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VINTAGE RADIO By RODNEY CHAMPNESS, VK3UG The 32V 5-valve Operatic Mignon Valve radios designed for remote country areas were quite different from their city cousins. They had to have far more sensitivity to cope with weak signals and also ran from 32V battery lighting system supplies. And just to make it even more difficult, they had to cope with the interference from the vibra­tor used to generate the HT. As the majority of the vintage radio enthusiasts are city-based, many have not had the opportunity of seeing and being involved with sets specifically designed to operate in remote rural locations. They are quite different, as can be appreciated when all of the conditions that these sets had to work under are taken into consideration. In the 1950s, commercial AM broadcast stations had an aerial power of 2kW in country areas and 5kW in the city. Nation­al stations were up to 10kW and many are now 50kW. In most country areas, there were usually only two or three stations within a radius of perhaps 160km. If you wanted to hear more, it was necessary to have a large outside aerial and an earth – and a set that was quite sensitive. I lived near Bordertown in South Australia and the only stations of good strength were 3WV with 10kW 110km away at Dooen, 3LK (3WM) with 2kW 130km away at Lubeck, and 3SH with 2kW 250km away at Swan Hill. The typical mains-operated 4-valve set was totally inade­quate and the average 5-valve (including rectifier) mains receiv­er was still struggling to do a satisfactory job. To get the necessary sensitivity, 5-valve sets were used in rural areas. They worked from 32V DC (ie, battery-powered home lighting sys­tems), so no rectifier was need­ed and all five valves were amplifiers. Typically, these sets had an RF stage, converter, an IF stage, detector and first audio amplifier, and an audio output stage. In normal sets, AC mains operation was quite easily achieved using a trans­former with windings to supply the various voltages required for the valves and a vacuum tube rectifier to convert the AC to DC. On 32V DC sets, things were nowhere this easy. Nominally, the receiv­er could be like its AC mains brother but to supply the high voltage (HT) for the valves, it was necessary to use a vibrator power supply. Vibrator supplies The 32V Operatic Mignon, made in South Australia, was really equivalent to a 6-valve mains receiver since it did not use a rectifier. Its RF stage was highly desirable since it was often used in remote rural areas. 86  Silicon Chip A vibrator a is solenoid-driven mechanical switch which opens and closes its contacts at between 100 and 150 Hertz (depending on manufacturer). The pulsating DC is applied to a transformer specifically designed to The Operatic Mignon used a vibrator to derive the HT from the 32V DC input. As with all vibrator sets, there is a trap in that if the 32V supply is reversed, all the electrolytics on the HT rail will be damaged. In fact, a series silicon diode (rated at 3A) would be good insurance. be used with a vibrator. The transformer steps this up in its secondary where a much higher square wave AC voltage is developed. A second set of contacts on the vibrator “rectify” the secondary voltage to produce the HT (high tension) for the plates and screens of the valves. This sort of vibrator with two sets of contacts is said to be “synchronous” because the contacts work in unison. Asynchronous vibrators have one set of contacts, to switch the primary current, and the secondary voltage is recti­fied by a valve rectifier. One trap with vibrator supplies is that if the 32V supply is reversed, the electrolytic capacitors get charged up with the wrong polarity and the set won’t work. It doesn’t do the electro­ lytics much good either! The transformer must be “tuned” using “buffer” capacitors so that there is minimal sparking at the vibrator points, otherwise the vibrator will have a very short life. Even with buffer capacitors, there is still some sparking at the vibrator contacts (also called “points”) and radio interference is produced. This interference would wipe out all radio reception if it were not dealt with. Typically, the whole vibrator power supply is shielded, as can be seen in the Operatic. Some sets had double-shielded sup­plies and also single point earthing was commonly used to prevent interference currents circulating around the receiver chassis. Some areas are very remote from radios stations which means that daytime radio reception on the medium wave broadcast band is virtually non-existent. At night, many stations are heard but suffer from selective fading and often there is more than one sta­tion on the same frequency. So that day-time reception could be achieved, at least one shortwave band was installed. This allowed the domestic shortwave stations to make up for the lack of medium-frequency reception during daylight hours. These receivers also had to work off a very variable power supply, which could be as low as 28V and as high as 40V. Sometimes extra cells were added to the 32V bank of batteries to make up for voltage drop in the cables and the voltage could reach 45V. Some sets had a 3-position power switch marked “Off”, “Charge” and “On”. In the “Charge” position, a resistor was placed in series with the supply to reduce it to around 32V when the batteries were being charged. As can be understood, remote country listeners really had it tough in regard to