9 • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • Vibrators and vibrator receivers - 1 There are many vibrator powered radios that find their way into swap meets and auction marts. There is a very good reason why they are still to be found in such numbers: they were very practical, for many years... T HAS BEEN mentioned here before In this context a vibrator is an electro- in electric sets were likewise used because of that reticulated 240V AC power did not mechanical device, which when used with the convenience of a 6.0V heater supply. I extend beyond the cities and the major an appropriately designed transformer, In an all-battery powered radio, a bank of country towns of Australia until about the formed either a DC to DC converter or a three 45V dry batteries would last from four middle 1960s. By implication, until then DC to square wave inverter. The former to six months, depending on current drain and most farmers had to rely upon other sources were referred to as a `synchronous type' hours of use, while the 2V accumulator lasted of power — notably the 32V home lighting vibrator, while the latter were the `non-syn- about 6-8 weeks. Vibrators eliminated the plant and the 6V vibrator powered radio. chronous type'. As a rule for radio work, need for costly dry B batteries, and changing Although radios were produced for 32V the synchronous types supplied 130V DC to 6V avoided the need for special charging DC, six volt vibrator-powered radios were far or so from a 6V accumulator, as a substitute arrangements for the 2V accumulator. and away the more numerous, and were real- for the expensive B batteries. The one drawback was that the 6V accu- ly only superseded by the advent of reticulat- Six-volt accumulators were chosen for mulator only lasted only two to three weeks ed electricity and the emergence of the new three good reasons. Firstly, most cars up to between charging. However, as we have cheaper Japanese transistor radio, which the FJ Holden, in addition to farm machin- seen, charging the radio battery was essen- made the purchase of a new accumulator or ery, used a 6V system. Charging the battery tially at no cost, to the enterprising farmer! the repair of the faithful old set uneconomical. was merely a case of swapping the radio battery over with the car battery, and by the The basic principle Why 6V, and vibrators? time the `cocky' had made a couple of trips Now let's have a look at the way vibrators Firstly, now is as good a time as any to lay to to town, the battery was charged. work. Fig.1(a) shows the circuit of a com- rest an unfortunate misconception. The term Secondly, in the case of the 2.0V series of mon electric bell or buzzer. The make-and- `vibrator' was a wholesome and proper part of valves, it was possible to design a very con- break action of the buzzer spring (hereafter a radio man's terminology and was freely dis- venient series-parallel network for supplying called the `reed') and the adjusting screw, cussed by Catholic and Protestant radio men the valve filaments. And thirdly, in the case of causes a pulsating or interrupted DC to flow alike. There was no hint of unseemliness. car radios, the same valves which were used through the armature coil. (TOUCHING ARMATURE R MAGNET Fig.1: In (a) we see the principle of an electromechanical buzzer, where an electromagnet interrupts its own current. In (b) we see how a vibrator is essentially just the buzzer in a different physical form, while (c) shows the first practical attempt at using a half-wave non-synchronous vibrator to generate the HT in early auto receivers. 72 ELECTRONICS Australia, May 1999 ••••••••••••••••• ••••••• •• tio ••••••... by Roger Johnson Fig.2: A closeup of a vibrator cartridge removed from its protective shielding can. The array of adjustments is quite complex, to be discussed next time. If a secondary winding was wound Following on from this was an even worse around the armature coil, a voltage would contraption designed to eliminate the rectifi- be induced in the secondary as a result of er valve. A second set of contacts was placed the collapsing magnetic field when the reed on the reed — electrically quite separate was in the `break' or `off' period of the from the primaries of course — to enable cycle. The frequency of the vibrating reed rectification. This model developed the could be determined by the adjusting screw. same faults as the early non-synchronous The vibrator cartridge is simply a more type of Fig.1(c), but with worse results. The sophisticated and elaborate development of secondary contacts tended to arc and pit this simple buzzer principle. even more so than the primary