Military Collectors Group Post

                     Backmail #37

Index;(18 pages) IT SEEMED LIKE A GOOD IDEA AT THE TIME; R-467/ALR by John Mackesy VK3XAO REMOTE CONTROL OF AVIATION ELECTRONICS; AN EXAMPLE, By John Mackesy VK3XAO POWER FOR AIRCRAFT AND MILITARY RADIOS; by John Mackesy VK3XAO RADIO COMPASS AN/ARN-6; By John Mackesy The AT5/AR8; From Australia, By Steve Hill **************************************************

REMOTE CONTROL OF AVIATION ELECTRONICS; AN EXAMPLE, By John Mackesy VK3XAO (email, Part 1 Many of us will be familiar with Bendix Radio Compass/ADF systems, typically MN-26, SCR-269G, ARN-7 and ARN-6. These all shared a common heritage, and were rugged, reliable and easy to maintain. The ARN-6 (1950) was particularly reliable, due in part to the level of development of the technology, and to the use of 28 VDC B+ voltage. All these receivers used standard octal-base tubes, the *K7, *SK7 series tubes being common to all types. Radio Compasses (Auto Direction Finders - ADF) differ from much aviation radio equipment in that they need to be continuously and steplessly tunable over a wide range. In the Bendix systems, this was accomplished by a mechanical tuning drive cable (similar to a speedometer cable) between the control box and the receiver. Although successfully used for many years, this method has a number of disadvantages. These include friction, limited bend radius and limitations on length and layout. Passage through pressure bulkheads is another problem. One ADF system which didn't use mechanical tuning drive was the '50s Marconi AD7092. This was a rather novel device used in a number of British-origin aircraft, including Comet, Meteor, Canberra, Viscount and DH Dove. Compared to the ARN-6, it was lighter and more compact, but in some ways rather more complicated. It also required more wiring, and (in the writer's opinion) is not as easy to operate as the ARN-6. The receiver is a typical aircraft "black box", compact and featureless, 8" (20.3 cm) H x 5" (12.7 cm) W x 12.5" (31.7 cm) deep - about half the size of the ARN-6. At 15.7 lb (7.1 kg) it also less than half the weight. In the words of the AD7092 manual T2216/3 (undated) "The receiver case is strongly constructed of light alloy metal. Quick release side covers provide ready access to the valves and components, which are built on to small easily accessible sub assemblies". It's probably worth mentioning that these "sub assemblies" are not of the plug in type. The basic receiver circuit is a single-conversion superheterodyne with an IF of 110 Khz and a frequency coverage of 100 Khz to 2.0 Mhz, in 4 bands. Receiver circuitry is fairly conventional, with 1 RF stage, a separately excited mixer, oscillator, 2 IF stages, BFO, detector and reverse grid current suppressor (!), 1st audio and audio output stages.20 When operating as a direction finder, additional stages are brought into action - a loop amplifier, ADF rectifier and AGC diode stage, 1st & 2nd loop motor amplifiers, loop amplifier output, DF switching and a 2-tube DF modulator stage. All tubes (total of 17) are standard 7 or 9 pin miniature types, with 6.3V filaments wired in series groups of 3 - filament power is derived from a 19V (carbon pile) regulator. 250V B+ is derived from a small dynamotor, which also acts as a cooling fan. The AD7092 requires 22-29V DC at about 5A, and 115V or 26V 400 Hz AC at 6W Other features of note are variable selectivity (3 steps - 5 Khz, 1.5 Khz and 400 Hz), a silicon diode rectifier to drive the tuning meter and use of a vibrator to generate 110 Hz modulation (for the ADF stage) This would seem to be sufficient circuitry to take care of most radio tasks, but not for the AD7092. Associated with the basic receiver is a plethora of small black boxes, each with its interconnecting cable(s). The numbers vary (depending on the installation) but would typically include, in addition to the receiver: Receiver Controller Type 1274 Power Factor Transformer type 1571 19V DC Regulator Type 1555A Voice/Range Filter Type 1275 Junction Box Type 1629 Loop Controller Type 1342 Sense Amplifier Type 1571 DF Aerial Transformer Type 922 Loop Bearing and Tuning Indicator Type 1630 DF Loop Aerial Type 1324 Receiver mounting rack ... and a bloody lot of interconnecting cables, most of which are terminated by Pye "Multipol" connectors. The Pye Multipol is a square multi-pin connector, with pins in multiples of 4, something like a Jones plug. Unlike Jones plugs, pin arrangement is symmetrical, with a key at the edge of the plug ensuring correct orientation. As mentioned earlier, the AD7092 uses electric tuning - no mechanical drive cables. The 3-phase AC remote synchronous positioning and indication system, variously known as Autosyn, Selsyn or Aysynn, is used for both tuning drive and frequency indication. In the AD7092, a synchro "master" generator coupled through gearing to the tuning knob on the Type 1274 controller causes a "slave" motor coupled to the tuning gang to rotate in step with the master. A second synchro master transmits the position of the tuning gang to a slave in the controller, which drives the dial pointer. Band indication is accomplished by lighting the appropriate (270 deg.) arc of the dial. I know it all sounds horribly complicated, but in practice it's mechanically quite straightforward, and works very well. The synchro master/slave pairs require 26V 400 Hz AC power, and consume about 2W a pair. 8 wires are required for the tuning drive and indication system. A similar system is used for the loop position indicator. Two different types of loop aerial were available, Types 1324 and 1264. The 1324 is a flat recessed iron cored aerial intended for high speed aircraft (Comet, Canberra) where drag must be kept to a minimum. The 1274 is a more conventional air cored loop, usually mounted inside a streamlined housing or inside the aircraft canopy. From a mechanical point of view, both types are a work of art - the 1324 has 3 different types of position transmitter!20 Part 2 BUILDING AN AD7092 SYSTEM The obvious question here is why would you want to? In my case, there were 2 reasons. 