The A.J.S. Symphony Seven Receiver - Inside Out

Background
 By 1926 radios were becoming simpler to use with fewer controls, and valves had a longer life, making it no longer necessary to mount them on the outside for easy replacement. A.J.S. radio sales at the time were falling and their receivers were starting to look old-fashioned. The decision was taken to develop a new range of modern looking receivers under the A.J.S. Symphony name in the hope that sales would return to their previous high levels.

Several models were produced including simple two and three valve T.R.F. receivers, a portable 5 valve T.R.F. receiver, a 5 valve superhet and the top of the range model the Symphony Seven, a seven valve superhet with an external frame aerial. This was one of the first commercially produced British superhets and was several years ahead of its time. This type of receiver wasn't widely adopted in the UK until the early 1930s. The Superhet was invented in Paris in 1918 by American engineer Edwin H. Armstrong. He sold the design to R.C.A. and the company's first superhet, the RCA Radiola was launched in America in February 1924.

Superhets easily outperformed the relatively simple receivers of the day, which needed an extremely long aerial to pick-up distant broadcasts. The Symphony Seven could receive weak signals without the use of such an aerial. The receiver described here was recently restored and is comparable in sensitivity and selectivity to many modern transistor portables. The audio output is high for the time as the amplifier can deliver several hundred milliwatts without noticeable distortion.


An A.J.S. Symphony Seven.
A super heterodyne, or superhet for short, is the still the type of receiver that we use today. It has few tuning controls and is highly stable, sensitive and selective. The performance of the tuned radio frequency receivers, or T.R.F. for short, that were commonly in use until the 1930s was far inferior to these early superhets.


An advert from 1926.

 Harry Stevens' design performed as well as anything else in the UK at the time and far better than the majority of the competition. It should have taken the industry by storm and been a landmark product.

Sadly this never happened, presumably because of the extremely high selling price of £67.10s.0d. Unfortunately none of the company's records have survived and so it's not known how many were made. The number was probably quite small.
Circuit Diagram


I would like to thank Graham James Richardson for the photos that enabled me to complete the diagram.

The circuit is extremely interesting as it shows one of the earliest UK superhets. A separate oscillator is employed with direct injection into the mixer via the centre tap on the aerial. Separate plug-in frame aerials were used for Long Wave and Short Wave (Short Wave is in fact what we now call Medium Wave). The IF frequency is 260Khz and the gain of the IF amplifier, like on all early superhets is carefully controlled to eliminate instability. Early IF transformers were not screened from each other and valves at this time had a relatively high internal capacitance.

The oscillator and aerial are tuned separately and so there are two tuning capacitors. A conventional leaky grid detector is used and the three audio stages are choke coupled. The final stage is powered from a 132 volt supply to provide a loud, undistorted output. The original valves had 6 volt filaments and were from the "Six-Sixty" range that was sold by The Electron Company Limited of London. They are impossible to find these days and so Mullard equivalents are used instead.
Circuit Description
L1 is a plug-in balanced frame aerial that is tuned by Cl. The output from the oscillator is injected into the mixer via the aerial's centre tap. The difference between the oscillator and aerial frequency is 260Khz, which is of course the I.F. frequency. The I.F. transformer has an un-tuned primary winding L2, coupled to tuned secondary circuit L3, C4.
Positive feedback is applied to the mixer by C2 (labelled Volume 1 on the front panel) and the centre tapped aerial coil. This increases the Q of the aerial tuned circuit, so increasing the gain and the selectivity. This is an early form of what became known as a Q multiplier.

The oscillator uses a strange arrangement when compared with more modern techniques. Anode winding L5 is tuned by C3 and feedback is provided by L4. Both coils are tapped for Short Wave and the output is developed across coupling winding L6 and fed into the mixer. The receiver operates over the following frequency range:

Long Wave: 150Khz to 346Khz
Short Wave: 376Khz to 1.192Mhz
The two intermediate frequency (I.F.) amplifiers are coupled via transformers T1, T2, and T3. Each transformer has an un-tuned primary winding and a tuned secondary winding.

The transformers are unscreened and so if the gain was not carefully controlled the amplifier would be unstable.

To prevent instability the amplifier is powered from just 24 volts and the gain can be adjusted by R1 (labelled Volume 2 on the front panel).

The detector is a conventional leaky grid detector, RC coupled from the secondary of the final I.F. transformer. C2 filters out any remaining carrier wave and the audio signal is developed across the primary winding of audio coupling transformer T1.

Audio 2 amplifies the signal from T1 secondary, and -1.5 volts bias is supplied from the grid bias battery. The output is developed across the primary winding of audio coupling transformer T2. C3 flattens the amplifiers' frequency response by filtering out any unwanted high frequencies developed across T2 primary.

Audio 3 amplifies the signal from T2 secondary and drives the loudspeaker. Minus 7.5 volts bias is applied from the grid bias battery and the loudspeaker is connected directly to the PM256 anode and supplied with 132 volts from the HT battery. C4 filters out some of the high frequency components giving a more mellow sound.


The audio sections of the receiver showing the two audio coupling transformers. Courtesy of Graham James Richardson
The construction is very advanced for the time. The chassis is modular, each receiver containing the appropriate modules, some of which were used in several different models. Each module is simply held in place by two screws.

The large coil is the oscillator coil with the wave change switch inside. It seems that most of the components including the switches, tuning capacitors, I.F. transformers and audio coupling chokes were made in the works. Possibly only the valves, knobs, and slow-motion drives for tuning were brought-in.

The valve holders have spring-loaded contacts and seem to be more reliable than earlier A.J.S. types.
A close-up view showing the later type of  A.J.S. tuning capacitor, with the oscillator coil on the left.

The modules are connected to each other via solder tags that can be seen along the front edge.
This view from the left-hand side of the receiver clearly shows the modules and the way they are connected together. There is also a row of bus bars that run along the front edge of the chassis.

The chassis varies in length in different models. This particular receiver is the most complex model that A.J.S. produced and so the chassis is quite long. The much simpler two and three valve radios have fewer modules, as does the 5 valve superhet.

Modules do not appear to have been used in the Symphony portable receiver, presumably because of size and weight.
A close-up view showing two of the I.F. transformers. In those days such transformers were not screened, which seems very strange today.

Two of the bus bars can be seen in the background.
Another view of an I.F. transformer. The two windings are at the bottom and the tuning capacitor is clearly seen at the top.

In the bottom right-hand corner is one of the sprung valve holder contacts. These appear to work extremely well and are superior to many of the commercially available valve holders of the day.

Again some of the bus bars can be seen in the background.
The front panel. On the extreme left is the aerial tuning knob with the oscillator tuning on the right. The two lower knobs are the gain controls.

The knob on the extreme right is the wavechange switch.
A front view of the receiver with all of the doors closed. The batteries are held in the lower compartment.

On the far side of the case is a quarter inch jack socket for the loudspeaker. The socket includes a pair of contacts that are wired in series with the accumulator. They act as the on/off switch. When the plug is inserted the radio is switched on.
Another view of the receiver with the top door open to reveal the control panel.
If anyone has one of these receivers, or any information about them, or photographs please contact the webmaster.
 
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