WS-122_review_2002.pdf

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1、Not exactly a thing of beauty, this WS122 war-time military transceiver was still being widely used in Victorian rural fire services until the 1980s. Its transmitter output valve is an 807. VINTAGE RADIO By RODNEY CHAMPNESS, VK3UG The Radio Corporation WS122 In May 2002, the AWA FS6 army transceiver

2、 was described. This month we cover another military transceiver, the WS122. This was a much more portable set, requiring only three men to carry it! The WS122 as a type or model number will mean nothing to many readers, while others will become dewy-eyed dreaming of their beloved WS122 that they us

3、ed many years ago. Commonly the WS was dropped off the type number; it sim- ply stood for Wireless Set. The 122 is one of several different portable high frequency (HF) radio transceivers used during World War II by the Australian Army. As men- tioned above, in May 20021 described the AWA FS6, a pop

4、ular HF trans- ceiver from the same period. After reading this article you will see that these two transceivers are as differ- ent as chalk and cheese. The WS122 (Aust) was the final unit in a series of sets which had their start in the British-designed 22 set. Radio Corporation (Eclipse Radio Pty L

5、td) was charged with the job of building an Australianised version of the English 22. Their first attempt was something like the British 22 and probably not much different in performance or fa- cilities. They then built the Yellow Band 80 ?SILICON CHIP ?.au This photo shows the WS122 ready for servi

6、ce with power supply and ancillaries hooked-up. The protective grille at the front made it difficult to operate and most users discarded these. OCTOBER 2002 81 The power supply used two vibrators. The third one is clipped in as a spare. 22 (Aust). It had an Australianised circuit using American and

7、Austral- ian designed valves and while still a grid-modulated transmitter, it used only one 807 valve in the transmitter output section. I doubt that this was a particularly common set. Ive never seen one anyway. However, Radio Corporation felt that they could do better. Their design engineers reall

8、y got the bit between their teeth and came up with the 22 (Aust) set. This was soon partnered by the 122 (Aust) set which was iden- tical except that it had provision for crystal control as well as variable fre- quency control (VFO) of the transmit- ter operating frequency. The 122 remained in servi

9、ce until the mid 1950s when it was progres- sively replaced by the No 62 set. Early interest In my mid teens, I became inter- ested in radio communications and was itching to be able to use radio transmitters and receivers. My chance came in 1957 when I became involved with the local Emer- gency Fir

10、e Service (EFS) in country South Australia. I was given a much- modified Type A MkIII transceiver (described in the October 1998 issue) to use at fires for communications back to base in a nearby town. I ultimately built my own trans- ceiver, based on designs in Radio all very handy. There are two s

11、ets (red and blue) of mechanical preset tuning adjustments that can be adjusted to allow rapid selection of two frequencies. The ad- .au Under chassis view showing the shielded RF section compartment and the crowded wiring. justments are similar in concept to the pushbutton tuning on older car radio

12、s. These are locked with the screwdriver located on the top right of the front panel. With the front protection grille in place, the set is not easy to operate but quite OK once it is removed. Most of us just threw these grilles away. Certainly it is far from an easy set to service. Some parts are e

13、xtremely difficult to gain access to. However, considering the amount of parts there are, the set is quite reasonable to work on, provided youre not in too much of a hurry to get the job done. Receiver valve line-up The receiver uses a total of seven 2V battery valves, with a 1D5GP ra- dio frequency

14、 (RF) amplifier, followed by a 1C7G as a mixer. The intermedi- ate frequency (IF) stages use a further two 1D5GP valves while a 1H6G func- tions as detector and delayed auto- matic gain control (AGC). Note that the triode section of the 1H6G isnt used in the receiver only in the transmitter. The aud

15、io stage is an 1F5G audio output type valve and in the receiver it only feeds two pairs of headphones. (The 1F5G has sufficient gain and out- put to feed a speaker in a modified set.) In Morse code (CW) mode, the beat frequency oscillator (BFO) is another 1H6G valve. The receiver is designed to rece

16、ive AM, MCW and CW transmissions which it does quite competently. With careful tuning and attention to the radio frequency gain, the receiver can also adequately tune single sideband transmissions. One attribute it has which is not common on portable military equip- ment is a meter measuring the eff