1. The AD7092 seemed like such a strange and deviant device that I just had to do it. 2. Who else would do it? It all started when I came across this "black box" among the bones of a derelict Canberra jet bomber. The black box was labeled "MF-DF" - what the hell was an MF-DF? After some thought, and a peek inside the box, I realized that "MF-DF" meant "Medium Frequency Direction Finder" - a device better known as a Radio Compass or Automatic Direction Finder. Having had some experience using and building Radio Compass systems, my interest was immediately aroused. Coincidentally, I happened on a supplement to the manual for the equipment shortly after this, further firing my enthusiasm. To cut a long story short, I eventually wound up with 3 receivers, mounting rack, voice/range filter, power factor transformer, and a box of Pye Multipol connectors. Chasing up the other parts of the system was a saga in itself, but eventually I had enough parts to start getting it together. As is usually the case, the controller was the elusive bit. This was acquired from an old mate, who phoned one evening to say that he had this dial-looking thing with a "Marconi" name-plate, and did I want it? He expressed some surprise at how soon I got there - fortunately, the police weren't about. Although there is (as I said earlier) a bloody lot of wiring to do, the manual has very comprehensive and easy-to-follow cable guides. All I had to do was build the cables, a slow and tedious business... there are 28 wires in the receiver control harness, 12 wires in the loop aerial control cable, 12 wires in the receiver to loop cable, etc., etc.20 After everything had been checked, double-checked, and checked again as a system, I fired it up - and it worked first try! I later had some difficulties with the loop controller, but it certainly worked well as a receiver. That was 1981. Since then, it's had the very occasional workout, spending most of its time in storage. It's just too messy and awkward to set up permanently; I prefer my Bendix ARN-6, which is both much easier to live with and to operate. This situation changed recently, after I decided to redefine the cable setup and physical layout of the system. It's one thing to build a system that works, something different again to build a system for display. As far as I know, this is the only example of a complete and operational AD7092 system in Australia, and possibly in the world. If anybody else has one, (or parts) I'd like to hear from you. Like most projects of this type, this would not have been possible but for the assistance generously given by the following people: Bill Babb VK3AQB Kelvin Date VK3DBZ The late Harry Wallace, of A.E.O.S, Moorabbin Airport Lynton Hayres, also of A.E.O.S. The late Ted Wilkes, ex-VK3UU Postscript... No article of this type would be complete without a look at the evolution of the technology described. In 1959 Collins introduced the 51Y3 ADF receiver, a device slightly larger and 5 Lb heavier than the AD7092. This used both tubes (11) and transistors, didn't have numerous associated black boxes - and was electrically tuned, with digital frequency readout. And furthermore... I discovered that there is an electrical remote tuning system available for ARN-6. This is somewhat different to the AD7092 in that it uses a motor and a position transmitter connected to the standard ARN-6 tuning input. The control box has a frequency INC/DEC switch, which controls the PD14 drive motor. Aircraft which used this (that I know of) were the C130A and the RA3B. If anyone out there has any of these parts, I'd really like to hear from you. ************************************************

IT SEEMED LIKE A GOOD IDEA AT THE TIME; R-467/ALR by John Mackesy VK3XAO One of the more engaging aspects of the radio hobby is its diversity. Some of us specialize in a particular era; others a specific application, model or manufacturer. My interests are in aviation - not necessarily military - and amateur radio. Aviation radio in this context doesn't mean the astronomically expensive "black boxes" of the modern airliner/bomber/fighter; rather the devices of a bygone, almost forgotten era. An age when aircraft engines had a multiplicity of cylinders, swung impressively large propellers and shook the ground with their thunderous roars, even with them at 15,000 feet. Of the aircraft radio equipment from that era, little survives. Of the survivors, few are operational, even fewer unbutchered. This is the story of "one that got away"*. It started as quest for BNC panel sockets. This turned up a "black box", bearing 6 of them, brand spanking new, never used, original packing etc. for only