INTRODUCTION

The Electrosound looks like an amenities receiver used by the Armed Services. It has a rugged wooden case, with a steel front and back panel, using feet on the bottom and a leather carry handle. It can operate from 240 volts AC or 6 volts DC. It has a retractable aerial on a spool on the rear of the case, and RF (Radio Frequency) amplifier stage, but has no BFO (Beat Frequency Oscillator) so it is not intended to receive morse. This radio has octal and some miniature valves, so is post WW2 vintage, and an ARTS&P license sticker on the back, that would date it as 1955 to 1960. It has 3 frequency bands, the broadcast band (500 to 1600 kHz), and 2 short wave bands (2.5 to 7.5 mHz, and 7.5 to 22 mHz).

An "Amenities" receiver is intended to provide radio news and music to off duty serviceman. In the USA it was known as an "Entertainment" receiver, or called a "Troop Morale" receiver. In the UK it was known as a "NAFFI" receiver because it was placed in the canteen, and also known as an "Invasion" receiver because that is where you would hear of the invasion landings. This type of receiver is normally a ruggedised version of a domestic receiver, with the additional ability to operate from several voltages, and in hostile climatic environments.

Front View

CONTROLS

The controls are very simple. There are only 5 knobs. The left hand knob is the TONE control, the next knob is the VOLUME control and ON/OFF switch, the centre knob is a spring loaded DIAL light ON/OFF switch, the next knob is the station TUNING knob, and the right hand knob is the BAND change switch. When the band is changed, a pointer moves up the right hand side of the dial to indicate which band is selected. This mechanisim is fairly crude. There are 2 lifting handles. On the left hand side is the speaker, and the dial is on the right hand side. The power cord comes out the back. Also on the back is an octal socket, containing a plug, that is jumpered for 240 volts AC. For 6 volt DC operation, a different octal plug with different jumpering is installed, and this has 2 wires coming out for connection to a 6 volt battery. On the back is a reel containing wire, and this can be unwound and used as the aerial.

DESIGN

The case is made from plywood, has a recessed leather handle on the top, and rubber feet on the bottom. There was a protective front and back cover, but these are missing from this example. The mains cord plugs into the rear panel, and can be protected when the rear cover is on. The radio itself has a conventional steel chassis, with the 2 octal valves mounted in the normal fashion. The other valves are miniature, but are mounted on a plate rivited in an octal valve hole. This suggests that the chassis was originally intended for all octal valves. The RF tuning section is a separate assembly that can be removed. It contains the mixer, tuning coils, and RF amplifier. There is a separate closed box that containing the vibrator power supply, and it has a plug and a flying lead so that the supply can be easily changed. The radio has a 240 V AC mains transformer. There is a 6X5 rectifier, with a capacitor and choke HT filter. There is a jumpered octal plug for AC operation. There is another plug that allows DC operation.

The audio output valve is an EL32 pentode which drives a large internal speaker. There is a TONE control circuit on the grid, with some plate feedback. The audio pre-amplifier, detector, and AGC (Automatic Gain Control) rectifier is a 6N8 pentode double diode. The single IF (Intermediate Frequency) amplifier is a 6BH5 pentode, and the IF transformers are slug tuned with fixed capacitors. The mixer/oscillator is a 6AN7 triode/heptode. The RF amplifier is a 6BH5. There is a spark arrestor on the aerial lead in. The receiver has AGC and this is applied to the mixer, RF amplifier, and the IF amplifier.

Top View

The radio uses nickel plated brass round head and cheese head screws on the front panel, and uses lock washers. One of the speaker bolts is too long and has been crudely cut off. The capacitors are mica, electrolytic, and paper types. The resistors are mainly the striped type, but there are a few that are the old body/tip/dot style. It uses plastic wire. The front panel is steel, and painted grey wrinkle. There is a thin plastic sheet under the knobs that carries the knob legends. This is buckled and yellowing, with age.

Back View

ARTS&P Detail

RESTORATION

When I acquired the radio, it was described to me as working very well. It worked straight away without any smoke or other problems. I ran it for some time, while checking the filter capacitors for heating. Upon examination, it showed that 2 capacitors had been replaced with modern types. I removed these and fitted some of the same style, that have been stuffed with new capacitors, so that the underside looks original. Some solder joints and the wiring was tidied up. I had no circuit, so I traced it out.

Spark Arrestor on Aerial

Dial Pointer Mechanism

I performed an alignment, the 455 kHz IF sensitivity improved, so that the injection level into the mixer grid was 50 microvolts. The oscillator and dial pointer were adjusted until the frequency readout was correct. The other coils were peaked, and the radio now has improved sensitivity (for a 10 dB S/N)……..
Broadcast band 500 kHz = 4 uV, 1500kHz = 8 uV
Shortwave band 3 mHz = 7 uV, 7 mHz = 6 uV
Shortwave band 8 mHz = 12 uV, 20 mHz = 14 uV

The wooden case had the thick white paint scraped off, then sanded, then painted the original black, as determined by the paint on the inside. The painting was sloppy, the front had been splashed with white paint, so it was re-done with grey wrinkle paint. It came up very nicely. The white paint was scraped from the handles, the dial glass and the piece of plastic under the knobs, that carries the knob legends. The chassis was cleaned, and the knobs polished.

Underneath View

CONCLUSION

The radio performs well for a domestic receiver, considering it only has one IF stage and one RF stage. It is easy to use, and fairly sensitive. The large speaker gives a good sound.

Circuit

Copyright
Ray Robinson VK2NO

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