The most popular vibrator in Australia was the MSP/Oak/AWA unit. It was exclusively used by numerous manufacturers including Ferris, Philips, Ferguson, and of course, AWA. In fact, it was used by all manufacturers except for Astor, Air Chief, and other Electronics Industries brands. The other Australian vibrator manufacturer was Van Ruyten, but theirs are high power 50 c/s units used for 240V AC inverters.
Patented in 1934 by Oak.
The Oak vibrator was first patented in the U.S. in 1934. Its main improvement over other vibrators was the series driven coil, having its own contact independent of the transformer switching contacts. In Australia, the design was manufactured by Manufacturers Special Products. They were the component manufacturing of AWA, named thus as a way to sell AWA components to competing manufacturers. Apart from types V5105 and V5124, it appears the Australian type numbers are not used by Oak in the U.S. and vice versa.
The design was patented by Oak in 1934. Adjacent photo shows
internal parts of a non synchronous vibrator. This particular type is a V4012.
The series drive design means the vibrator always starts, even if the main switching contacts are worn or the voltage is a bit low. Another advantage of the separate connection to the driving reed is that it is possible to operate the vibrator off a higher voltage simply by putting a resistor in series with the driving pin. For example, a V5105 which has a 6V coil can be run off 12V with a 27R 5W resistor in series. One model of Philips car radio actually had this facility built in so that the radio could be used in a 6 or 12V car simply by changing links under a panel. Furthermore, the entire power supply can be switched on and off by means of the driving coil alone, reducing the switch current considerably.
From notes in Radio Engineering, April 1935. Their prophecy of longevity turned out to be correct.
It is because of the prevalence of the
Oak design in Australia, that reliability problems which seem so common
in the U.S. where the shunt drive type is standard, did not exist here.
Out of my hundred odd items of vibrator power equipment it has never been necessary to actually replace a vibrator; and this is over more than 35 years.
So, fear not fellow Aussies; do not take what you read in overseas magazines and vintage radio forums as gospel, regarding vibrator reliability. Our series drive MSP units do not have those issues, instead lasting as long as any other electronic component. And, if you're into restoring Astor/Air Chief car radios, it's worth looking out for the MSP replacements for the original Ferrocart types. These are discussed further on.
Of course, it's important to note that unsuitable circuit design, loading, or defective buffer capacitors will ruin even the best vibrator. With the correct component selection and attention to power rating, an MSP Oak vibrator is completely reliable with an extremely long life - so long that it's really a non issue.
Unfortunately, vibrators often fall victim to the incessant 'tweaker' who is drawn to its moving parts like moths to a light. Oblivious to the precision construction and careful setting up in the factory , they twist the contacts this way and that, attempting to repair a defective unit. The teeth marks of long nose pliers can often be seen...
In actual fact, randomly "adjusting" a vibrator is one of the worst things one can do. One needs an oscilloscope to do it properly, for the timing and duty cycle cannot be accurately set otherwise. It's hard to get some people over the "it's just a buzzer" mentality, which incidentally is so prevalent in the Model T world with ignition coils.
Run into a resistive load, the vibrator primary contacts are adjusted thus. A tweaker who jumps in with a pair of pliers and no instruments cannot hope to duplicate this. Add secondary contacts and the mess can be imagined! Yet, they're the first to complain about the reliability of vibrators...
As with most vibrators taken out of storage after many years, the insulating film that builds up on the contacts needs to be cleaned off. This is usually the cause of a vibrator not providing output, whether it starts or not. The driving coil contact seldom is affected this way, because it is made of silver and does not tarnish. It is also normally closed, and thus nothing gets in between the contacts when not in use. The insulating film appears to be a decomposition by-product (sulphur is one possibility) of the sponge rubber used to support the vibrator stack and to provide acoustic insulation. The film can be physically cleaned off the contacts by means of 600 grade sandpaper or a fine blade such as an X-acto knife. However, care must be taken not to upset the contact spacing. That means not unscrewing the stack as well! For this reason, I prefer to electrically clean the contacts where the film build up is fairly minor. Current limited high voltage will break through the insulation and burn it off.
Another sensible aspect of design is that
service access for the MSP vibrator is easy. Unsolder the earth tag and
remove the circlip. The guts can then be slid out. Usually, it is not necessary
to open the vibrator unless it has been damaged, or after a long period
the contacts in a synchronous or dual interrupter type need to be retimed.
If the driving coil does not function it's either: 1) the wire broken off just near the coil, 2) the adjustment screw for the contact is a bit out, 3) dry joint in pin or at bottom of vibrator frame.