ten bucks(thats about $7.50 US) - an offer too good to refuse. Unusually, the black box came with its matching remote control - this one looked like a useful source of switches and pots. According to the data plate, it was a C-1933/ALR-3; the front panel bore the legend "CM X RCVR" Detailed analysis of the black box (on the dining room table)* revealed that it was a "R-467/ALR" - an airborne electronic countermeasures receiver from the '50s, (according to its label) intended for use on Neptune ASW aircraft. Inside the box (127mm W x 165mm H x 330mm D*) lurked a forest of useful parts, including valves* like, 2 6X4's, 2 6AQ5's, 2 0A2's and half a dozen 5670's (2C51). The 5670 is an HF twin triode, and was widely used in VHF applications. There were also motor-operated rotary switches and a useful quantity of high quality resistors, capacitors and miscellaneous hardware. It was all most beautifully constructed, to the usual exacting MilSpec standards. Following some reflection, I decided that it was ethically unsound to convert this equipment to components. Which, logically, raised the thorny issue of what to DO with it. The usual tendency is to put things like this on an out-of-the way shelf, thereby avoiding the need to make a decision. Not this one! This was going to be a goer - after all, who else would do it? Lacking a schematic (or any other data) the interconnecting wiring between the control box and receiver was puzzled out. Each end of the cable terminated in the multipin circular metal shell connectors commonly known as "Cannon plugs"* - MS3106 24-28S in this case. A pair of these were found, and a wiring harness built - 26 wired - on the long-suffering dining room table. Not a difficult job, but rather tedious. On completion of this task, the R-467/ALR was ready to be powered up - for the first time in some decades. As is usual with aviation electronics, it required 28V DC and 115V 400 Hz AC. In my situation, the 28V DC source is a transformer/rectifier supply and the AC is derived from 28V DC input/115V output rotary inverter. But I digress... A multimeter was connected to front panel B+ test point, and the "POWER" switch moved to the "ON" position. Filaments OK, B+ OK, B- (!) OK, all functions OK. Seems like Murphy* took the day off. But what does it do, you may well ask. Good question. It appears to be a very broad band receiver - no tuned circuits, you can't get much more broad band than that. The circuitry is mostly classical video amplifier, the output video and demodulated audio - the 2 6AQ5's. There is one clue - there are 4 antenna inputs, the control box allows you choose any one of the 4, or accept input from all at the same time. I recently discovered that Neptune aircraft had 4 rod antennae spaced at 90 deg. intervals around the rear fuselage, which suggests the following scenario*. With the antenna input set to "ALL", a radar* signal would provide an output from the receiver. Thus alerted, you'd (quickly) switch through the antennae, thereby divining from which quadrant the threat was coming. With any kind of luck, this would lead to the avoidance of a potentially embarrassing triple-A/missile situation. What makes this workable is the fact that radar emissions are detectable from a much greater range than they can provide a usable return. So there you are, sitting in a large, slow moving target with an impressive radar cross-section and no defensive armament. What do you do? The short answer is - hide behind the horizon! Although it was an interesting project, there didn't seem to be any practical use for the beast. That situation changed recently when it was pressed into service as a preamplifier for my Panoramic Analyzer. This is used in conjunction with a Collins airborne HF transceiver... but that's another story. As for the BNC panel sockets, I found this piece of commercial junk, y'see, and it had these BNC sockets... Footnote: Electronic countermeasures, commonly abbreviated 'ECM', come in 2 forms. These are (1) 'active' countermeasures, i.e. jamming, chaff, and various sorts of decoy techniques, and (2) 'passive' countermeasures, which is basically maintaining a listening watch. This provides a basis for more effectively pursuing (1). It also lets you know when you're being illuminated by radar-controlled gunnery and missile systems, among other things! For more information on this fascinating subject, read "Instruments of Darkness" and 'The Radar War'. Finally - my feeling is that the equipment described is part of a more grandiose* system. If anyone out there has info - manuals, schematics, personal experience - I'd like to hear from you. ed)* "one that got away"*, a phrase normally associated with fish stories, so I suppose it does apply here. (on the dining room table)*, an elaborate diagnostic test fixture, normally only available to the most dedicated if scientist, or confirmed bachelors. "Cannon plugs"*, a term often synonymous with masochist, or masochistic, usually associated with an abnormal sex life, or tendencies. This conclusion can be confermed in the part numbers used with these devices,I/E "MS3106 24-28S" "mm*" is the abbreviation for millimeters, a unit of measure often use by foreigners when a good yard stick isn't available. "valves*" is Limy for tubes. I'm desapointed, he didn't once use the words "earth", or "aerial"! "Murphy*" is the butler, often blamed when things go wrong, as he had the day off, extra care was required in this project do to the lack of a scape goat. "scenario*", a pet word often used by the actor, Robert Culp, in the now defunct TV comedy series "Greatist American Hero", when planning the days commic events. "radar"*, A much love central character in the Movie, & later TV series "MASH". He was famous for his ability to detect flying aircraft at great distances, & his sleeping with a Teddy Bear. Until this time it was unknown that counter-measures had been devised to defeat these abilities. "grandiose*", a river separating the United States, & Mexico, I fail to see it's connection with this article. Dennis ****************************************************

