some extent.