The Wireless Set Number 128 is an Australian made backpack radio used by the Australian Army. The frequency coverage is 2.0 - 4.5 mhz in one band, and has variable frequency tuning. It also has 3 crystal channels for the transmitter and receiver. It uses 9 miniature valves, and has an IF of 1.6 mhz. The transmitter power output is 0.25 watts, AM, MCW, and CW. It can load into a normal whip or into a long wire aerial. The radio was made by Tasma Radio (Thom & Smith Ltd, Mascot, NSW) in 1946 and the Mark 2 version in 1952. The physical form was copied from the American BC-1000 transceiver. It operates from a dry battery block type WBA0200 providing 3 volts DC, and 162 volts DC. For fixed or vehicle operation, there was a vibrator power supply available (Vibratory Unit No.3) that was built into the battery box. The radio was tropicalised and was waterproof. It was designed in 1944 as a replacement for the WS No. 108 (the Australian version of the British WSNo.18), and entered service in 1946. It was used by the Army in Korea, and was eventually replaced from 1955 by the A510.

Figure 1: FRONT VIEW (Mk1)

Figure 2: FRONT VIEW (Mk2)

The front panel is covered by a fold down protective cover, similar to the BC-1000 and Wireless Set Number 31. The obvious physical differences are that the aerial is on the left hand side, and the front panel has many more knobs and a meter. Above the aerial insulator, is the EMISSION switch, which selects R.T. (Radio Telephony meaning voice AM), and M.C.W. (Modulated Continuous Wave), and C.W. (Continuous Wave) used for morse code. Next to this, is a large knob with a crank, labeled AERIAL TUNING, and this turns a roller inductor used to match the aerial. There is an arrow to show which way to turn the knob for LOW or HIGH frequency. There is a lock near it. Above it is a spring loaded knob that turns on the DIAL LIGHT. To the left is a knob to adjust the HET (heterodyne or BFO) tone. Below it is a knob that selects the METER function, note this only works during transmit. The possible meter functions are H.T. battery voltage, L.T. battery voltage, P.A. which is the power amplifier current, and AE which is the aerial current. Between the aerial knob and the meter switch is a small window to show the aerial turns counter, but this is only on the Mark 2 model. In the centre is another large knob and this is for the COARSE frequency TUNING, and it has a dial window above it. Above to the left is the DIAL LOCK. Below to the left is the FINE TUNING knob, and below this is the NET switch, to enable the transmitter to be tuned to a distant station. On the far right is a sealed meter, graduated to 2.5, and it has 2 marks on the scale for the battery voltages. Below this is the transmitter oscillator switch, labeled M.O./XTAL, which allows 3 crystals (X1, X2, X3) or variable tuning, marked M.O. (Master Oscillator). Only 1 crystal is required for each channel, as it is used for transmit and receive. The frequency must be 1.6 MHz higher than the channel frequency. Underneath this is the VOLUME control. Between the meter and the dial, is a knob to allow 3 fixed frequencies to be set, labeled FLICK SELECTOR, and A, B, C and OFF. At the bottom is the FLICK LOCK knob which allows the 3 flick settings to be adjusted and set. This knob rotates as well as being able to be pulled out to select the 3 positions. The Mark 2 has a nameplate riveted over the original engraved name, in the lower right hand corner. There is an EARTH terminal near the meter. Note that the meter has radium on the dial and on the pointer, but it is contained within the meter case. However, each dot on the knobs and front panel also has radium in them. The fluorescent paint is old and does not glow anymore, but it is still radioactive. There is an instruction plate on the inside of the front cover.

Usually the FLICK setting is set to OFF, and the COARSE TUNING and FINE TUNING knobs are used to tune in a station. There is a FLICK function that allows 3 dial positions to be set, such that when turning the COARSE TUNING knob, it will stop at a preset frequency. The FLICK SELECTOR knob can select 3 stations, labeled A, B, and C. Setting a FLICK preset is somewhat complicated.

