Sunday, 11 December 2011

RC-14 - An Old Project Revisited

Way back in June 1987, the RSGB published an article in RadCom describing the "RC-14", a relatively simple "beginners" single band (20m) direct conversion receiver.    This was "kitted" by the then popular "Cirkit" emporium, and was offered at an attractive price to RSGB members.

This was, in my view a very well designed piece of equipment, and contained only three integrated circuits (one of the famous and rather splendid Plessey "SL" ICs and a couple of op amps configured as active filters with characteristics very similar to those of a high-performance crystal SSB filter) plus a varactor-tuned VFO, and the end result was a very pleasing receiver.

I built the receiver not long after it was published using the approved kit, and adapted it to my "bespoke" requirements - I disliked the flimsy enclosure provided and moved it to a more mechanically sound but less aesthetically pleasing aluminium die-cast box, and replaced the linear slider tuning pot (remember it was a varactor based design) with a rotary multiturn unit, albeit at great expense, and used it for a while (as you do) and moved on . . .

My version of the RSGB RC-14

Recently, I have become interested in the "QRSS" aspect of the hobby which is based around the transmission of very low power, low signalling rate beacon signals for extended periods which are received at various locations around the globe using PC-based "grabber" software.

Participation in this requires either the setting up of a "grabber" receiving station, or the use of a low power "MEPT" (Manned Experimental Propagation Transmitter), or in some cases both, though not necessarily at the same time!

Over the last few weeks I have been doing either and even occasionally both (!) using conventional equipment, and by "conventional" I mean "High Performance Japanese Transceivers", certainly as the receiving station, but it occurred to me recently that it was possible overkill using such high-spec equipment to perform such relatively mundane tasks.   In a flash of inspiration (or something!) I remembered my RC-14, and began to wonder if that could be dragooned into service as a QRSS receiver.

The built-in VFO, though adequate for general receiving purposes was certainly not of the required specification as a QRSS receiver, where frequency stability is a primary requirement, but the substitution of the built-in VFO with a crystal-based, or even synthesiser-based local oscillator would seem to be a Good Idea.

If you are following the argument, then by now you will surely have picked up on the mental thread . . .

And so, another project is born!