Vacuum Tube (Valve) Radio and Audio Repair Guide: Automobile receivers

For servicing purposes the receiving circuitry of automobile radios resembles closely that of con temporary mains powered domestic sets, and like the latter the vast majority settled into the four- tubes (valves) plus rectifier superhet configuration (with, however, some interesting exceptions); therefore the same fault-finding techniques are applicable. It is in the power supply section that the two types differ because in the automobile receiver both LT and HT have to be derived from the car battery. In this respect it has to be remembered that during the vintage radio period both 6 V and 12 V systems were in general use and most manufacturers produced separate models for either voltage.

The LT presented no problem as tubes (valves) with 6.3V or 12.6V heaters could be powered directly from the battery. (In fact some manufacturers preferred to use 6.3 V heater tubes (valves) in both cases, wiring them in parallel for 6 V systems and in series-parallel for 12 V use. Where necessary, shunt resistors were added across certain heaters to balance the current flow.) The big difficulty for the designers of early automobile sets was how to step up the 6V or 12V of the battery to around 200V for the tube (valve) anodes and screen grids, etc. Initially motor generators were tried but these were both noisy and inefficient as regards battery consumption, and were soon abandoned in favor of what became the universal provider of HT, the vibrator power pack.

The basic non-synchronous vibrator is basically similar to an electric buzzer, powered by the car battery, with the moving reed acting as the centre pole of a two-way switch. It works in conjunction with the centre-tapped primary winding of a step- up transformer, the tap of which is connected to the 6 V or 12 V supply. All the time the reed is moving to and fro, at around 115 times per second, it earths alternately one end or the other of the primary, causing interrupted DC current to flow first one way, then the other, in constant repetition. This induces an HT voltage in the secondary winding, usually of around 200 V to 250 V. Incidentally, the fact that vibrators run at 115 Hz means that the size of the transformer core need be only half that of a type intended for 50 Hz use, giving a useful saving on space.

The HT output from the transformer has, of course, to be rectified, and for many years the favorite tubes (valves) for the job was the 6X5/6X5GT, a full-wave type with a 6.3V heater and highly insulated cathode. Another popular rectifier was the 0Z4, a gas-filled tubes (valves) needing no heater supply; the cathode reached its operating temperature from ionic bombardment when the anode voltage was applied.

No separate rectifier at all is required when a synchronous or self-rectifying vibrator is used. Extra sets of contacts are arranged on the vibrating reed which switch the HT output at either end of the HT secondary winding in exact synchronism with the LT input. Contrary to what is familiar in most HT rectifier applications, the centre tap of the secondary winding is positive, not negative. As the current flow in a particular half of the primary induces one of the same polarity in the secondary, the appropriate outer end is earthed to complete the circuit.

The 115 Hz output of the vibrator has another advantage for the car radio designer apart from that of transformer size. The output from the rectifier, of whatever type, pulses at 230 Hz and a much simpler HT smoothing arrangement is called for.

The efficiency of the vibrator pack is evidenced by the fact that the estimated HT wattage of a four- tubes (valves) + rectifier set is about low whilst that of the heaters is about 12W, a total of 22 W. The actual consumption of such a set from a 6 V or 12 V battery would be about 30 W total.

Negative or positive earthing

One terminal of a car battery is connected to the chassis of the vehicle (called ‘earth’, but not strictly so), which is used as a return path to avoid the need for two supply cables to every piece of electrical equipment. In the early days of motoring negative earthing was popular until the 1930s when informed scientific opinion declared that positive earthing gave a better sparking effect at the plugs and resulted in less chassis corrosion. It was almost universal until the mid-1960s, when for some reason negative earthing made a comeback. Is it only coincidence that cars now seem to corrode very easily? Be that as it may, the double changeover has caused much trouble for almost everyone concerned with car radios. From a technical point of view an ordinary non-synchronous vibrator pack should be indifferent to the type of earthing, but the synchronous type is definitely polarity-conscious. If one intended for use on a positive earth system were to be connected to a negative earth system the HT voltage too would be reversed in polarity with disastrous results for the smoothing capacitors. To guard against this it was usual to make the vibrator reversible to suit either polarity. All that needed to be done was to withdraw the vibrator from its socket, turn it through 1800 and plug it back in. To show which polarity was which, + and - signs were printed on the top of the vibrator to be lined up with an arrow pointer.

Constructional aspects

Sets built before the Second World War, especially the archetypal Philco models, used standard sized tubes (valves) such as the American UIX series, which called for considerable ingenuity in packing them into small cases. The convention at that time was to have the main ‘works’ in a floor-mounted box, with a control head on or near the steering column carrying the tuning, wavechange and volume controls. They all acted in a purely mechanical manner, the two units being interconnected with heavy-duty cable drives. The same system was revived after the war by a number of makers, but using the much smaller GT octal tubes (valves). Later two- unit types usually had the complete RF/IF section of the receiver in a dashboard mounting with an amplifier/output/power supply fitted remotely (possibly in the engine compartment) and inter connected with a multi-cored cable. There was sound reasoning (literally!) for installing the power supply, at least, outside the passenger area, since this virtually eliminated the characteristic buzz of the vibrator which could be annoying in an otherwise quiet car.