17、ect of the AGC voltage in controlling the gain of the RF and IF sections of the receiver. This is very handy for deter- mining the relative strength of re- ceived signals. In amateur radio, this feature is usually called an S-Meter which equates to signal strength meter. Its reading is useful in ali

18、gning the re- ceiver and transmitter circuits. Transmitter valve complement The transmitter has two RF stages. The VFO and crystal oscillator is a 6U7G (V5A). This stage drives the output valve which is an 807 (V7A) small transmitting tetrode. The FS6 described in May 2002 uses grid modulation but t

19、he 122 uses the much more efficient plate and screen modulation method. The modulator is much more elabo- rate than that used in the FS6. The 1D5GP used as the 2nd IF amplifier is also used as the first audio amplifier in the transmitter, followed by the triode section of the 1H6G (receiver detector

20、/AGC). The next stage is the receiver audio output stage and modulator driver (1F5G). This drives the modulator valve which is a 6N7 run as a push- pull class-B stage. To my knowledge, the 22 (Aust) and the 122 (Aust) were the only port- able, mobile, man-pack military trans- ceivers that used such

21、a sophisticated (for that time) modulation system, ex- cept for the ATR4. The transmitter has three modes of operation: voice (radio telephony - RT, AM); Morse (CW) and Modulated Continuous Wave (MCW). On voice, a press-to-talk button on the microphone is used to change over from receive to transmit

22、. The microphone has a dynamic in- sert which explains why so many . audio stages are necessary to achieve full modulation; a carbon microphone has greater output but poorer audio quality. In addition, the quality of the audio being transmitted can be moni- tored through the headphones. On CW, the M

23、orse key is depressed and the unit changes over automati- cally to transmit. It has what is called semi-break-in keying. A tone (sidetone) in synchronism www. siliconchip. corn. au ?OCTOBER 2002 ?83 Photo Gallery: STC Model 5017A 4111., ? .ha, A product of STC (Sydney), the model 5017A uses the same

24、 chassis as the STC 5017 but is housed in an alternative cabinet style. This example is from 1936. This set covers the medium wave band and uses the following valves: 6A7 frequency changer; 6D6 IF amplifier; 6B7 first audio/detector/AVC amplifier; 42 output and 80 rectifier. supply is used to provid

25、e the HT (150V) for the receiver. On low power transmit, the same supply is used and it is switched to provide 180V to the transmitting valves. On high power transmit, the two vibrator power supplies are con- nected in series to provide 250 to 260V on MCW and voice and 320 to 360V on CW. It is well-

26、filtered which reduces vibrator hash to a very low level. General Overhaul As mentioned earlier, my 122 had been modified to suit work on the Country Fire Authority radio net- works. Some of the modifications could be easily reversed but some had to be left as is, hence the non authentic look of the

27、 set in places. Generally, the components in the sets have been quite reliable and very few needed replacement. If you are going to work on one of these sets it is imperative to obtain a handbook, as the sets are very complex (for their time) and difficult to work on. In the back of the transceiver

28、case is a circuit diagram of the set and in the power supply is a copy of its circuit. These are better than nothing but are hard to follow. I replaced a few paper capacitors in the audio sections. On transmit, I found I could tune the transmitter up and obtain good output but then the output would

29、just die away. The plate current of the 807 was dropping but the HT voltage was remaining con- stant. The problem was that the 807 had lost its emission and a replacement soon fixed that. I suppose that after 40-odd years it was entitled to be tired. The 1D5GP receiver RF valve was also slightly wea

30、k and was replaced. The 1F5G valve seems to be the one most likely to require replacement from my experience. Every other valve has proved very reliable and long- lasting, despite being bumped around in a vehicle for many thousands of kilometres. The crash limiter is a pair of di- odes wired anode t

31、o cathode so that any voltage AC or DC above around 0.2V is clipped to that level. It can be switched across the headphone out- put and is designed to limit the effect of static crashes. This it does but it also severely limits the audio output and distorts it. Radio amateurs thought it was use- les

32、s and I didnt use it. I wired another four diodes (1N4148) in the same way as the origi- nal diodes and put these in series with the existing ones. This improved the audio quality while still retaining the static reducing ability. I decided that it would be a good idea to check the alignment of the

33、transmitter and receiver circuits. I tackled the receiver first. I found that the intermediate fre- quency (IF) alignment was peaked on 460kHz so it was left alone. Thats close enough to the designed 455kHz anyway. The RF and oscillator sec- tions are not so easy to get at, particu- larly for the 2