For example.....
select FLICK A, then turn the COARSE TUNING knob until the FLICK stop engages
lock the tuning DIAL LOCK
pull the FLICK LOCK knob out to the A position (indicated by a red line on the shaft)
rotate the FLICK LOCK to unlock position A
push the FLICK LOCK back in flush with the panel
unlock the tuning DIAL LOCK
tune in the station with the COARSE TUNING knob
lock the tuning DIAL LOCK
pull the FLICK LOCK out to the A position
rotate the FLICK LOCK to lock position A
push the FLICK LOCK back in flush with the panel
unlock the tuning DIAL LOCK
The preset FLICK LOCK position A is now set to the desired frequency

Figure 3: Top View (Mk1)

Figure 4: Top View (Mk2)

The transceiver uses miniature valves, and some have a dual purpose, being used for transmit and receive functions. When the radio is turned ON, the transmit receive relay energises, and is always on during receive, but unenegised during transmit, which seems a reversal of conventional operation. This is because the radio only has a 3 volt and a 162 volt dry battery, so powering the relay would drain the battery. The relay is in the negative HT lead, so that receive current energies the relay, and at the same time provides negative bias. Also, the receiver HT is about 20 volts lower, which is adequate for the receiver. Additionally, the relay is out of circuit for transmit, so the full battery voltage is available. The receiver is a superheterodyne type, with one RF amplifier, two IF amplifiers, a refelexed audio amplifier, a BFO, and a separate oscillator for the receiver. The receiver and transmitter can be continuously tuned, or crystal locked to 3 channels.

The aerial is connected to a roller inductor for aerial loading. It then goes through a relay contact to the RF amplifier in the receiver. The circuit incorrectly shows it going through the aerial metering transformer, but it is only in circuit when the equipment is in transmit mode. The RF amplifier uses a 1T4 valve (V2C), which uses a tuned circuit in both the grid and plate circuits. This is capacitor coupled to the mixer which uses a 1R5 valve (V1A). There is a separate oscillator using a 1R5 (V1B), that allows variable receiver tuning, or three crystal locked frequencies. This oscillator is coupled from its filament circuit, to the mixer first grid. There are two IF stages at 1.6 mhz using 1T4 valves (V2A and V2B) and three IF transformers. This then goes to the diode of a 1S5 valve (V4A) which is the AGC and audio rectifier. The AGC is applied to both IF stages and the mixer. The second IF amplifier acts as a reflexed amplifier and the audio is developed across a transformer that drives the headphones. There is a dual diode across the transformer as a “crash” (or peak) limiter. There is an RF gain control that adjusts the screen voltage on the mixer and the first IF amplifier. Valves V3A and V3B (3A4) are not used on receive. The valve V4A (1S5), acts as a BFO when switched to CW. Valve V2D (1T4) is used for netting, as it is a 1.6 MHz crystal oscillator.

The transmitter circuit is a transceiver type, so that transmitting on the receive frequency is automatic. An oscillator using a 1T4 valve (V2D) and a 1.6 mhz crystal, is mixed with the MO or crystal frequency, injected into the filament of V1B (1R5), and the output taken from the plate. This then goes to V2C (the receiver RF amplifier) which now acts as a tuned transmitter driver stage. This is coupled to V3B (3A4) power output valve and connected to the aerial via the ammeter sensing transformer, and the roller inductor. The AM modulator is V3A (3A4) which choke modulates (Heising modulation) the RF output, producing high level AM. For transmit, the valve V4A (1S5) acts as a microphone amplifier on RT, or an audio oscillator for MCW. Valves V1A, V2A, V2B, are not used for transmit.