POWER FOR AIRCRAFT AND MILITARY RADIOS; by John Mackesy VK3XAO Unlike domestic radios, all aircraft radio equipment is powered by something other than 115/240V 50/60Hz. This also applies to a significant proportion of military ground equipment. For the collector who wishes to restore these (often complex) systems to their full operational glory, exotic power requirements can be a major difficulty. There are 2 solutions to the problem: 'conversion', and 'roll-your-own' power. 'Conversion' generally means techniques like re-arranging filament wiring, outboard B+ supplies, and reworking internal power supplies. When done to a high standard, conversion can produce very acceptable results. Unfortunately, not all equipment lends itself to this approach, particularly devices which contain servomechanisms and/or position indicators. 'Roll-your-own' power usually means 24-28V DC and/or 115V 400 Hz AC. A high proportion of mobile (transportable, vehicular) equipment uses 24V, this being a common standard for military vehicles (and heavy vehicles generally). Aircraft use 24-28V DC extensively, many larger aircraft also using 115V 400 Hz AC power systems. 400 Hz is used to reduce the iron requirement in generators, motors and transformers, as 400 Hz devices needing much less iron to be resonant, than its 50 Hz counterpart. This all reducing weight, & during WW-II iron, when supplies were critical. Some military ground equipment also uses 400 Hz power. Most receivers (and low power transmitters) will not need more than 150W (total), so power sources are reasonably manageable. Typical examples (in the writer's collection) are the WW2 vintage Bendix MN-26 & SCR269 Radio Compasses and the '50s Marconi AD7092 Auto Direction Finder. More grandiose devices tend to require more power, especially higher powered transceivers. The Collins 618S-1A Airborne HF Transceiver is an example of this. A sample of typical power requirements appears below: EQUIPMENT 24-28V DC 115 VAC 20 MN-26 RADIO COMPASS 3.0 A NIL20 SCR-269 RADIO COMPASS 1.0 A 100W (320-1000 Hz) 20 AD7092D ADF 3.0 A 6.0W20 618S-1A (Receive) 3.0 A 160W 618S-1A (Transmit) 30.0 A 160W There are a few devices (usually test equipment) which will operate over a wide range of power supply voltages and frequencies. A common example is the British AVO CT-160 Valve Tester, which can be used between 50-250V and 50-500 Hz AC. DC power sources, usually based on a simple transformer-rectifier arrangement, are relatively straightforward and inexpensive. Some equipment will operate satisfactorily on 'raw' DC, but well-smoothed regulated power is desirable for most applications. A point to remember is that although the DC requirement may be quite modest (4 - 6A), cold filaments and dynamotors tend to look like dead shorts at switch-on. For 400 Hz AC power generation, an inverter (usually 24-28VDC in, 115VAC out) is the most practical solution. Inverters are widely used in aircraft with modest AC power requirements, where the bulk, weight and complication of an engine driven AC generator is not justified, and for standby purposes. Until the '60s the aircraft inverter was an electromechanical device (a combined DC motor and AC generator), commonly called a rotary inverter. These have now been largely displaced by the 'static inverter', solid-state, much lighter, more efficient and reliable. As a result rotary inverters have become available at reasonable prices.20 A 150W inverter takes care of most things, with a 28V DC input current of about 12A. Rotary inverters are not particularly efficient, rotate at high speeds (typically 8000 RPM) and have a large cooling air requirement. Internal fans move the air, the overall construction being very similar to a siren - so is the noise, usually a high-pitched resonant whine. Solid-state inverters have much higher efficiency (and better regulation) with much less noise. In my situation, 27.5V DC power is supplied by several small (2 - 10A) regulated supplies. For AC power, the main source is a 190W Eclipse Type 778 rotary inverter, with a '60s 770W Rotax Type S3303/2 for occasional higher power operations. The Rotax inverter is surprisingly quiet (modern fan design!) and is a type originally used in Argosy and Canberra aircraft. A 1.2KW 115/208V 3-phase inverter is 'in progress'. There is also a 'Nova' Frequency Changer (solid-state), 240V/50Hz in, 208/115V 400Hz @ 1Kw out. This was sidelined for a number of years due to its RF hash-generation tendencies, but after a recent intensive noise-suppression campaign is now the primary AC source..20 Notes: (1) 400 Hz is the nominal AC frequency. Most equipment is specified for operation between 380 to 420 Hz. (2) Some equipment