INTRODUCTION
The WS No.108 is a backpack radio covering the HF band. It can be carried by one man, and used while walking. It can provide R/T voice communication, and send 450 milli watts of AM (Amplitude Modulation). It has 3 dry batteries, 1.5 volts DC for the filaments, and 90 volts DC for the HT (High Tension). The HT battery is two 45 volt batteries in series. It covers the frequency range from 8.5 Mhz to 8.9 Mhz and has an IF frequency of 455 Khz. The radio is 9.5 inches wide, 11.75 inches high, and 9.5 inches deep. It weighs 26.25 pounds. It was made by the Radio Corporation Pty. Ltd. in Melbourne in 1941.
Picture1: WS No.108 Front
The radio can be operated while being worn on the operators back. There is a TRANSMIT and RECEIVE switch, which is operated by pulling on 2 leather laces. There are 3 jacks on the side of the case. Two jacks are for headphones, and one jack is for the microphone. The plugs have a different depth shank, so they cannot be plugged into the wrong jack.
The aerial is a telescopic whip type, and can be extended to 6 feet. It is normally operated at a 3 foot length. The aerial is mounted at the front of the case, at a 45 degrees angle. This is suitable for operation while walking, or operating while lying prone. The aerial socket has a terminal on the bottom to allow the connection of extra wire (if required to extend the range), with up to 15 feet extra.
The receiver is continuously tunable. The transmitter is continuously tunable, or it can be switched to 4 separate preset channels. The transmitter and receiver are independent, and can be use on the same frequency, with the NETTING function, or with the transmitter and receiver on different frequencies.
CONTROLS
The controls are on the front panel. At the right is the receiver tuning dial, the volume control (VOL), and the nameplate. Above the nameplate is a thumbwheel for fine tuning of the receive frequency. At the left is the transmitter frequency control (which has a reduction drive, and a dial lock) and the aerial tuning control (labeled ANT). There is a small plate on the left hand side that can be removed. Behind this are 4 ceramic trimmers, which can be screwdriver set, to allow 4 transmit frequencies (A, B, C, and D). In the centre is a 10 milliamp meter that shows the power amplifier HT current. Below this is a switch that can select the 4 preset transmit channels, or the DIAL, for setting the transmit frequency. At the top is the net switch, labeled HOLD FOR NETTING. When receiving, this switch can be turned to the right, and the transmitter DIAL adjusted, until a beat is heard. Then the transmitter and the receiver are on the same frequency. Alternatively, when receiving, the NETTING switch can be turned to the right, and the receiver DIAL adjusted, until a beat is heard. Then the transmitter and the receiver are on the same frequency.
Picture 2: WS No.108 Front Controls
MECHNANICAL
The radio is in one case. It consists of 5 valves in the receiver, and 2 valves in the transmitter. They are all on one chassis. The receiver is on the right, with a dual gang tuning capacitor. The transmitter is on the left with the oscillator tuning capacitor and the tank coil. Underneath the chassis are the 2 audio transformers, also the RF transformer, the oscillator coil, and the aerial tuning capacitor. Most components are wax covered. There is a 6 pin male plug on the back of the chassis to supply the LT and HT, and the headphones and microphone connections. This plugs into a female connector inside the case. There is corner of the chassis that has a bevel, to clear the cable from the 6 pin connector, to the batteries and audio jacks. On the left side of the case is a spring loaded connection for the aerial. When the radio is in the case, it connects the aerial. The controls are on the front panel. There is a switch on the left hand side that is the TRANSMIT/OFF/RECEIVE rotary switch. This has lever on a rotating shaft. The shaft can be rotated with 2 leather tapes, to the transmit or receive position. The case has 1 shelf inside it. The radio sits on top of the shelf. The batteries are below the shelf. A panel on the front, covering the batteries can be taken off, by removing 1 screw. When the battery cover is off, the batteries can be changed, also the radio can be withdrawn from the case, if required. On the right hand side of the case, there is access to the receiver tuning gang. A Bowden cable can be plugged in here, to enable the receiver to be tuned, while being worn on the operators back. The cable is normally clipped to the battery cover, when not required. There are 2 canvas harness straps at the top of the case, and 2 straps at the bottom. This allows the radio to be attached to the operators harness, for carrying purposes. The case has a metal protecting cover that attaches to the front. Inside the cover are clips for holding, the microphone, the headphones, the aerial, and the operating laces and lever.
