INTRODUCTION

This is an Australian Army Amenities receiver made by AWA (Amalgamated Wireless Australia Ltd) in 1945. It is a dual wave superheterodyne receiver, using 5 valves. It covers the Broadcast band and one Shortwave band. It operates from 6 volts DC using an internal vibrator power supply. It is contained in a sturdy pressed steel case with rounded corners, and is waterproof. There are 2 carrying handles, and a hinged speaker protection cover. The radio weighs 51 pounds, but when installed in its wooden carrying case, it weighs 95 pounds, and with the battery, weighs 145 pounds. The manual recommends a long wire aerial 50 to 100 feet in length, and an earth wire if necessary.

CONTROLS

The front panel is dominated by the speaker on the left hand side, and the large tuning dial on the right hand side. The power input cable comes into the radio through a grommet, and is located below the speaker. All the receiver lettering is screen printed onto a large plate behind the dial knob. At the bottom on the right are 4 knobs. The left hand knob is a VOLUME control. The next knob is the 2 position power switch labeled SWITCH OFF ON. The next knob is the 2 position band change switch and is labeled RANGE MW SW (Medium Wave and Short Wave). At the right hand side is the TONE control. In the middle of the frequency scale is the main tuning dial and this is 2 speed vernier knob, the rear section is 1:1 and the front section is 23:1. It has a plastic pointer. To the right of this are the Aerial and Earth terminals. There is a handle on each side.

FRONT VIEW

FRONT VIEW (speaker cover open)

DESIGN

The radio is a conventional superheterodyne with one IF (Intermediate Frequency) amplifier and one RF (Radio Frequency) amplifier. The aerial is connected to an untuned primary, the secondary of the transformer being tuned, which is then capacitor coupled to the grid of the RF amplifier. The valve is a 1M5G variable mu pentode amplifier, which has AVC (Automatic Volume Control) applied. It shares a common screen supply with the IF amplifier. The anode circuit consists of a transformer which has a tuned secondary, and this is capacitor coupled to the mixer grid. The mixer has AVC applied only for the MW band. There is a top coupling capacitor for the SW band in the grid circuit. The mixer is a 1C7G which has a 455 kHz IF transformer in the anode circuit. The oscillator is part of the mixer valve, and the coils are capacitive coupled to the oscillator grid, with the grid return resistor going to the filament pin. The oscillator anode supply is extensively decoupled, with 2 dropping resistors and capacitors, as well as an extra individual resistor for each band. The mixer valve base is also rubber mounted to reduce microphonics, vibrator and mechanical vibration. The IF amplifier uses a 1M5G pentode, and uses AVC. There is a neutralizing capacitor C31, which is actually 2 bent plates on top of the tuning capacitor.

TOP VIEW (showing piston trimmers, green back panel with patents sticker, valve bracket)

The detector and audio preamplifier is a 1K7G pentode double diode. One diode is the audio detector from the IF transformer secondary. The bottom of the IF transformer secondary, has an RC filter network going to the VOLUME control, which is then capacitor coupled to the grid. The other diode is the AVC rectifier and is capacitor coupled from the audio diode. There is RC coupling to the audio power amplifier. The output valve is a 1L5G pentode, with minus 3 volts bias on the grid. There is a little right angled bracket to hold the valve in its socket. The other valves have shields to hold them in. The output goes to a speaker transformer and a 7 inch speaker. There is a feedback potentiometer to the grid to provide TONE control. There is no BFO (Beat Frequency Oscillator) so the radio is not intended to receive morse code. The power supply uses a synchronous vibrator to provide both primary and secondary switching. It is a split reed type so that back bias can be developed. There is extensive filtering to reduce interference.

UNDERNEATH VIEW

PHYSICAL DESIGN

The radio is contained in a sturdy pressed steel case with rounded corners, is waterproof, and contains two dessicators inside. Each control has a dust cap and grommet on the shaft and a flexible coupling. The tuning shaft has a gear drive to the 3 gang tuning capacitor. Each nut and bolt, is either welded to the front panel, or has a grommet, to prevent moisture ingress. The front panel has a gasket around its edge. The speaker has a protective door and a strong grill. The door and grill can be removed by removing 4 nuts. This allows the speaker to be withdrawn, unplugged and replaced. Behind it is a large rubber boot, so that even with the speaker removed, the radio is still sealed. The rubber boot behind the speaker also allows the case to function as a speaker baffle and improve the sound. The outside of the case is painted grey wrinkle, and the inside of the radio has a grey painted chassis and a green painted underside. The inside of the case is painted green, as is the back of the front panel. It also has a patent list label attached. The vibrator supply is in a sealed metal box, mounted on rubber. The fuse is located between the front panel and the chassis and is a little difficult to access. All solder connections have a dab of blue paint applied by the inspector. The chassis could be from a domestic receiver, as there are several spare holes stamped in it. The mixer valve is rubber mounted to isolate it from vibration. The ON/OFF switch is interesting as it appears to be a domestic light switch, which is activated by a lever from the front panel. The dial scale is a flat plate, painted grey and screen printed with the knob legends.

TOP DETAIL (showing neutralising capacitor, gear drive, fuses, and dessicator)

CIRCUIT

Note that there are 6 circuit errors. On the schematic, wire joins are shown as dots where wires cross, but 5 dots are missing. The IF and RF screens should be connected to R11 and C33, and this not shown. The audio preamplifier grid resistor R16 appears to join the HT bypass capacitor C41, but they are actually connected to ground where the wire crosses the ground, and this is not shown. The vibrator HT output from L21 appears to go only to R19, but it should be connected to the HT bus where it first crosses it, and this is not shown. The back bias resistor R27 should be connected across C28 where is crosses the wires, and both connections are not shown. There is a bypass capacitor C61 that is across V4 filament, and this is omitted on the circuit, however, there is a note on the circuit diagram, saying that it is not shown. The circuit has the valve symbols drawn upside down, which was an AWA standard at the time (not because Australia is at the bottom of the world), and shows the valve pin connections with pin 1 at the top and the other pins shown in a clockwise manner. This makes it easy to see which pins are which, but difficult to see the internal wiring of the valve envelope. Also note that there is a pin missing on valves V1, V3, and V5. This practice was later changed, and valves were drawn conventionally, with the anode at the top.

