May 2007: Here's a good one - the conversion of an old Yaesu FT690 6-metre multimode rig to 4 metres. This was meant to be a long-term project but as usual I went at it like a bull at a gate, resulting in failure within days. Then I calmed down and started a 'Mk2' version...
Here's the Mk2 transverter board with the 20 MHz oscillator, NE612 balanced mixer and 70MHz bpf. Fed with 50MHz from my sig gen, it worked a treat at this stage.
The transverter board almost finished, and still working! Those unused pads and blobs of solder are left over from an earlier failure. The smaller board is the 200mW PA - it used a VN67 fet, but it was plagued with instabilities. So was the next version. I suspect the VN67 is not a good choice for this job, but I'm too thick to realise it.
The final PA, using a good old 2N3866 bipolar. In fact, this amplifier was designed as a wideband power amp (you can see the little ferrite transformers on the input and output) with a 75MHz lpf on the output. The finished board has the antenna change-over relay on the end.
A view of the transverter installed in the battery tray. Starting at the top right, we have: RX converter (2 dual gate fets and 3 70MHz tuned circuits), bottom right is the screening box containg the 20MHz osc, mixer and 70MHz bandpass filter. Over on the left is the tuned fet amplifier and just above it (the little black dot) is a MMIC amplifier. This delivers about 20mW to the PA.
Spectrum analyzer plot from 50 to 150MHz. The 70MHz output is at 0dBm and the second harmonic is -68dB.
Nothing else is visible.
The block diagram (below). The TX input to the transverter is taken just after the dual-gate amplifier/AM modulator in the 690. On receive, the down-converted output from the transverter goes directly to the 690's receiver input.
June 2013. A few changes at G1HBE have meant the removal of my 4m antenna and the retirement of the old converted TS780. It now looks as if my access to 4m will be /P or /M from now on. So I've decided to give the transverted 690 a bit of extra urge, and what better way than to use the nice Mitsubishi PA block removed from the 780? A quick scour of the shack produced a 4" by 2" Eddystone box and a heatsink that could have been made for it.
Here it is, with a PCB of my own making. These PA blocks are easy to use, just a bias pot and a simple LPF on the output. Plus a change-over relay for the aerial of course. 20-30mW from the transverter was enough to get 20w out!
Round the back. Seperate RX and TX co-axes, PTT and supply make up that bunch of wires. I've throttled back the output to about 8 watts, as the heatsink is a bit small.
And below is the output spectrum showing +/- 25 MHz from 70.400MHz. Amazingly, there's virtually nothing coming out that shouldn't!
So there we are - another enjoyable project that works well. Only the cutting of the square aperture in the Eddystone box caused me problems, but patience and sheer bloody-mindedness paid off in the end.
July 2013: The recent spell of good weather has got me out in the hills with my little rig. A couple of photos to prove it.
Parked up on the hill at Harrop Edge in Mottram, Cheshire. The antenna is just a quarter-wave mag-mount, but the clear take-off across Manchester and the Cheshire plain ensures good sigs.
Here's the rig being slowly baked by the hot sun. I moved it into the shade after taking this picture to reduce the chances of a fire!