EMI built an excellent example of the genre for sale under the Radiomobile trademark in the late 1 940s, and this appeared in cars such as the Standard Vanguard, the Austin 16, the Jaguar 3 liter and the exotic Invicta Black Prince Byfleet Drophead Coupé. This receiver had the loudspeaker mounted in front of the main unit, slightly slanted down wards, whilst a subassembly carried the controls and dial even further forward. Two slim knobs flanked a narrow but attractive dial, above which were two rows of four pushbuttons each. Four were for preset station selection, the rest for wavechange (long/medium) and tone (speech/music). The circuit included an untuned RF amplifier but was otherwise conventional. The tube (valve) were of the Marconi/Osram 81 series, having bases similar to the American local. In standard basic form the output stage was contained in the main unit, but the power supply was in a separate case which could either be bolted directly to the other or, more usually, connected via a multi-way cable. There was an alternative output stage, employing two KT81s in push-pull, which could be added on to the main unit, in which case the original output tubes (valves) was modified to become a driver stage. A special 24 V model was made for use in coaches, and this had the option of a microphone input for sightseeing commentaries en route.

A later Radiomobile had a small dash unit containing an RF amplifier and a frequency changer only, the IF stages being in the main unit. This too could be had in single-ended or push-pull output form. It used the small M-OV ‘70’ range of B7G-based tubes (valves).

Permeability tuning

The general adoption of permeability tuning brought about a useful reduction in size of car radios, and in the 1950s it was possible to standardize the size of the hole needed in the car dash for mounting. It was also much easier to include a tuned RF amplifier in the specification, but few makers took advantage of this. One which did was Ekco, with the CR152 model. This has an ingenious manual/preset tuning system using three small combination knobs and dials connected to individual sets of RF and oscillator coils (the frequency-changer grid circuit was not tuned). One set covered long waves, 1150 to 185Dm, the other medium waves in two sections, 190 to 34Dm and 330 to 57Dm. The dials could be set on certain stations, selected by turning the wavechange switch or rotated at will. Incidentally, a loud speaker muting switch, ganged with the wave- change, was fitted to this model, and should be checked if the set appears to be ‘dead’.

Some unusual sets and features

Both Ekco and Pye produced car radios having a number of band-spread short-wave ranges. A separate dial was provided for each band, changed in unison with the wavechange switch.

The continental firm of Becker made some extraordinary radios featuring automatic, self- seeking tuning with AFC. When used manually, the permeability tuning was operated conventionally by a knob, but on ‘self-seek’ it was power driven by a clockwork mechanism. The spring for this was tensioned by a solenoid which was switched on automatically as the tuning reached the high frequency end of the dial. This threw the tuning instantly back to the LF end, ready for another traverse of the dial. Included in the clockwork was a nylon-bladed paddle wheel, similar to that used in a musical box, which revolved at high speed during tuning, and which could be braked by a catch moved into position by the action of a small relay. Because the paddle made so many turns relative to a small movement of the pointer, extremely accurate braking was assured. The relay was connected into the anode circuit of a double triode tubes (valves) controlled by the detector; as a station came on tune the relay closed, so quickly that the mechanism stopped at virtually the precise point. To overcome any slight error the AFC came into action in the same manner as described elsewhere in the text, by means of a reactance triode. A switch gave the choice of three levels of sensitivity, so that only very powerful, or those plus fairly strong, or all, stations would stop the tuning. The automatic tuning was brought into play by slight pressure on a hinged bar above the dial, but some models had provision for remote control when the set was installed in a chauffeur-driven car. The output and power supply sections were in a separate unit for engine compartment mounting. If the normal MW and LW coverage was not sufficient for the owner an optional multi-band band-spread SW adaptor was available. These sets were strictly the province of the wealthy car driver!

AM/FM automobile receivers

One or two AM/FM sets appeared from time to time, such as the Philips model X61V. It covered MW, LW and VHF, had pushbutton tuning for three AM and two FM stations, would work on either 6 V or 12 V systems of either polarity and incorporated a facility to enable the owner to use a mains-type Philishave electric razor from the HT line. ‘What it didn’t have, alas, was any great appeal to the public. The truth is that FM reception in cars met with even less success in the UK than the domestic variety, for, with the best will in the world, the performance on FM in BBC-only days was not consistent enough under varying driving conditions to make it really attractive even if the programs available had given any choice from those available on AM. Nevertheless, a set of this type is worth restoring now that there are far more local stations with transmitters sited closer to the areas where many people have to take their cars; and the adoption of slant polarization has made the traditional car aerial more suitable for FM reception. The only problem, as with all UK FM receivers in the tube (valve) era, is that the VHF coverage was restricted to between 88mHz and 100 mHz, rather frustrating for anyone wishing to receive Classic FM!