The circuit is slightly confusing, as the switching of parts of the circuit for TX and RX is not exactly obvious. The three position switch for CW, MCW, RT, and the T/R relay contacts, together with the NET switch, require some understanding. The EMEI notes shows the circuit in its several roles, simplified (the switching removed), so this makes the circuit clearer to understand, and makes fault finding easier. Some valves are shown with an arrow through the symbol, which indicates these are variable mu valves. The valves are 1.4 volt filament types, and are in a series parallel arrangement, across the 3 volt battery supply. The dial light is a special miniature screw in type, and quite difficult to source. The meter is only active on transmit, and shows the LT and HT battery voltage, the PA current and the aerial current.

Figure 5: Underneath (Mk1)

Figure 6: Underneath (Mk2)

Prominent in the center of the radio, is the 4 gang tuning capacitor and the roller inductor. The radio is built on a light aluminium chassis, and has 2 enclosed boxes, one for the BFO and one for the oscillators. The BFO box is very small, and it is difficult to work on those components within. The RF box is down the RHS (viewed from the front) and this contains the RF circuits, valves and crystals. The bottom of this box is recessed above the chassis, which makes the components hard to reach. The wires are neatly loomed and follow angular paths. The components are hand labeled with an ink pen, and the chokes are coated to protect them. When the radio is inserted in the case, a gasket makes a watertight seal, and there is a desiccator inside the case. The rear of the chassis has an octal socket, which mates with a plug fixed to the inside of the case. (Figure 9). From here, on the outside of the case, a cable is attached to another octal socket, and this plugs into the battery box. (Figure 10). A lead with a 5 pin connector then plugs into the battery. The battery box has the ON/OFF switch, and two connectors. One is for the handset. The other is for a control box, which mounts on the harness shoulder strap. The control box has 2 small connectors, one for headphones, and the other for a throat microphone. There is a S/R (Send and Receive) switch also on this box. The control box and handset each contain a link, which connects the negative LT battery circuit. The handset or the control box must be plugged in, or the radio will not work. The MK 1 had a metal frame with shoulder straps attached. The MK2 had shoulder straps attached directly to the radio.

Figure 7: Frame (Mk1)

Figure 8: Backpack (Mk2)

I restored a MK1 and a MK2 at the same time, and compared the differences. The Mk1 has a low serial number 3, and the Mk2 has the serial number 731. The case and battery box are made of light aluminium, and are easily dented. Both these showed signs of a hard life, and it was difficult to extract the chassis as the case was crushed and touching the top of the IF transformers. The radio is held into the case by screws, threaded trough small brackets. The brackets are often bent, or do not line up, and the screws may be missing. The battery box is held on with spring latches. The battery box contained rubber foam lining, which had melted, and then hardened, so a lot of runny solid rubber had to be chipped out with a chisel.

Figure 9: Case Inside View (showing octal plug and dessicator)

Figure 10: Case External View and Battery Box Inside View (showing switch box and battery lead)

The 1.6 mhz crystal was missing from both radios, and this is a normal problem when acquiring these backpacks. New crystals were fitted.

Figure 11: Large 1.6 mhz Crystal

I made an extension cable (octal to octal) to enable easy tuning and fault finding. Some cut wires were rejoined, and two large capacitors were replaced. The 3A4 modulator filament was O/C so it was replaced. The side brakets were floating, as all the screws missing, indicating some dismanteling by a previous owner. These were screwed back on with new screws. The dial light was missing, so a new one was put in. It is a MAZDA CD33 (323) which is a 3 volt tiny bulb in a metal screw-in housing.