contacts. Fig.1(b) shows a modified arrangement. Fuses and secondary delay relays were This time the armature coil is at the free sometimes incorporated to give the sec- end of the reed, so that the reed can more ondary contacts that fraction of a second easily oscillate, and with a greater ampli- delay, in order to get the reed moving tude. Not only that, the primary contact has before the load was placed through the sec- a certain resilience. This means that it can ondary contacts. accommodate variations of the amplitude of the reed vibration, so that the 'gap' Parallel coil vibrators remains more constant. This is important A big improvement came with the full wave for reasons that will become apparent soon. parallel coil vibrator, of the type seen in The system of Fig.1(b) also allows the Fig.3(a) which happens to be a synchro- addition of extra contact points. nous (i.e. rectifying) type. This is the type in As you can see, Fig.1(b) is simply a common use from about 1936 until the end rearrangement of Fig.1(a). The chief of the vibrator-powered car radio (1958 or advantage is that it permits additional flex- thereabouts) and many a receiver was kept in ibly mounted contacts on each side of the service until the mid 1960s. Not only that, vibrating reed. These in turn can control they were extensively used in military other electrical circuits, independent of the receivers (in particular in WW2) where driving armature coil. much development took place in insulating, This is indeed what happens in a vibrator moisture proofing, sound proofing, and most cartridge. The close-up photo in Fig.2 shows adjustable resilient contact, in which the important of all, providing reliable contacts. a complicated array of contacts as used in a energising coil was in series with the pri- With this type of vibrator the armature synchronous, or rectifying, cartridge. mary of the transformer. Half-wave rectifi- coil is wound with many turns of fine wire Like the ignition points in a car engine, cation only was possible, and was achieved the contacts of a vibrator cartridge open using a conventional valve. and close literally millions of times during Since the armature or magnetising (inter- their service life. A vibrator cartridge oper- changeable words) coil was in series with ating at 115Hz will have over one million the transformer primary, this meant that the contact movements every 2-1/2 hours. total primary current was drawn through Special electrical and mechanical engineer- the armature coil. It therefore had to be of a ing must be undertaken if any sort of very low DC resistance. Not only that, the longevity is to be anticipated. The engi- DC resistance of the transformer, and the neers of the day aimed at 1500 to 2000 imbalance of the reflected load during hours of operation. operation, meant that the operation of the reed could easily be disturbed unless the The early attempts load conditions, transformer primary and The development of the ultimate `modern' armature coil were carefully designed, vibrator cartridge came as a result of the matched and maintained. demand for automobile receivers in the early Furthermore, if the `points' (synony- 1930s. In this instance, the vibrators were mous with `contacts') became in any way used as shown in Fig.1(c). Until then, what pitted of corroded, there was an excellent car radios there were had to be supplied by chance that the reed would not commence separate HT or `B' batteries (see, for exam- vibrations at all. This is a classic example ple, Vintage Radio in Electronics Australia of a case where `giving it a slight whack' for June 1998). may have fixed the problem, by jolting the These early attempts comprised a single reed into operation! ELECTRONICS Australia, May 1999 73 PR MARY CONTACTS :ECNDARY.:. CONTACTS PRI REED REED DRIVING REED, &CAN &CAN COIL, COIL, CAN OFF-CENTRE 6 PIN BASE ITH LIX :BASE. SMALL 7 PIN PIN FERROCART SIDE PINS REMOVED BASE OAK Fig.3: In (a) is shown the basic circuit for a synchronous (rectifying) vibrator supply, while (b) shows that for a non-synchronous vibrator, where a rectifier valve must be used. In (c) is shown how the uncommon split-reed synchronous vibrator was used. Representative vibrator cartridge base connections are also shown, for reference. to give a much higher DC resistance, suit- whilst the primary voltage is connected. ously be negative for the high tension. The able for parallel connection with one side Under these conditions, contact is made secondary centre tap is therefore positive. of the low resistance DC winding of the alternatively between the reed and the two When the reed travels to the top set of transformer primary.