34、to 4MHz band. The slugs are horizontal, going into the large below-chassis shielded en- closure and are well down in the chassis amongst a lot of components. Great care is needed to avoid shorting anything. I couldnt even get a screwdriver onto them and had to use a pair of long-nosed pliers to la-

35、boriously rotate the slug cores; a bit of butchery but the only way of ad- justing these coils. The 4 to 8 MHz cores are accessible for use with a screwdriver. with the Morse key is heard in the headphones. This sidetone makes it so much easier to check the quality of the signal. The modulator is in

36、active in this mode. MCW is the same as for CW, except that the modulator is operating and a tone is transmitted in synchronism with the operation of the Morse key. Vibrator power supplies The power supply is really two sup- plies, both based on vibrators. One 84 ?SILICON CHIP ?.au Vintage radio fee

37、dback As an old radio man who cut his teeth on valve radios, I always enjoy Rodney ChampnessVintage Radio feature. But Rodney, my un-met friend, something you said in the June 2002 issue is driving me bananas because I dont understand it. In describing the Tasma M290 superhet radio, you point out th

38、at the local oscillator padder capacitor works best if placed where the Tasma M290 has it, in series with the oscillator tuning gang rather than in series with the earthy side of the oscillator coil. Whats the difference? In either case, the oscillatory circuit consists of the oscillator coil with t

39、wo capacitors in series across it. Seeing that the oscil- lator is not tickled into activity by phase changes across the parallel resonant circuit (it has a separate tickler coil), what does it matter to circuit operation where the earth point is placed in the circuit? Thats the only physical differ

40、- ence I can see, the effective place- ment of the earth point. The only technical difference I can see is that the padder placement you call best, in fact puts the dynamic Miller capacitance of the valve grid across the whole oscillator circuit in- stead of (with the other padder place- ment positi

41、on) across just one of the series capacitors (the tuning gang). For circuit constancy, wouldnt it be better to put the padder capacitor in series with the earthy side of the oscillator coil to slightly improve the dynamic stability of the oscillator cir- cuit? Stan Hood, Christchurch, NZ It is alway

42、s good to get comment from readers and I appreciate Stan Hood of New Zealand for taking the time to do so. At the outset I certainly dont claim to be the font of all knowl- edge on vintage radio or design. These are my thoughts on why the local oscillator does work better when the padder capacitor i

43、s in series with the capacitor and not the earth end of the oscillator coil. One side of the original padders was earthed, there- fore using them in the earthy end of the coil was convenient and it worked. When fixed padders be- came common most manufacturers carried on the convention. However, some

44、 put the padder in series with the tuning capacitor. I used to wire all my receivers with the padder to earth just as Ra- dio & Hobbieshad done. I accepted is as the way to do it and never questioned it. However, I ran into trouble with a receiver that would drop out of oscillation on the low freque

45、ncy end of the dial. All I did was to shift the padder to be in series with the tuning capacitor and the problem vanished. Ive since done this modification to a few re- ceivers and the results have all been favourable. It may be remembered that 2A7s and 6A7s were prone to drop out of oscillation on

46、the low frequency end of tuning ranges. This modification has cured any sets that Ive had this problem with. It also seems to im- prove the sets sensitivity. I do believe that either the phase of the feedback is changed or the amount of feedback is reduced or maybe both. The effect may also vary dep

47、ending whether the feed- back winding is near the grid end of the tuned winding or near the so- called earthy end. Looking at the typi- cal circuit redrawn, it does look like a cross between a Colpitis and a tick- ler feedback type circuit. With the tuning capacitor fully meshed, the coil would appe

48、ar to be centre tapped. When tuned to the high end, the electronic tapping point has moved down near to the padder capacitor. With the padder in series with the tuning capacitor the coil always has the bottom of the coil referenced to earth and therefore the feedback would be more predictable. All I

49、 can say it works better with the padder in series with the tuning gang. Rodney Champness, Mooroopna, Vic. Surprisingly the adjustments were quite close, only requiring a touch- up. The alignment of the transmitter involves making sure that the receiver and transmitter tuned circuits, par- ticularly the respective oscillators, all track one another. I wont go into all the procedure necessary to achieve this, suffice to say the alignment and adjustment details are straight-forward and un- ambiguous. As some of the tuned circuits are common to both the