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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.

'Mezzanine' Box Design

Now came the break-through, the invention of the 'mezzanine' box. This little rig, has turned out to be my all time favorite for no other reason than that it sounds great, and works great! It was made with cut pieces of thin aluminium. The big thing to note about the construction however, is an internal 'middle floor' to the box. Both the front and the back can open to expose separate modularised units can be easily tested and individually removed for modification or maintenance. It is a 2W CW/DSB TX, DC receiver with a very smooth VFO and audio section. Added onto the top in PCB box design are a 10W linear and frequency counter display. The tin shielding hides the VFO and is necessary to prevent frequency shifting from pressing on the box lid! The linear can be switched out for QRP opoperation Audio derived S-Meter and TX microphone meter are included. The PCB construction has been very robust and I have taken this rig on many field trips and it has never let me down.   

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.

A 1kg DSB/CW SOTA rig

The last rig in this article is interesting in that it has combined many of the best features previously discussed. It was made as a SOTA (Summits On The Air) (lightweight) rig to take mountaineering and weighs less than 1kg. The box is made from a special thin grade PCB, and designed in the 'mezzanine' style. It is a 2W DSB with partially suppressed sideband. The TX/RX is all break in keyed and timed and muted with CMOS analogue gates for click-less listening. It incorporates many other features such as VCO tuning, sine-wave side tone and 3 stage audio filtering. The audio chain is designed with hi-fi low distortion care. No PCB's were made and all the modules are simple breadboards using the Dremmel tool prototyping method discussed elsewher on this site. Modifications and updates are very easily done.