Figure 12: Dial Lamp MAZDA 323 (3 volts)

The missing valve shields were replaced. The TX aerial relay contact was shorting to the roller inductor frame, so the relay was moved slightly to fix this. The IF was aligned, but it was a little difficult, as it was picking up broadcast stations at 1.6 mhz. The RF aligned well, and the BFO aligned normally. The roller inductor was noisy so it was cleaned with DEOXIT. Some wiring modifications were removed, and a capacitor wiring error in the PA was corrected. The transmitter was aligned, and showed 2.5 units (or amps?) of aerial current on the meter, at 2.5 mhz. It increased when modulated. The NET control had low volume, so a high resistor was replaced (R25A). The radaio needs the external Control Box for correct operation, otherwise the RT works on transmit and receive, but CW and MCW will not work properly. The case and front panel had been roughly repainted with a different colour green, and was done by hand. It proved hard to touch up, so I reluctantly decided to sandblast the front panel, case and battery box. This required removing the front panel, which is no small task. It was undercoated, sprayed the correct colour green, and the lettering filled with white paint. It came up well.

Figure 13: Front Panel Sand Blasted

Figure 14: Front Panel Painted

Figure 15: CIRCUIT (Mk1)

This was in better condition than the Mk1. The bias resistor was intermittent, and causing strange behavior, until it was re-soldered. All the valves were fine. A new crystal was fitted, the closest I had was 1620.42 khz, so the IF frequency was adjusted slightly. The receiver gain was not good enough, as the common screen supply voltage for the receiver mixer and first IF was low. The volume control (actually an RF gain control) was fine as were the resistors and the bypass capacitor. It turned out that the mixer screen was pulling too much current. A new valve raised the screen voltage and the gain significantly. The IF and RF stages aligned easily, giving 2.2 uV at 2.2 mhz, and 1.8 uV at 4 mhz. The slugs were very touchy. Make sure the BFO and RF covers are fitted, as they shift the frequency. The NET was slightly off frequency and was adjusted. Then it failed, as C13L shorted, so it was replaced as well as the series resistor R25A. The LT and HT meter readings were low so the appropriate resistors were replaced. The relay contacts and roller inductor were cleaned. The roller inductor coupling between it and the front panel knob was broken, as the boss had come adrift. A new boss was made of brass and soldered on. The roller inductor counter was not working as the advance pin was broken.

Figure 16: Broken Coupling

Figure 17: CIRCUIT (Mk2)

A small canvas pouch (Bag, Accessory, No.1) attached to the back of the radio to hold, the 8 aerial rods ”B” and accessories. These sections are 12” long and when assembled become a 7 foot 4 inch long tubular whip aerial. The 1946 manual lists Antenna rods "B" and "X" but only “B" is listed in the 1952 manual. The rod diameter is too small for the aerial connector, so an adapter is used. An "Adaptor, aerial base, flexible (Aust.)" is also listed, so Figure 18 may not be the correct adaptor.

Figure 18: Aerial Adapter

The canvas bag contains a long wire aerial, a throwing weight, and a counterpoise. There was an Officers handset and a morse key.

Figure 19: Morse Key (cable is not original)

The handset is an STC type, chunky and heavy. The original issue is suspected to have been a bulky telephone type, appropriately modified. The Mk. 2 handset was replaced by the later version. Several handsets were found to have the earpiece windings reversed. They were rewired in the correct phase, and the audio output was very much louder. Several handsets were connected like this, so it is either a factory fault or a refurbishment error. The headphones are an American HS30 type. The microphone is a British throat microphone, low level, No.2.

Figure 20: Accessories (Mk1) showing headphones, switch box, throat microphone, and handset.

Figure 21: Accessories (Mk2) showing bag, control box, headphones, throat microphone, adapter, and aerial Rods B.

The Mk1 control box and accessories use a push on bakelite 3 pin connector, similar to a “drop” cord. The Mk2 accessories use a more robust metal 3 pin connector.

Figure 22: Control Box (Mk1)

Figure 23: Control Box (Mk2)

There is a small box that contains spare valves.