To electrically clean the contacts, the following test jig can be assembled:
A battery of appropriate voltage is used
to get the reed vibrating. While a bench power supply can be used, there
is a risk of inadvertent damage if it should somehow come into contact
with the high voltage. High voltage is obtained from a 240V isolating transformer
and current limited to 400mA by a 100W incandescent light bulb. This is
applied to each contact in turn until the contaminants burn off. Obviously,
once this happens the bulb lights up. Because of the vibrator frequency
not being related to the mains supply, the bulb will flicker once the contacts
make. This is quite normal.
It goes without saying that no part of the circuit, or the vibrator should be touched while the mains is connected. The can of the Oak/MSP vibrator is connected to the reed. Although the transformer provides isolation from the mains, there is still 240V between its secondary terminals.
Disclaimer: There are restorers who don't appreciate the dangers of a direct mains connection and will run this circuit without a transformer. No reponsibility is taken for undesirable results that occur.
Check all contacts
It's important to see that all contacts are functioning, because if the vibrator works in half wave, those contacts which are working will be damaged. Simply observing the transformer waveform will confirm this, but the output voltage is also revealing. A potential problem is someone who gets a vintage radio, turns it on and find that it appears to work. If the vibrator is working in half wave, the radio will indeed be working, deluding the owner into thinking all is well. However, damage will soon set in, and if the contacts are badly burned and the spring temper damaged, the long term reliability could be poor despite efforts to repair the vibrator.
MSP / Oak Vibrator Data
Non Synchronous types.
The basic vibrator, most commonly used in car radios and small inverters. Where DC output is required, a valve or other rectifier is required. Most commonly found are the V5105 and V5123 types. These were used in every AWA car radio, of the valve rectifier type, up until the last model in 1965. Note that with all the vibrators types listed (not just the non - synchronous types), the contact current rating goes down as input voltage increases. This is because arcing is more prone to occur with an increase in voltage.
These are fitted with secondary contacts for self rectification. Commonly used where it is undesirable to use a valve rectifier, such as where dictated by space or power consumption constraints. Some car radios did use synchronous types, but mostly they were used for accumulator powered domestic radios. Note that the timing of the primary and secondary contacts is not identical. The secondary contacts open and close slightly later than those of the primary. The 4 volt type might cause some curiosity. It had two popular applications. One was for portable camera flash units where a small two cell lead acid battery was used, but more commonly they were used for 6 volt accumulator powered radios. Here, the vibrator is connected across the 4 volt section, and the valve heaters across the remaining cell (2V). The idea was that by supplying the directly heated filaments from a separate cell, it was easier to keep vibrator interference out of the system.
Synchronous Split Reed types.
These are mostly used in domestic accumulator powered radios, because with directly heated battery valves an external bias supply is often required. As the valves filaments are also the cathode, it is not possible to use cathode bias in the same way as indirectly heated types.
It is also possible to configure a split reed vibrator so that it works as a full wave switch into a non centre tapped transformer primary. The Bland shaver inverter takes this approach. It is worth keeping this in mind where it is necessary to convert a 6V car radio to run on 12V. By changing the vibrator to a split reed type, it is possible to keep the original 6-0-6V transformer on 12V by not using the centre tap.
6-12V conversion with a Split Reed
Vintage cars having their 6V electrical system converted to 12V are a fact of life, as much as the practice is undesirable. When it comes to the radio, several options are available. A 6 volt radio can be simply run from the new 12V supply by means of a resistor. The resistor is selected so that with the battery 12V (i.e. not charging), the radio receives 6V as measured on an analog meter. Why analog? Because the current draw is not steady DC and the peculiar waveform is more likely to give a less accurate result with a digital meter. In practice the resistor value is somewhere around 2.5 ohms and dissipates a fair bit of heat; 30W is not uncommon. The exact resistor of course depends on the particular radio and has to be selected on test.
The disadvantage of this scheme is the wasted power and that the nominal 6V changes as the radio warms up. This may or may not be a problem in the particular situation.
The more efficient method is to convert the radio to 12V. For the valve heaters and dial lamp, these can be replaced with their 12V equivalent; e.g. 12X4, 12AQ5, 12BA6, etc. If this is not convenient, the existing valves can be rewired in a series parallel circuit. See here for further details.