getting reasonable radio reception. The radio set designers had quite a task to design suitable receivers for these remote locations. That they succeeded can be seen in the Operatic Mignon and sets produced by other manufacturers. The Operatic Mignon RF A12 Bland Radio of Adelaide may not be a manufacturer known to many but the Operatic brand name was well known in South Austra­lia and Western Victoria for many years. In country areas, their 32V radios gained a reputation, over several decades, as reliable and sensitive receivers that were well-suited to rural conditions. The Mignon was quite a standard set with nothing unusual in its appearance. It was a good solid brown Bakelite set of 1951 vintage. It is of January 2001  87 moved, then two screws, one at either end of the cabinet are removed and the chassis is just slid out with the dial and all the works attached. I wish all receivers were as convenient as this to disassemble. The chassis can be tipped onto its end where the vibrator box is located or even tipped upside down with no damage to components. A view underneath the chassis shows that it is not unduly cluttered, despite being dual wave and having an RF stage. Vibrator supply & series heater wiring The metal box on the righthand side of the chassis is the shielded vibrator supply. This shielding was crucial in minimis­ing interference to fringe area reception. average size, has the usual slide type dial, and four controls to operate the set. Yes, the wrong knobs are on this set, as it was purchased without them. I’m on the lookout for the right knobs. A view of the back of the receiver shows a metal box on the right which is the shielded vibrator power supply. To the far left is the 3-gang tuning capacitor. The set uses a 6N8 RF stage, 6AN7 converter, 6N8 455kHz IF stage, a 6BD7 detector/AGC and first audio stage, followed by a 6AQ5 audio output. The Mignon is very easy to remove from its case. The four knobs are re- How to Power A 32V Radio Power supplies that put out 32V at an amp or so are quite scarce. However, it is possible to build quite a simple supply that will easily power this and other sets. The following parts are needed: a transformer with a 24V <at> 2A secondary, a bridge rectifier rated at 100 PIV or higher and a current rating of greater than 2A, a 4700µF 50VW electrolytic capacitor and two .01µF 200V greencap or polyester capacitors, plus any necessary mounting hardware and cabling. This supply will comfortably provide up to 1.5A at around 32V DC. 88  Silicon Chip The vibrator power supply has been found to be quite reli­able. Some vibrator supplies are extremely reliable, rarely, if ever, needing a replacement vibrator while others need a new one relatively frequently. If you do replace a vibrator, it is a wise policy to replace the buffer capacitors as a matter of course. In this vibrator power supply, the buffer components are the .004µF capacitor and 10kΩ resistor in series and the 0.5µF capacitor – all these being connected to the vibrator trans­ former in the lower right of the circuit diagram. The voltage ratings of these capacitors must be strictly adhered to as must their capacitance values. The voltage rating on the .004µF capacitor may be as high as 2kV working. WES Components in Ashfield, NSW have suitable capacitors, which are normally used in TV receivers. A value of .0039µF is near enough to .004µF but a 0.47µF capacitor should have a .027µF capacitor placed in parallel with it to nearly equal 0.5µF. This is a 32V DC receiver but the vibrator is rated at 24V. However, this only applies to the reed drive of the vibrator and a 100Ω resistor is used to drop the voltage down from 32V to 24V. Throughout the receiver it can be seen that cathode bias is used, instead of the more popular “back bias”. With most vibrator sets, it is not possible to separate the low tension and the high tension circuits and they have a common negative which goes to chassis or earth. As a result back bias cannot be used. However, it does mean that it is quite practical to measure the current drain of each valve by checking the cathode to earth/chassis voltage. Note that the valve heaters are in series across the 32V supply and there are also resistors across some heaters. ELECTRONIC VALVE & TUBE COMPANY The Electronic Valve & Tube Company (EVATCO) stocks a large range of valves for vintage radio, amateur radio, industrial and small transmitting use. Major current brands such as SOV-TEK and SVETLANA are always stocked and we can supply some rare NOS (New - Old stock) brands such as Mullard, Telefunken, RCA and Philips. Hard to get high-voltage electrolytic capacitors and valve sockets are also available together with a wide range of books covering valve specifications, design and/or modification of valve audio amplifiers. PO Box 487 Drysdale, Victoria 3222. Tel: (03) 5257 2297; Fax: (03) 5257 1773 Mob: 0417 143 167; email: evatco<at>mira.net New premises at: 76 Bluff Road, St Leonards, Vic 3223 The under-chassis wiring is relatively uncluttered. Quite a few of the old paper capacitors were replaced with polyester or metallised polyester types. The resistors are there to balance the voltages across each valve. The 6BD7 and the 6AN7 only draw 0.23A of heater current and this is padded out to 0.3A by the 175Ω resistor. If a 6AN7A was to be used as a replacement, the heater equalising resistor would need to be changed so that it was only across the 6BD7 and be reduced to 90Ω. Likewise