is specified for use on aircraft with "frequency wild" AC power systems, where the frequency may vary between 320 and 1000 Hz. (3) Although 115V 400 Hz is the generally accepted standard, other voltage/frequency combinations may be encountered, e.g. 80V 1600 Hz, 115V 800 Hz. (4) Static inverters are available with sine wave or square wave output, sometimes both.20 (5) Disadvantages of 400 Hz power are a significant line impedance (not a problem on the aircraft scale) and a pervasive 400 Hz background whine. The latter is particularly annoying when it gets into the audio. (6) The means whereby an engine operating at varying RPM can drive an AC generator at a fixed output frequency is technically fascinating, but sadly, outside the scope of this (necessarily brief) article. ed) My thanks to John for his much needed contribution. And to the area of aircraft equipment specifically as there has been an obvious neglect of our/my attention to this field. Typically, a 100 watt, dynomotor driven transmitter can draw as much as 135amps for a split second until the thing gets spinning. This can spell immediate death to a solid state regulated power supply, even one capable of relatively high current output. One remedy used by an old friend is to strap about 1 farad(yes that's a bunch) across the output terminals of his 40 amp regulated supply. The capacitors start the dynomotor spinning, & once this is done, the current needed usually drops to a manageable 35 amps. This system has yet to fail with any load it's been used with. High current supplies were also available for such power mongers as the T-195 for operation from AC mains. This was a monstrous thing, unregulated, & had a no load output of over 60 volts. If the companion R-392 were turned on before or without the transmitter, it would be instantly fried because it's low current drain could not pull down this extreme no load voltage. Many early supplies will act in the same manor so use extreme caution, & common sense when trying to utilize them. Dennis *****************************************************************

RADIO COMPASS AN/ARN-6; By John Mackesy Dennis, I can just hear you saying "Shit! Aircraft crap!" Oh well, it's a heavy habit, why not share it around? ------------------------------------------------------------------

Back in the '60s - does anybody remember that far back? - my colleagues and I spent many happy hours flying low and slow above the North Atlantic. These were the days of the Cold War, a chapter of our history which today seems rather tinged with absurdity. My role in this madness was servicing Anti-Submarine Warfare (ASW) aircraft. These aircraft (sadly, now all gone to their reward) patrolled 'the Pond', maintaining the peace. Like so many of our doings of the past, it all seemed to make a certain amount of sense at the time. Our rather large, comfortable and well-equipped aircraft bore a wide range of electronic equipment, used for offensive, defensive, navigation and - well, other purposes. You wouldn't want to get lost over the 'Pond' (particularly when pursuing 'other purposes'), so the navigational equipment included things like LORAN and a couple of AN/ARN-6 Radio Compasses (obsolescent even then). It could be rather boring droning along in the darkness above the (curiously remote) sea. One of my occasional diversions (apart from reading, drinking coffee and making toasted turkey sandwiches) was roaming up and down the broadcast band on the Radio Compass. What a great performer! I could hear stations on both sides of the Atlantic, all through the Caribbean and beyond. Three decades further on, I have my own AN/ARN-6 Radio Compass, and it's still a great performer. As a military collectible, the ARN-6 has a lot going for it. Let me quote the opening paragraph of the ARN-6 manual, USAF T.O. 12R5-2ARN6-2, issued 4 August 1950. "1. GENERAL PURPOSE.- Radio Compass AN/ARN-6 is an airborne navigational instrument. It is designed smaller and lighter than other automatic radio compass equipments for the purpose of using it in small aircraft." As a restoration project, especially for the beginning military collector, the ARN-6 is definitely worth considering. 1. Parts are readily available. 2. It was widely used. 3. Needs only 26.5 VDC. 4. Covers the broadcast band. 5. Reliable and easy to work on. 6. Relatively compact and lightweight. Now that I have your attention, we'll look at the interesting aspects of the ARN-6. First the name... 'A' stands for 'Airborne', 'R' for radio, 'N' for 'Navigational. -6? That's a number suggesting that it was the 6th of this type of equipment accepted into this series. As the name would imply, the device uses the directional properties of radio signals to aid in navigation. It does this by exploiting the directional characteristics of a LOOP aerial. As you may recall, such an aerial will exhibit maximum signal strength (PEAK) and minimum signal strength (NULL) characteristics dependent on its orientation to the signal source. This allows the operator to home on a signal, or to derive a position by triangulation. Back in the old days (before about 1941), the directional loop aerial was driven by hand, indicating its bearing on a mechanical device looking much like a compass. This was a fairly slow and complicated business, requiring a fair degree of operator skill for best results. One of the traps of this system was the dreaded 'Reciprocal Bearing Anomaly' - is the station ahead of or behind the aircraft? 