Picture 3: WS No.108 Side View
ELECTRICAL
The receiver is a super hetero dyne type with an IF frequency of 455 kHz. The aerial is switched between the receiver and the transmitter by a wafer switch. The aerial coil has an 8-84 pf tuning capacitor, which has a trimmer capacitor in series with it. There is also a trimmer capacitor across the coil. The RF amplifier is a 1N5GT pentode, which has AGC applied to the grid. The RF amplifier anode uses an un-tuned primary and un-tuned secondary RF transformer, to connect to the mixer grid. The mixer is a 1A7GT heptode, even through the circuit only shows 4 grids, it actually has 5 grids, a pentagrid. The oscillator uses grid 1 and grid 2. The oscillator coil has only one winding with the tuning capacitor paralleled by a trimmer, and with another trimmer across the coil. The main receiver tuning capacitor only has 2 sections.
Picture 4: WS No.108 Top View
The anode of the mixer goes through an IF transformer to the grid of the first IF amplifier, a 1N5GT pentode. This also has AGC on the grid. The anode is coupled to the second IF amplifier, also a 1N5GT pentode. This has no AGC control. The second IF amplifier anode, then goes through the third IF transformer to the diode in the detector and audio amplifier, a 1D8GT diode/triode/pentode. The circuit is drawn in such a misleading way, that this appears to be a septode, but it is not. It is a triode and a pentode. The rectified audio is connected to the volume control, and then to the triode grid. The AGC is also extracted here. The triode anode is capacitor coupled to the pentode grid. The pentode grid is also connected to the HT negative line, which goes through some resistors to generate back bias. There is a circuit drawing error here, where R12A should be shown to be connected to the HT negative line, pin 5 (on the rear connector). There is a connecting DOT missing from the circuit. This is on the combined receiver and transmitter circuit. It is correct on the receiver only circuit. The manual shows the complete circuit, also a separate receiver circuit, and a separate transmitter circuit. The anode of the audio pentode goes through a transformer, then to the headphones jack.
The transmitter is a MOPA style (Master Oscillator and Power Amplifier). The oscillator uses a tuned grid, with a secondary winding connected to the filament. There is a grid leak (capacitor and resistor in parallel) in the grid circuit. There is a 5 position switch to select the oscillator tuning capacitor. There are 4 fixed capacitors, which can be adjusted with a screwdriver. These can be individually set, for 4 different frequencies (for presettable channels A, B, C, and D). The fifth position is connected to the transmitter main DIAL for manual setting of the transmit frequency. The oscillator valve is a 1A5GT pentode.
Picture 5: WS No.108 Back View
The power amplifier is a 1Q5GT pentode. The anode of the oscillator goes through an untuned coil. This is coupled to the grid of the power amplifier. The anode of the amplifier is supplied with HT through a choke and from the secondary of the modulation transformer. There is a meter to measure the PA anode current. The anode is capacitor coupled to a tuned circuit, and connected to the aerial. The meter is used to determine the correct coupling, as evidenced by a dip in anode current, when correctly tuned to the aerial.
The modulator is the 1Q5GT audio amplifier, normally used for the receiver. The amplifier anode enters the primary of the output transformer, and the secondary supplies the anode and screen of the power amplifier. The modulator grid is fed from the secondary of the microphone transformer, which is connected to the microphone.
The radio is turned ON, or to transmit or to receive, with a three position rotary
wafer switch. The center position is OFF. There are 7 three position contacts on this switch.
The aerial is switched from transmit to receive.
The LT voltage is switched to the transmitter filaments or to the receiver filaments (or off).
The HT voltage is switched to the transmitter or to the receiver (or off).
The LT voltage is switched to the audio amplifier, in both the transmit and receive positions.
The microphone is switched to the audio amplifier grid in the transmit position, through a transformer.
The headphones are connected to the audio output transformer, in the receive position.
One of the back bias resistors is bypassed in the transmit position, as the HT current is greater.
The NETTING switch is spring loaded to an OFF position. It can be held in the NETTING
position. It has 4 two position contacts. When NETTING .....
The transmitter filaments are turned on.
The HT to the oscillator is turned on.
The HT to the PA is turned off.
The back bias resistors are paralleled with another resistor, as the HT current is greater.
The meter used in the WS No.108 is a TRIPLET 10 milliamp meter of the moving coil type. It has the Radio Corporation name on the meter scale.
Picture 6: WS No.108 Left Hand Side
Picture 7: WS No.108 Right Hand Side
RESTORATION
The radio was in a very clean and complete condition. The case was extremely rusty. The radio was removed from the case, and the valves were removed. A bench power supply attached to the filament connector, and this was set to 1.5 volts DC. A torch bulb was used to test the presence of voltage at the filament pins on the valve sockets. The HT was connected, and the torch bulb was used again, to test the presence of voltage at the filament pins on the valve sockets. The wiring appeared to be correct. The valves were tested, and one of the IF amplifiers had an open circuit filament. It was replaced. The valves were inserted. A pair of headphones was plugged in. The HT was attached and slowly increased. Noise was heard in the headphones.