The next thing is to deal with the vibrator and transformer. If possible, the transformer primary could be rewound with twice as many turns using thinner wire. Under the assumption that the vibrator is an MSP type, a 27R 5W resistor can be connected in series with the drive coil. Simply installing a series resistor to power either the transformer alone, or transformer and vibrator is not recommended. The problem is that until the valves warm up, there will be no load on the inverter, causing excessive voltage in the transformer secondary winding which can cause insulation failure, damage to the buffer capacitor and arcing at the rectifier socket. Where the whole radio is fed from a resistor this is less problematic as the valve heaters provide a constant portion of the load.
However, there is a much more ingenious way around the problem. It allows the original transformer to be used without modification.
How to use a split reed vibrator to enable a 6V vibrator transformer to operate on 12V. Note that the driving coil is not shown for simplicity.
Here, a split reed vibrator is connected
so it works as a DPDT reversing switch, applying +12V then -12V to the
full primary. In effect, the primary receives the same peak to peak voltage
that it did on 6V.
The centre tap is no longer used. The advantage is of course the regulation and efficiency is as per original, and the transformer needs no modification. The vibrator and its socket do have to be replaced however, but as these items have the same appearance as the originals, the radio will still look original. The Bland shaver inverter uses this technique.
Of course, if the radio used a synchronous vibrator for rectification in its original 6V form, then either a solid state or valve rectifier will have to be fitted, but this is likely to be preferable to replacing the transformer.
Dual Interrupter types.
These are commonly used in 240VAC inverters or high power amplifiers, radio transceivers, etc. They are constructed the same as synchronous types but with the contact timing adjusted so both sets open and close at the same time. The contacts can be connected in connected in parallel to increase current rating, or more effectively, be used to switch two separate transformer primaries, or two separate transformers with the secondaries in parallel. These latter two configurations provide better current sharing between the contacts. In reality, it is impossible to ensure both pairs of contacts open and close at exactly the same time over the life of the vibrator, so the current rating is not actually doubled as might be imagined. The Ferguson VT146 transformer as used in a number of Radio & Hobbies inverter projects is intended for this kind of vibrator and has two primaries when used on 6V.
Ferris car radios and inverters.
A type that seems to be unique to Ferris is the six pin based V4012 and V4006. These are simply the same as their 4 pin counterparts except for the base. Internally, the V4012 is the same as the V5123 and the V4006 is the same as the V5105. Why Ferris chose to use a 6 pin base is a mystery. Perhaps it's because the extra pins exert a firmer grip on the vibrator in the socket.
With the standardisation of pin connections, these types actually have the same pin connections as the synchronous and dual interrupter types, except there's no extra set of contacts. This means that one often finds dual interrupter or synchronous vibrators plugged in instead, even though the extra contacts are not being used. It's a worthwhile modification to simply link the other socket pins to bring the extra contacts into being in parallel. While this has limited merit with synchronous types due to the later closure of the secondary contacts, it means that although current rating won't be greatly increased, the secondary contact will take over if something goes wrong with the primary contact. No doubt radio servicemen found it convenient that any MSP six pin vibrator could be plugged into these sets without having to do any socket rewiring.
The reliability of Ferris car radios was partly due to their use of the MSP Oak vibrator.
Astor car radios.
Type V4010 and V4016 are interesting types. They also have a 6 pin base and are non synchronous. They are designed as a replacement for the shunt driven Ferrocarts used in Astor car radios. V4010 is 6 volt and replaces the Ferrocart PM237, and V4016 replaces the 12 volt PM238. These vibrators are immediately obvious because adjacent pairs of pins are strapped together on the underside.
Otherwise, they use the same components as the normal V5105 and V5123 series drive types, except that the drive coil takes its feed from one of the power contacts. The drive coil contact only serves as physical adjustment for the reed; it does not serve any electrical purpose.
Despite being shunt driven, the other characteristics of the Oak design such as reed and contact design, mean that these are still far more reliable than the Ferrocart types. They are in fact, an example of good shunt drive vibrator design - where such must be used. Although I have not done tests, it would appear necessary to check the buffer capacitor value when using an MSP to replace a Ferrocart in view of the PM237 and PM238 operating at 150c/s. On the lower frequency of 100c/s produced by the MSP, the buffer capacitor would probably need to be increased.
Prefixed by "A", these are painted black to enhance cool running. A typical type is AV5948. They are run for an ageing period prior to final adjustment. Mostly, they appear in mobile two way radio applications, such as the AWA Carphone.
Reverse polarity types.