'Radio Compass' was an appropriate title for these devices. Time and technology marched on. It was realised that it was quite possible, even desirable, to automate the process of orienting the loop towards the transmission of interest. Electrical means of indicating loop position also allowed much greater freedom in equipment layout, with the additional benefits of reduced weight and maintenance. In this way, the simple 'Radio Compass' became an 'Automatic Radio Compass' - somewhat more complicated, but much more useful. Best of all (through use of a non-directional "sense" aerial and a bit of extra circuitry), it didn't suffer from the reciprocal bearing problem. In more recent times the term 'Radio Compass' has been largely replaced by 'Auto Direction Finder', commonly contracted to 'ADF' RECEIVER So much for ancient history. Let's talk about the ARN-6, more specifically, about 'Receiver R-101/ARN-6'. Like most aircraft electronic equipment, it presents as a compact, somewhat featureless 'black box' - all the interesting stuff is inside. Frequency coverage is 100 Khz to 1750 Khz (in 4 bands), it uses 16 octal tubes (2 of which are thyratrons), has 26.5 DC on its plates, electric motor-driven bandchange, and much, much more. As a receiver, conceptually it isn't very different to the more familiar domestic radio - it's just that there's more of it, and it's built to much higher standards, from both the hardware and performance angles. A single conversion superheterodyne, it has 2 RF stages (12SK7s), a separately excited mixer (12SY7), oscillator (1/2 12SX7), 2 IF stages (12SK7s) and a detector/1st audio stage (12SW7). Audio output is provided by a pair of 26A7GTs in push-pull parallel, driven by 1/2 of a 12SX7GT. The other half of this 12SX7 is used as a tuning meter amplifier. Half of another 12SX7 is used as a BFO. The tuning capacitor is a 5-section type, driven by spring-loaded anti-lash reduction (120:1) gearing. Tuning drive is by the usual flex drive cable, cables used on previous Bendix equipment also fit ARN-6. Electric servo tuning has been used on a few installations (C130A, RA3B are two I'm aware of), typically when the receiver is located in an unpressurised area of the aircraft, or where very long control runs are involved. The IF signal path is somewhat unusual in that there are 2 IF channels, IF channel switching being integrated into the bandswitch system. Band 1 (100 to 200 Khz) uses a 142.5 Khz IF, bands 2-3-4 a 455 Khz IF. An "IF Trap" is fitted to prevent feedthrough on the IF frequencies. The bandswitch motor drive mechanism is a work of art (all ARN-6 gears are stainless steel!), and has to be seen to be believed. Despite this, it's relatively simple to both comprehend and remove. COMPASS OPERATION In 'compass' mode yet more circuitry is brought into play. There is a 12SK7 loop amplifier stage (for the directional loop aerial), a 12SX7 modulator, a 12SK7 compass amplifier, 1/2 of a 12SX7 used as a tone oscillator, and finally a pair of 2050 thyratron tubes associated with the loop aerial servomotor system. Thyratrons need a substantial AC plate voltage to operate; this is supplied by a vibrator/transformer arrangement, which also produces AC for the autosyn loop position transmission and indication system. OTHER PARTS OF THE SYSTEM As is usually the case with aircraft radio equipment, the ARN-6 is remotely controlled. There are several different types of controller used with ARN-6, but all are conceptually similar. Tuning is via a flexible drive cable, bandswitching is electric; all controls (and the tuning meter) are located at the control box. The wiring loom from the control box contains 20 wires. LOOP AERIAL AS-313/ARN-6 This is a compact servo-operated directional loop aerial sealed into a nitrogen-filled glass-topped housing. There are 2 connecting cables, containing 7 & 3 wires respectively. It should be noted that the loop to receiver wiring forms part of the tuned circuit of the loop input stage. This means that the L & C of the connecting cable should be to the required spec. for best performance, although in practice this doesn't seem to be too critical. Just use at least 10' of shielded wire. LOOP INDICATORS 2 types are used, the ID91*/ARN-6 (3 1/4" dia.) and the ID92*/ARN-6 (5" dia). 4 wires are required for the position indicator input; several indicators may be wired in parallel.20 MOUNT MT-273*/ARN-6 This is where everything comes together. The receiver slides into this mount (also known as a "rack") and is retained by catches at the front of the mount. Connection to the receiver is accomplished via a 22-pin female plug on the mount engaging matching male pins on the rear of the receiver. In the bottom of the mount, accessible via a removable cover plate, is a forest of screw terminals. These connect the mount to the control box, loop, indicators, and DC power source. Audio output is also available. It is possible for one ARN-6 receiver to be controlled from two positions, typically pilot and navigator. Mounts intended for dual-control installations incorporate rotary solenoid switch to make the necessary connections. A complete ARN-6 system weighs approximately 60 lb, and requires 22 - 30 VDC at about 4.0A. BUILDING AN ARN-6 SYSTEM Probably the most daunting aspect of assembling an ARN-6 systems is