WHAT I LEARNED THE HARD WAY ABOUT CONSTRUCTION METHODS
This article is meant to be a step by step expose of all the mistakes and bad ideas I had in the evolution of a
successful homebrew construction methodology, and eventually the invention/discovery of a very versatile 'mezanine method'.
By sharing this I hope I can help you by learning from my experience.
(Each image can be clicked to see a more detailed view)
80m CW rig
The very first solid-state rig I built was this 80m 1W CW rig based on a Pat Hawker's design from Wireless World,
with a few tentative first steps of my own ideas, notably in this case a Varicap tuned VFO with a 10 turn pot.
I had the idea that to increase the density, I would mount all the board modules vertically in a nice die cast aluminium box.
The modules were separated with screened sections and the wiring went from cucubicle to cubicle via small holes.
it worked fine .... so long as nothing went wrong, because to do any kind of fault finding or adjustment required
the complete removal of the PCB in the module to get at the components, very awkward and a big mistake!
The other problem is the die cast box only gives access from the very top. We shall see a better idea later.
Another mistake was the inclusion of a rechargeable NiCad battery pack inside the rig, I later learned how inconvenient
this is compared to having an external battery pack with push-in individually replaceable AA rechargeable cells,
and on that note, I later gave up on NiCads altogether in favour of more reliable NiMH cells.
Having built-in cells is also a leakage hazard.
30m CW only
The next rig was a great leap forward, a 30m CW 1W rig also VCO tuned - this time using cheap 1N4004 diodes instead of
varicaps, and as the 30m band is very narrow, no 10 turn pot was required. The whole transceiver is on one very nice PCB and
maintenance was easier. However, any unsoldering to remove a component - which happens a lot when you are evolving a
design - entailed the complete removal of the PCB again, and in this case, several of the plugs and pots had to be
removed as well to get the board past.
Once again very awkward. Another mistake was the placement of a passive (inductor) LP filter between the
mixer and the 100dB gain AF amplifier, leading to problems taming microphonics and stability. I later learned to break
the gain up and have at least 20dB gain before the filters.
This project was published in ZS and Sprat and was
sold as kits by the Liverpool ARC.
Using PCB for boxes
The following rig began a new construction method, in that I broke away from the die cast box and began making boxes
with double-sided PCB Board. They are very easy to make, are very strong and light-weight, and offer good RF screening.
This rig was designed originally as a mountaineering rig and was the size of a 20 pack cigarette box,
with simple Xtal control. The lack of VFO proved a great limitation in use, as there is very little 40m activity
in South Africa, so the whole little Xceiver was lifted out, and put into another PCB Box with a very nice VFO.
The beauty of this VFO is that it runs at half the TX frequency, and drives a push-pull doubler,
this creates very little keying chirp. This was so successful, I used it on all future rigs.
You can see the little transceiver board in the middle, but now it's surrounded with various additional LP and CW filters,
as well as a 1W audio amplifier. There is a small speaker built into the roof of the box. A very interesting feature of this ig was the use of a very low noise op-amp (OP-27) dierctly after the diode mixer, in an inverting mode with the input resistor set to 56Ω to match the mixer. A simple but little used idea, it worked extremely well.
40m Xtal filter SSB Hi DR rig
This rig was a fairly serious project: a full
dual filter SSB rig on 40m, 2W TX and a very high dynamic range receiver (92dB - achieved using high power FET front-end).
The IF used the classic Plessey SL600 series, but I was not that impressed with their AGC handler.
The box is using the mezzanine design and each module is on pins, which plug in and can be easily removed.
The large closed die cast box contains the
VFO and Xtal Carrier oscillators. This was also my first rig to incorporate a digital frequency readout as part of the
initial design. All rigs from here on also included counters.
Other than this rig only has a 2W output it is a very high performance transceiver, and has given me much pleasure in use
as well as in modifying it over the years. The VFO based on the classic Wes Hayward design is truly amazingly stable
and has practically no drift after initial warm-up. It set the stage for a box/chassis type that I have continued to use.