TV Sept p652-653 Vintage Radio 8/6/04 4:32 PM Page 2

Vintage radio: tuning indicators

Tuning indicators were often a feature of the more up-market receivers that graced the nation’s homes from the Twenties to the Seventies. Several different types were used over the years. J. LeJeune describes their mode of operation and circuitry

uning indicators were never lamp with two wire electrodes resistor being included to reduce really necessary with the AM inside, a fairly short one and anoth- loading on the detector circuit. Treceivers that graced the er that extended almost the length When the grid voltage became nation’s homes from the Twenties of the tube, see Fig. 1. One elec- more negative, an increasing shad- to the Seventies. They were never- trode was grounded while the other ow was cast over the fluorescent theless often a feature of the better one was connected via a resistor to anode coating. As a result the ‘eye’ class of receiver, along with push- the anode of a variable-mu IF valve closed. A small disc at the centre of pull output stages and, in later to which AVC (automatic volume the aperture hid the hot cathode models, band-spread tuning on control, i.e. AGC) was applied. from the user’s view. Fig. 3 shows some wave ranges. The negative-going AVC voltage the display and Fig. 4 a typical cir- increased as a station was tuned in, cuit. The most common types used Early indicators biasing back the valve and thus were the EM4, EM34 and Y61. The very first indicator was the reducing its anode current. As the After a time the fluorescent coating Tuneon. This was a long, thin neon valve’s anode voltage rose, the lost is brightness and the eye began length of the glowing neon gas to dim. increased. So you tuned in for max- FM tuning indicators Negative electrode Neon gas imum length of the orange-pink Positive electrode glow. The arrangement worked A variation on the magic-eye fairly well, but reduced the effec- device appeared in the US to tiveness of the AVC action because enable FM receivers to be tuned of the change in anode voltage accurately. Its type number was required to operate the indicator. 6AL7GT. The end of the glass Fig. 1: The Tuneon neon tuning indicator. Less troublesome was the sim- envelope was again coated with ple current-meter connected in fluorescent powder, but inside there

HT series with the anode of the gain- were two tetrode electron guns side controlled IF valve, see Fig. 2. The by side. They produced a pair of Tuning meter was at full-scale deflection illuminated green strips on the meter 0-20mA under no-signal conditions, the nee- ‘screen’. The anodes of the guns dle sliding back as a station was were connected to a supply of some tuned in. You simply tuned for a 250V. Fig. 5 shows the idea. The minimum meter reading. grids controlled the brightness of Output the strips, while the deflection The magic-eye plates varied their length. By the Thirties the magic-eye tun- The grids were biased so that ing indicator had become extremely the beams were cut off when no Variable-mu popular. This was a thermionic signal was present. This was easy IF amplifier valve device, basically a triode with a to arrange, using the AGC line and Input conical anode. The open end of the cone faced the top of the glass envelope, which was coated with a fluorescent powder on the inner surface, similarly to a CRT screen. The grid was a pair of wires or, sometimes, small blades that were AVC spaced 180° apart and close to the cathode. It was connected to either the AGC line or the output from the Fig. 2: Tuning meter in the anode circuit of a AM detector circuit. The latter was variable-mu IF amplifier valve. generally preferred, a high-value Fig. 3: The magic-eye display.

652 September 2004 TELEVISION TV Sept p652-653 Vintage Radio 8/6/04 4:33 PM Page 3

a polarity-inverter stage. When a two separate illuminated ends HT viable signal was found the grids moved towards each other until the were driven positively and the edges met, indicating excessive 1M 1M screen was illuminated. The recording level. deflector electrodes were connect- In many recorders these devices 5 3 6 ed to each side of the FM ratio were replaced by a cheap moving- detector circuit so that, when the coil meter movement, often driven tuning was slightly off to one side from simple transistor circuitry. EM34 of the centre frequency, one A meter didn’t deteriorate and deflector was more positive than need replacement. The advent of 3M3 Input 4 the other, making the illuminated reasonably good automatic level from AM strip longer. As the correct tuning control in cassette recorders finally detector 0.047 point was reached, the two fluo- eliminated the need for a record- 2 7 8 rescent strips became of equal level meter. length. Should the tuning drift to 6.3V 1k5 the other side of the centre fre- Tuning-scale indication quency, the strips would change in There were other arrangements of length in the opposite sense. The course, but perhaps the one most user of this type of tuning indica- worthy of mention was that used in tor would know whether the some early valve receivers. It Fig. 4: Typical EM34 magic-eye indicator circuit. Grid drive could be obtained from the AVC line or the output receiver was tuned high or low of appeared as a strip of light across from the AM detector. the correct point. the top of the tuning scale. As a These indicators were never station was tuned in, the length of Cathodes used in the UK, despite being a the strip varied. The shutter that Common grid very useful addition to a domestic produced it was a curved vane of Screen FM receiver. The moving-coil lightweight material which was meter with a centre-zero took over painted black. This was mounted from this indicator which, as with on the needle of a moving-coil the magic-eye type, suffered from meter: when there was no signal, display fading with time. The meter the vane (shutter) blocked the light arrangement is shown in Fig. 6. from a dial bulb – the light from (b) this was projected through a slot (a) Deflector plates Battery-operated receivers aperture on to the rear of the Fig. 5: The US dual-beam indicator for tuning FM receivers, Before the advent of the LED, bat- translucent tuning dial. (a) display, (b) internal arrangement of the indicator. tery-operated receivers used the As a station was tuned in, the DM70, DM71 and DM160 indica- meter needle moved and the shutter tors, which had 1·4V filaments. The allowed light to reach the tuning DM70 and DM71 had a display scale: the length of the illuminated like an exclamation mark, which strip varied with signal strength. Centre-zero was actually the shape of the con- The meter movement was, of Input tuning meter trol-grid aperture through which the course, included in the anode cir- fluorescent strip could be seen. The cuit of a variable-mu IF valve. single filament passed directly in front of the aperture, so that the flu- In conclusion orescent display was viewed These are all now relics of a through it. bygone age. In radio sets they were Output The DM160 could be classed as marks of a ‘de-luxe’ receiver, along Fig. 6: FM tuning-meter connections in a ratio-detector circuit. a sub-miniature display and was with other desirable accessories again a triode. But instead of an that were aimed at tempting aperture-plate grid it had a helical prospective purchasers to spend a grid between the filament and the little more than they had originally fluorescent anode. At only 5 x intended. 25mm the DM160 fitted easily into A look at today’s audio equip- the miniature receivers of the day. ment shows that little has changed The DM70/71 were 9 x 45mm. in this respect. Marketing people Fig. 7 shows the two types with call them “features” and, amongst a Y61 magic-eye indicator for themselves, speak of “bells and comparison. whistles”, the product being a “mug’s eyeful”. Ostentatious in Tape recorders presentation, and when in operation For a time these indicators and the flashing like a fairground in full EM80 series were very popular spate, much modern consumer both as tuning indicators and as audio equipment works well inexpensive recording-level indica- enough without any need for such tors in domestic tape recorders. The extras – which can make servicing EM80 had a fluorescent strip along a nightmare. the side of the envelope. This nar- In comparison, the old tuning rowed as bias was applied to the indicators were examples of good Fig. 7: Three generations of thermionic tuning indicator: a control grid. In tape recorders the taste and moderately useful extras. ■ Y61 (top), a DM70/71 type (centre) and a DM160 (bottom).

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