the wiring to/from/inside the mount. The wiring loom to the controller contains the bulk of this, but there is still a considerable amount of wiring to do. Cables to the loop, indicators (and certain types of control box) are terminated with MS 3000-series (Cannon plug) connectors. Although tedious, the wiring is not particularly complicated. The secret is to have a detailed plan to work to, and to check off details as you go. All wiring should be checked, double checked, then checked as a complete system. Once this has been done you'll be able to find out if the thing works or not! Assuming your wiring contains no serious blunders, the system should function as a receiver - receiver-mode problems are rare. Audio quality from the push-pull parallel 26A7GT varies from tolerable to awful, due to audio filtering in this stage. Taking the AF output from the 1st audio stage and using an outboard amp is recommended. Loop and compass-mode problems are more common, and may require some hair-tearing to solve - but that's a story for another day. Notes: Autosyn 3-phase AC position transmission and indication system. 12SK7 Metal Octal-based remote-cutoff RF pentode 12SX7 " " " twin triode (similar to 12SN7) 12SY7 " " " pentagrid converter (sim. 12SA7) 12SW7 " " " dual-diode-triode (sim. 12SQ7) 26A7GT Glass " " twin beam power pentode 2050 Glass " " thyratron ARN-6 (1950 onward) replaced the earlier ARN-7, which entered service in 1945. This was the successor to the very similar SCR-269. SCR-269 & ARN-7 were heavier, bulkier and required 115V 400 Hz AC power. A Bendix design, ARN-6s were also built by Magnavox. There is also the ARN-44 version of the ARN-6, which tunes HF frequencies with 4 bands. JOHN MACKESY VK3XAO *********************************

The AT5/AR8; From Australia, By Steve Hill The AT5/AR8 was designed by Amalgamated Wireless Australasia (AWA) according to a Royal Australian Air Force (RAAF) specification. This company (based in Sydney) was a radio pioneering company, and was responsible for a large proportion of Australia's radio production during World War 2. The RAAF specification called for a general purpose 50 watt air/ground transmitter reciever, also to be used for D/F and homing applications. What resulted was a set that was used in a huge variety of applications by all three Australian services, and also by foreign services. The set itself consists of three main units and power supply. The transmitter, receiver and ATU are in boxes each roughly 12x13x10 inches. Power is derived from a separate power supply. The two power supply types were the type G and the type S. The type G runs on either 12V or 24V DC. HT is derived from two motor generator sets. The type S power supply runs on 240V AC, and uses 866s to make transmitter HT and a selenium rectifier for reciever HT. HT is 550V DC for the transmitter and 250V for the receiver. A voltage divider is used to make 300V minor HT for the transmitter. A vibrator power supply is also known to exist which could power the reciever. A junction box was used to interconnect the control signals from the various units. Other units which could connect via the junction box were a remote control unit and a pulse generator. Connections for an intercom system with up to five telephones are provided on the junction box. The AR8 receiver is a MF/HF superheterodyne receiver, and covers the ranges 140-740kc, 765kc-2Mc, 2-20Mc. The unit is fitted with connections for a loop antenna, and can perform DF operations on the MF bands in both AM and CW modes. The receiver consists of two separate tuning sections, one each for the MF and HF bands, mounted next to each other in the upper part of the receiver. The front panel of the reciever is dominated by the two round vernier dials which AWA used in much of its equipment. These tuning units feed into a common IF/AF unit. The front panel has all controls for DF operations, as well as band switches for the MF and HF bands. The MF and HF bands are each split into three sub-bands. Tone and volume controls are provided, as is a BFO pitch control. A primitive front end protection system is also included. The receiver is normally muted by the keying relay which opens the cathodes, but this can be overidden to allow the monitoring of sent signals to be heard. The AT5 transmitter also covers the MF and HF bands. In HF it consists of a 6V6G as oscillator, another 6V6G as the modulator/sidetone oscillator, an 807 as the buffer amplifier, and two 807s as the finals. In MF the HF buffer amp 807 is used as the master oscillator. The transmitter has a VFO, or can operate on one of six crystal positions. Keying is achieved by grounding the cathodes of the valves, which is done by the keying relay. The bandswitch selects harmonics of the oscillator, thus highest power will be achieved on the fundamental frequency (about 50watts). A switch allows the rf signal to be connected through to the ATU, when the switch is off, a resistor is added to the screens of the finals allowing tuning of the transmitter itself to be performed at low power. When the transmitter has been tuned, the rf is switched to the atu, also increasing screen voltage so that full power is output. The atu is designed to handle a random wire on HF, and a trailing wire antenna (longwire?) on MF. A rather large variometer and tapped coil dominates the unit, taking about half the space inside the