RESTORATION

The outside of the radio was very grubby with flaking paint and rust. There was no alternative, but a strip and repaint. The knobs, dial, speaker, and front panel, were removed. The front was sanded, rust inhibited, undercoated and then sprayed grey wrinkle. The back of the font panel was masked as the green paint and patents sticker were in good condition. The handles were painted black. The case was also sanded, rust inhibited and painted grey wrinkle. Once again the green painted case inside was masked. The rubber shock mounts on the case bottom were re-attached afterwards.

The inside of the radio was generally very clean, with no rust. There was mould on the rubber, plastic, and capacitor cases. These were cleaned. One high voltage capacitor had burst so the residue was cleaned off. All 9 electrolytic capacitors were re-stuffed with new capacitors, so they look original. Four were high voltage (8 uF 500v) and 5 were low voltage (500 uF 16v).

The dial scale was rusty and hard to read. It was scanned to a computer file, and edited. The legend was enhanced and all rust and blotches were removed. The dial scale was then printed full size as a white decal. The actual metal scale was sanded and painted grey. The dial scale decal was applied, and then given a transparent protective coat of lacquer.

The rubber power cable was perished so it was replaced. The speaker was seized and full of dirt, as the felt cover was rotten. The voice coil area was cleaned up, the rotten felt and the dirt were removed. This was done by applying plenty of alcohol and a thin piece of plastic film worked in and around to remove the dirt. After several attempts, the voice coil was free to move without binding. A new felt cover was glued on to protect it. One side bracket was straightened because it was bent.

The vibrator supply was removed and opened. The vibrator was diss-assembled and the contacts cleaned. The internal high voltage electrolytic was re-stuffed with a new capacitor. The filter capacitor on the primary was shorted so it was replaced. The supply was tested and provided 200 volts positive HT. An oscilloscope showed the waveforms were good.

FRONT VIEW (showing switch and gear drive)

The radio uses 2 volt filament valves and they are connected in a series parallel string, across the 6 volt supply. I removed all the valves and tested them for continuity. The filament wiring was carefully checked, and found to be correct. The bypass capacitors were checked. Then 2 volts was applied, and the sockets checked for equal distribution of voltage. It was slowly increased, checking the whole time, until 6 volts was reached. All seemed well.

Testing was done with the vibrator disconnected. A bench supply was used to apply 200 volts DC. There was no sound from the radio. I checked around with an oscilloscope, and found the audio was working. There was positive voltage on the audio output grid, so the grid capacitor was replaced. Now there was negative 3 volts from the back bias, which was correct. The IF amplifiers were working, but there was nothing coming from the mixer. I checked all the voltages and replaced 2 capacitors but still nothing. I replaced the 1C7 mixer valve and suddenly it worked. I aligned the IF transformers and the gain improved. Upon connection of the vibrator supply, the load of the radio caused the high voltage to reduce to160 volts, and the radio stopped. I reconnected the bench supply, and reduced the HT voltage in steps, from 250, to 200 volts which was fine, but on further reduction, the radio stopped at 160 volts HT. At first I thought it was the mixer oscillator dropping out, but the oscilloscope showed me it would work down to 80 volts, and the frequency counter showed it varied only by 200 Hz. The IF worked down to 80 volts. It proved to be the audio preamplifier which was not performing properly. The 1 meg ohm screen resistor was measured at 12 meg. After this was replaced with a resistor of the same style, the whole radio now worked down to 80 volts HT.

The vibrator supply was reconnected, and the front panel and knobs put back on. An RF alignment was performed. This radio has only 3 coils, an aerial coil, RF coil, and oscillator coil. Each coil has the windings on it for the 2 bands, on the same former, and sealed in pitch. There would normally be a slug for the low frequency end and a trimmer capacitor for the high frequency end of each band. However, there was only one slug (I would expect 6!) and this was for the broadcast band oscillator. There were 6 trimmer capacitors. These are the brass rod piston type. With only one slug, the alignment was going to be a compromise. For a 10B signal to noise ratio on the broadcast band, I managed to get 6 microvolt sensitivity at 700 kHz, falling off to 25 microvolts at 1600 kHz. For the short wave band, I managed to get 3 microvolts at 10 mHz, and falling off to 15 microvolts at 6 mHz and 17 mHz.

OPERATION

The radio is easy to use, with just 4 controls. The sensitivity is reasonable but not wonderful. The reduction dial makes it simple to tune in a station. There is ample volume for the broadcast stations. Single sideband stations and morse transmissions cannot be tuned in easily. It performs well enough for its intended role. It would be possible to get far better performance, if the design included additional slugs, and separated coils. Even the use of elongated formers would be an improvement. So my assumption here is that the manufacturing tolerances were so good, the costs kept low, and the performance was satisfactory but not crucial. This is only an amenities set, so the premium components would have been reserved for the military grade radios. Hence the use of a domestic light switch.

ORIGINAL VIEWS

VIEW (front original)

VIEW (front original with speaker cover open)

VIEW (top original showing mould)

VIEW (underneath original showing mould)

DIAL (final artwork)

REFERENCES

C171020 Instruction Manual
Dial Scale Artwork

Copyright
Ray Robinson VK2NO

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