Figure 24: Spare Valve Kit

Figure 25: Spare Valve Kit (open)

The Mk2 has a nameplate riveted over the Mk1 engraved name on the front panel. The Mk1 used thicker wire. The thinner wire used in the Mk2 makes the wiring looms smaller, and there is more room making it easier to work on. The Mk2 uses physically smaller capacitors, so again there is more room under the chassis. The Mk2 has a turns counter on the front panel for the roller inductor, so it is easier to tune. The Mk2 is tropicalised, with all components varnished, and the transformers encapsulated rather than dipped. The chassis is covered in a yellow varnish, whereas the Mk1 is raw aluminium. The adjustment of the TX FREQ CAPACITOR is easier, as it is vertically mounted in the Mk2. The harness on the case is smaller and more comfortable and the spine pad on the case is better.

The roller inductor can be used to tune for maximum noise on receive and maximum aerial current on transmit. The tuning of SSB signals is easy, as the volume control is actually an RF gain control. The TUNING is touchy, and the HET tone and VOL need to be adjusted. The receiver is very sensitive, and easy to tune. The meter only works in the transmit mode, but if the HT battery is flat or missing, then the TX/RX relay will not pull in for receive, and the meter will then show the LT battery voltage. Plugging in the throat microphone, replicates a morse key, so if you switch to CW or MCW, it appears to be a “key down” situation. If only the Officers handset is used (with no control box) then only RT operation is possible.

The modulation is very good, it is clear in the headphones as a sidetone, and you can see the aerial current increase when speaking. Amplitude modulation using the Heising modulation method is superior to grid modulation. Both the Handset and Throat microphone give adequate modulation, but the handset has a better high frequency response. It is easy to achieve 100% modulation, with the troughs touching zero, without over-modulating. The peaks are flattened, with both microphones.

Figure 26: Modulation 50% (Handset)

Figure 27: Modulation 50% (Throat)

Figure 28: Modulation Speech (Handset)

Figure 29: Modulation 100% (Throat)

The tuning is variable and covers the 80 meter amateur band. The wiring is neat and loomed into angular runs. The components are hand labeled, and the chokes and transformers are dipped. There is only one crystal required for each frequency, not 2 crystals per channel as in some radios. The output stage and the IF is reflexed which reduces the valve count. The Aerial current meter is easy to use and gives a good indication of tune, even with the whip. The operators instructions are on a plate under the cover.

Figure 30: Instruction Plate

The chassis is too light and flimsy, causing a frequency change if bumped or flexed. It is prone to vibration, and microphonics can be heard. If the receiver does not have the dial lock tightened, a bump can shift the frequency. The case and battery box is made of thin aluminium and dents easily. The case screws protrude into the inside and foul or dent the IF cans. The FLICK mechanism is too complicated. The frequency range is small, only covering 2.0 to 4.5 mhz. The BFO can is so small it is very difficult to work on the components inside. The spring loaded front cover, was always snapping closed on your fingers, and it cannot be removed as it is riveted on. I had to tighten (squash) the pivots in the cover support arms, to keep the cover open. The Mk1 pack frame looks heavy and uncomfortable. The dots on the front panel are radioactive. The relay logic is reversed. The switching is very complicated, and the circuit is difficult to understand. The handset is heavy and large. The IF cans and valves are so close together, that a tube extractor is needed. The many brackets and screws on the outside of the case and battery box catch on everything. The springs in the DIAL LIGHT knob and HET knob are so strong that you get a sore wrist trying to hold them on. The position of the FINE TUNE and the HET knob (or FLICK knob) need to be swapped, as you are cross armed when netting.

When fitting the screws to hold the dial bezel on, do not use long screws as they will scratch the dial. These screws have a screw thread tapped into the plate behind them, so they are easy to remove and replace from the front. The screws on the Mk2 roller inductor window have nuts behind them, so if you remove these screws, the nuts and lock washers fall off, and it is difficult to replace them without removing the front panel.

Figure 31: Scratched Dial (due to long screws)

Handbook ZI/ZAA8609 Wireless Set No.128 Operating Instructions 1946
EMEI (Aust) F351 Wireless Set No.128 Technical Handbook 1948

Ray Robinson VK2NO

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