Suffixed by "R"; for example AV5948R. These are synchronous types that have the secondary contact connections swapped over, so that the output polarity is reversed from normal. An application for this is where a radio might be transferred to a positive earth car from one which was negative earth, or vice versa. Instead of swapping over the transformer connections, a quicker alternative is to simply change the vibrator to the other type. They are also used where split reed synchronous power supplies are used in series. This is because the secondary centre tap of one transformer has to be positive (the normal situation), while that of the other has to be negative in order for the voltages to add.
On this note, the AV5948 is used with the AV5948R in one of the AWA Carphone power supplies to provide 150V on receive and 300V on transmit.
The final, rather obscure type, is one in which AC is applied to the driving coil. The vibrator functions as a switch, synchronised to the mains. So far, the only application I have seen such a type in is a mains operated ignition coil tester.
Type VAC5124 has a 6V AC driving coil.
Strange MSP Data.
In the January 2002 issue of the HRSA Radio Waves magazine are tables of data for Ferrocart and Oak vibrators.
It is a curious thing as to where this data was obtained because there are differences to that shown above. For instance, the 12V vibrators are rated at 2.5A input current, instead of 4A. The frequency for all types is shown as 115 c/s instead of 100 c/s (as per the above data, and what I have measured). And, the Ferrocart replacement types are shown as having the separate drive contact in use, although still shunt driven. Admittedly, while I have only closely examined one such type, it did not have the separate drive contact in use.
The assumption is that this data may be early and it was found that the 12v current ratings could be increased to 4A. I do vaguely recall something about revised ratings being issued. This doesn't answer the question about the difference in frequency which could actually be problematic because of buffer condenser values. The source of data is not known, but it is noted that the Radiotron Designer's Handbook mentions 115c/s being the standard frequency - however the information here was largely sourced from Mallory who did standardise on 115 c/s.
It is possible that whoever compiled this data assumed that because the RDH was an AWA publication, that therefore AWA vibrators would be 115c/s.
The data I have provided above is from genuine AWA literature that I physically own, and tallies up with my examination of the vibrators. Nevertheless, this other (older?) data is interesting.
Oak Vibrators in the UK.
The English company, Wright & Weaire was another establishment outside the U.S. making Oak vibrators. However, unlike the Australian MSP version, the Wright & Weaire version does not externally look like the original Oak, and the numbering system is totally different. Instead of V5123, for example, W&W will use NS/12. This is a more meaningful numbering system; "NS" being "Non Synchronous" and "12" being the operating voltage. The can is of a wider diameter, and the base is crimped in place. The can is aluminium instead of zinc. Looking at the cut away diagram above, we can see that the can contains much more rubber for acoustic insulation. The entire can is lined, whereas the original design is not insulated against noise except for at the top and bottom. Presumably the thicker zinc stops most of the noise transmission by itself, whereas thin aluminium is not so effective. It is unfortunate that the crimped can construction has been used, but internally these vibrators should be as good as any Oak design.
Unbranded vibrator with Oak mechanism, and Wright & Weaire vibrator. Side view of the vibrators. The W&W shows the Services number.
It was the opinion of some manufacturers
to seal the base because they thought it would prevent the entry of air
which supposedly caused contact oxidation when not in use. This was a fallacy,
because in my large collection of open based and sealed vibrators there
is absolutely proof of this. After 40+ years of disuse, the contacts in
the sealed units in most instances still need cleaning.
Other manufacturers admitted it was to discourage "tampering" with the adjustments.
Alarm bells rang when I saw this written
on the side of a V6606 dual interruptor vibrator:
It says " OK 12 Volt". Some clown had decided
to use a 6V vibrator on 12V because it appeared to work. I dreaded what
I'd find inside, and indeed the mechanism and sponge rubber was coated
in a black substance.
Prior to taking the photo I had cleaned it up, but the black deposit is still evident around the sponge rubber.
This kind of component abuse is always appalling, in this case almost ruining a perfectly good vibrator. Thankfully, the driving coil appears to have survived, although something seems to have exuded from it particularly at the top of the bobbin. It would appear that someone needed a 12V type but didn't have one, and discovered that a 6V type would work instead. Yes it will, but with excess driving coil current and the resultant overheating, the arcing at the driving coil contact, and of course the excess reed swing giving an extra hammering to the contacts. Who knows how long it would have taken to burn out the driving coil. If only a 27R resistor had been connected in series with it. It's rather reminiscent of people with vintage cars that feed 12V into 6V starter motors.
And yet, you can be sure whoever did this would be the first to complain about vibrators being unreliable or some such nonsense.