My Focus Set By the time of the Focus series (March-April 1959), I had moved to secondary school and was probably busy each evening with homework!   So I have no memory of watching the series, and I didn’t make the set then.   When I decided to build a Focus receiver for this web site, I didn’t have a suitable case to house it, so I have made my own.

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My Focus radio, front view.     Top: stowage of leads.     Bottom: battery access. Construction The idea for the case was sparked by a solid-dielectric tuning capacitor with a spindle tapped for a fixing screw, and a companion clear plastic knob with a milled edge.   (These came from a kit radio my brother bought in the early 1960s.   He failed to get the kit to work, and that cured him of radio construction for life!) The intention was to build a case that a reasonably competent “Mr 1959” do-it-yourselfer might have built for his lad, in imitation of the stylish transistor sets of the day.   You will guess that this project was almost as much about developing the idea for the case as about the radio itself.   Mostly I have used only materials and methods that would have been available then.   (I cheated a bit here and there, but I’ll own up as I go.)   Size was to be as small as possible using the specified components. My case is 2 7/8 x 5 3/4 x 1 3/8 inches overall, and consists of: * Front and rear panels, in 3mm Perspex - see pictures above.   (I considered painted plywood for these, and indeed a nice case could be made this way, but as I used to buy Perspex offcuts back then from a local sign maker’s factory, I allowed myself some bright red Perspex for this case.)   The front and rear panels are oversize at top and bottom   The projections allow the radio to stand upright steadily, protect the switches, and serve as a bobbin to stow the earpiece and aerial leads.   The front panel has a circular hole to clear the tuning knob; this hole is cut through to the edge to provide a thumb recess.   I cut slivers from a stainless steel erasing shield to form two bright index marks above and below the knob.   The front and rear panels are fixed with chrome raised-head screws which form decorative elements. * The tuning knob has a paper dial showing through it - see pictures above.   (I confess – I generated this on my PC.   “Mr 1959” could have produced it with a ruling pen, Indian ink and a steady hand.)   The knob fixing screw is concealed behind an aluminium disc.   This was temporarily stuck to the end of a piece of dowel and spun in the electric drill to apply the brushed finish with a fine wire brush. * Two softwood side-pieces, slotted through to receive the circuit board - see pictures below for these and other case parts.   The right-hand side-piece is also slotted to receive the edge of the tuning capacitor, allowing the edge of the knob to line up with the outside of the case.   For this piece, I cut a single large through hole and divided it with a piece of lolly-stick glued in.   The slots on both side pieces are closed off on the outside by pieces of hardboard glued in place.   Once assembly had been tried, I covered the side pieces in Fablon.   (OK, it's fake Fablon - I generated a 1950s-style pattern on the PC, printed it off, covered the print with matt plastic film, and applied spray adhesive to the back.)

* Case top, of white plastic from an old appliance.   This carries two "table-lamp" switches for on-off and wavechange, and has a hole for the captive earpiece lead.   I also fitted aerial and earth terminals, rolled from thin brass sheet and secured on the underside with wire rings soldered in place. * Internal divider, white plastic, located in half-thickness slots in the side pieces.   It has a small "mouse-hole" in one edge, to clear the battery lead. * Battery cover, white plastic.   (“Mr 1959” might have used offcuts of Formica or similar for the latter three components.)

Case parts ready, and     in trial assembly.

Circuit board The circuit board is 1/16-inch Paxolin, drilled to receive the tuning capacitor boss.   I decided to mount most of the small components tightly round this boss, in the space between the board and the underside of the tuning knob; this demanded a different layout from that shown in the leaflet.   I initially fitted nine solder tags (above, left) to receive the small components.

Wiring complete, with additional     components fitted after trials.

Resistors and capacitors are modern, but the OC71 transistors are new old stock.   The diode and crystal earpiece have been in my spares box for years.   I fitted a 5.6kΩ resistor across the earpiece connections for adequate Tr2 collector current. As related below, I fitted further components after trying the set.   These pictures show the set with the additional components fitted. Trials and modifications On first switch-on, output was quite distorted - the first transistor stage was clipping.   The Focus circuit shows both emitters connected directly to ground.   In an effort to control the gain of each stage, I decided to add the emitter circuit components as shown for the very similar receiver in Chapter 14 of Fun with Radio, second edition (1959). See the "Circuit and practical layout" section on The Focus Transistor Set page.

Circuit diagram from the Focus     leaflet, with the changes I made.     Copyright © British Broadcasting Corporation;     reproduced by kind permission.     Images courtesy BBC Written Archive Centre.

I bolted in three more solder tags to receive these components.   I used Davey’s resistor values (Tr1: 1kΩ Tr2: 470Ω), but used two 10μF capacitors that I had handy in place of the 8μF ones he specified.   This cured the distortion.   I repeat here the leaflet’s circuit diagram, showing (in red) these additions, together with the collector load resistor added as mentioned above. The set is not very selective, as I expected, and there is little than can be done about this without fundamentally altering the “front end”.   I tried aerials varying in length from 10 to 50 feet, and various combinations of aerial length and trimmer capacitor value.   I found that, for my reception conditions, the set needs an aerial in excess of 20 feet.   An earth connection is needed for adequate volume during daytime listening, but quite a few stations come in at night with a 50-foot aerial but without an earth. Instead of connecting the aerial to the red tag as specified (reaction winding used as aerial coil), I tried connecting it to other points on the coil (green and yellow tags).   I did also try out the Repanco coil in a “test-bed” one-valver with reaction, where - as expected - it performed very well. I have ended using the radio mostly with a 50-foot loft aerial connected to the yellow tag (tapping on medium-wave winding, also connected to the diode).   With this setup, reasonable results are obtained with BBC Radio 5 Live on medium wave, and BBC Radio 4 on long wave.

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Despite this little radio’s performance limitations, I have enjoyed building and using it.   It has whetted my appetite to try one of Gilbert Davey’s more complex transistor designs.   I am quite pleased with the appearance of the case, and I may well re-use it - or a similar one - for another set. If you built the Focus set at the time of the BBC series, or if you decide to build it now, please get in touch.