the 150Ω resistor bleeds off 0.15A so that the 6AQ5 gets the right current through it (0.45A) and the other valves get 0.3A through their supply line. The dial lamps are fed off their own series 62Ω resistor. Getting it up & running As has been said, the Mignon is a quite conventional re­ceiver designed for use on 32V DC. All of the usual critical capacitors were replaced. It is quite important before turning the set on to make sure that the negative line of the supply goes to the chassis. If it is positive to chassis the HT voltage will be reversed. The set was then tried out – it was rather sick, with the high tension (HT) relatively low. No shorts were found on the HT line so the vibrator was thought to be the culprit for the lack of voltage. The vibrator was removed from the power supply and the mechanism itself removed from its case. To do this, it was necessary to desolder the small lug on the side of the base. The next step was to remove the circlip inside the bottom of the base using a screwdriver and then slip the vibrator out of the case. Vibrators aren’t easy to come by so I decided to clean up the points. This was done by running a points file between each set of points until they appeared reasonably smooth. Fine wet and dry paper was then used to polish the contacts. During this process, the points were closed together under slight pressure to help the polishing action. January 2001  89 vibrators. If you have any vibrator set, I recommend that you always replace the buffer capacitors, except where they are mica and test OK. A general check-up The vibrator was removed from its metal case so that its con­tacts could be cleaned up with an automotive points file. There are a total of five gaps to clean in these synchro­nous vibrators. I checked that the points were reasonably smooth and shiny, by using a magnifying headset. If the points are very pitted, it will not be possible to get them into first class condition. Be careful not to bend the points out of position if you decide to overhaul a vibrator. This vibrator had obviously had a long and hard life, as there was quite a bit of black around the insides, and still is. The foam rubber buffers and the insulated rubber sleeves on the leads to the vibrator plug had all disintegrated. I used contact adhesive to glue some thin rubber strap to either side of the top of the vibrator to act as a buffer so that it wouldn’t bang against the side of the mounting can and make a noise when it was operating. It was not practical to re-sleeve the braid wires coming into the vibrator so 10mm plastic tubing was cut and placed so that all the flexible braid leads were kept apart. This work can be seen in the photograph. A vibrator in poor condition will not provide as much output voltage as a new one but since the receiver is unlikely to be used much, a slightly 90  Silicon Chip dodgy vibrator is not worth replacing. The vibrator in this set isn’t 100% but is still quite adequate. Another problem that sometimes occurs with vibrators is that they vibrate well but there is no output from the supply. I have found some that haven’t been used for years develop an insulating film on the contact points, hence the contacts never make electrical contact with one another. The exception is the reed drive circuit, so a thorough clean even of new vibrators is needed, if a fault like this shows up. As stated earlier, the buffer capacitors are critical to the long life of This circuit shows how the pick­up (crystal cartridge) connec­tions should have been wired, to avoid hearing the radio program when listening to records. Now that the supply was producing a voltage somewhere near the 160 volts expected the receiver started to perform. The IF stages were aligned and no problems were found, with all the adjustments being close. This was done by forcing a strong signal through the set with the signal generator on 455kHz attached to the aerial terminal, the receiver on the broadcast band and the gangs closed. By doing it this way, test instruments do not interfere with the tuning of the IF channel. The tuning peak can be located by placing a digital multimeter (with 10MΩ input resistance) across the volume control and tuning for maximum voltage. By the way, there is an error in the circuit diagram around the pick-up terminals; it won’t work without radio programs also coming through loud and clear along with the record you are playing. The correct portion of this circuit is shown separately. It just goes to show that draughtsmen and proof readers didn’t always get this correct. A liberty was also taken in the way that the IF transformers were drawn in that the resonating capacitors for each winding were omitted. It was always assumed that anyone reading the circuit would know this. The tuning of the RF sections is a relatively complex task which we don’t have space to cover here. Suffice to say that the stages all peaked nicely and the set performed well. Summary A receiver with an RF stage is always desirable; the extra stage of radio frequency amplification really does make a dif­ference on the broadcast band as well as on shortwave. The Oper­atic Mignon is no exception. It is sensitive, has a good delayed AGC system and a moderate audio output level. Operatic receivers do not have any whiz bang circuitry or anything that appears exotic but they work well and just keep on going. They are part of our rural radio heritage. I’m pleased SC to have it in my collection.