A signal generator was connected and set to 455 Khz. The IF gain was low, so the voltages were checked. It appeared that the audio coupling capacitor C10D was allowing HT to pass to the grid of the 1D8GT pentode. This capacitor was replaced, and the output was much better. The screen voltage on the two IF amplifiers was low, so the screen bypass capacitor C7B was snipped out. The gain improved again as the voltage rose. A new capacitor was inserted, and there was a further gain improvement. The IF was aligned.
Picture 9: WS No.108 Circuit
The signal generator was moved to the aerial terminal, but there was no signal heard. The generator was moved to the 1A7GT mixer grid, but still no signal, just the IF signal was heard. The mixer was not oscillating. The coupling capacitors C6E and C8A had a low resistance so they were both replaced by mica capacitors. The oscillator was now running, but was off frequency. There are 2 trimmer capacitors to set the frequency. One is across the coil. One is in series with the tuning capacitor. Each capacitor was adjusted one quarter of a turn, and the frequency range checked. This was written down, as they interacted. Each capacitor was adjusted, until the correct tuning range was achieved. This took quite some time.
The aerial coil trimmers were adjusted in the same manner. For a 10dB signal to noise,
the gain was adjusted to close to 2.5 micro-volts. Then suddenly, the gain reduced
to 15 micro-volts. This was confusing. I suspected that one of the trimmers had an
intermittent contact inside. They are a piston style trimmer in a plastic case.
I replaced both trimmers, but there was still the sudden change in gain during adjustment.
I finally replaced the C21A padder capacitor, and the intermittent was gone. I put the
2 removed trimmers back in, and the set now aligned properly. The gain for a 10 db S/N was ....
8.5 kHz 8 uV
8.6 kHz 5 uV
8.5 kHz 2.5 uV
8.8 kHz 2.5 uV
8.9 kHz 4 uV
The set was set to transmit, and the output was checked and appeared to be about 400 milli watts. The output was also checked on an oscilloscope, and with a microphone was plugged in, and the modulation looked acceptable.
Picture 10: WS No.108 Accessories
THE CASE
The case was extremely rusty, but the front cover was on, so the front panel and the radio were in good condition. The wiring, the aerial terminal, the jacks, and the canvas straps were removed. The case and covers were painted.
Picture11 : WS No.108 Case (LHS)
Picture11 : WS No.108 Case (RHS)
Picture11 : WS No.108 Case front
FRONT COVER
The rusty front cover was sanded and painted, and the legends stenciled inside. The microphone, headphones, aerial, and Bowden cable were fitted.
Picture 12: WS No.108 Cover
COMPARISON TO THE WS No.18
The WS No.108 is a copy of the English WS No.18. If this radio was a faithful copy,
it would be called the WS No.18 (Aust), but there have been some major changes.
It provides the same function as the English radio, but has been built differently.
It was made in Australia, as a locally produced man pack radio, using local parts for
construction. It differs from the English original radio, in many respects.
It covers the frequency range 8.5-9.5 mHz, where as the English radio covers 6-9 mHz.
The English radio covers the 40 meter band, so it is useful.
The radio is on one chassis only, containing the transmitter and the receiver,
not on 2 chassis like the English radio, a transmitter on one, and the receiver on another.
It has a telescopic whip, not a sectioned whip aerial.
The batteries are easy to access. They are in the lower compartment, with an easily
removable cover. The cover is secured by one knurled screw. When the screw is removed,
the batteries can be accessed, and the radio can be withdrawn from the case, if required.
The WS No.18 uses a difficult method to change the battery.
It has a metal rain cover, not a canvas cover.
The headphones, microphone, whip aerial, transmit control and fine tune cable,
are all stored inside the metal cover, and a separate carrying bag is not required.
It uses American valves, not English valves.
CONCLUSION
The manual is dated 1941, and this covers the Mark I and Mark II versions. The Mark I was not used a great deal, so not many have survived. The limited frequency coverage meant that it could only communicate with radios like itself, and not the WS No101, or WS No11. The controls are simple and good, except for the transmit/receive switch, which appears to be primitive. The power output is very good for a set of this size. It would be better if it could perform CW or MCW, as well as just AM.
REFERENCES
Australian Pamphlet Number 5, Wireless Set No. 108. Marks I and II, 1941
Wireless Set No.108 MARKS I, II, and III, https://www.qsl.net/vk2dym/radio/N0.108.htm
Copyright
Ray Robinson