atu. This is used for MF tuning. The tuning procedure involves first selecting whether parallel, series, or direct coupling is required. Control signals from the MF/HF switch on the transmitter determine whether the MF or HF tuning section is used for tuning. The transmitter can then be tuned by using the antenna tuning and coupling controls. An aerial ammeter gives a tuning indication. The set saw widespread use with the RAAF. I know for certain that it was used in Hudsons, Beaufighters and the early Catalinas. The RAAF also used the set for ground-air control, and in many other ground based applications. For example; No 1 Air Support Unit, Madang ADHQ control tower, 89 OBU for point to point communications. Many larger units used the AT5/AR8 as backup equipment. For example ADHQ Morotai had 4 units for emergency use. The RAAF not only used them in the air and on the ground, but also at sea. The RAAF crash launches used the AT5/AR8. Service was not restricted to the RAAF however. The army version was known as the 112 set. I know very little as to the army use of the set, but I do know that it was used by 9th division signals. It saw applications mounted in radio trucks and also mounted in the back of jeeps. The Navy also used the AT5/AR8. HMAS Diamantina had one as an emergency set in her emergency radio room. I can only assume that this was the case on many other ships, and that the set was used as a primary set on smaller vessels. Many a radio mechanic cut his teeth on the AT5/AR8. One person I spoke to trained on them, and was required to draw the circuit from memory as part of the training. The AT5/AR8 also saw service with the US in a modified form. It was known as the TW12, and was basically a transmitter and ATU mounted together. I believe it was HF only, but can't be sure. Another US version was the AMT150, but I know nothing about this set. The AT5/AR8 also saw service with the British forces, but I don't know if this was official or not. I do know that an RAF unit procured a set to go into a truck as a homing beacon from their Australian counterparts. This was done by swapping it for a peice of their own equipment. After the war, as with other types of equipment, there were large amounts of radios that weren't required. Many AT5/AR8s were used by hams in the fifties. Modifications were available to make them more ham friendly, such as changing the keying system to grid block keying. I have also been told that they were used on commercial airplanes after the war. I have one example of the AT5/AR8, which was my first ever, on the air rig. Initially I used it aboard HMAS Diamantina. When I got my ticket I brought it home and still use it on the air occasionally. I didn't have a lot of trouble getting it going, just an open circuit resistor in the transmitter's oscillator HT line. It uses the 26V type 'S' power supply with two MG sets, and draws about 15A on transmit. I've found the receiver to be quite ordinary. Its not particularly sensitive, and the tuning is too coarse. It drifts for an hour or so after being turned on. I use another receiver whenever using the AT5 (an Admiralty B40 reciever, 1946 vintage), and use the antenna changeover relay in the atu. I don't know how the reciever would be if I serviced it. It's still full of paper caps, and I haven't done on work at all on it. The transmitter is quite good. The VFO is very chirpy, so I have to use crystals which minimizes this tendency. It is only slightly chirpy with a crystal. I've put a zener on the oscillator HT supply which has slightly reduced chirp. I believe the chirp happens for several reasons. The oscillator does not have sufficient isolation. The motor generator set slows down every time you hit the key (ie voltage changes). The oscillator HT is derived by a resistor divider, that is when key up the voltage rises. When the cathode is grounded on key down the voltage has to fall to the 60V ht supply. Thus every time the key is pressed, the main HT voltage falls (mg set), the voltage on the anode falls due to the resistor divider, and the oscillator is pulled. I tried to alleviate this by running the oscillator and buffer amp all the time, and only keying the finals. This removed most of the chirp, however it caused the metering to be interfered with, and running the oscillator tends to deafen the receiver. Thus I use the rig as it was intended but with a crystal, not the VFO. According to an RF power meter I get 45 watts out on 7mhz. I've worked the US using the rig when it was aboard Diamantina. My biggest challenge with the rig came recently. I'm relatively new to radio, and missed out on the valve era by a long shot, so I'm a bit lost when it comes to repairing this stuff. Well, one day I thought I'd give the old AT5/AR8 a run. Fired up the MG sets as usual, put the thing into tune mode. Went key down and the pa anode current meter deflected. Tried to tune it, nothing happened. Hmmm, why is there smoke billowing out the back of the transmitter! The problem I had with trying to fix it (a problem I still have with other broken rigs) was not knowing where to start. I eventually bit the bullet, and decided firstly I'd pull all the tubes, and measure the HT volts. Did this, volts looked OK, smoke still billowing from transmitter. Of course you seasoned valve techs have probably guessed what the problem was by now, but not me. Don't forget this is the first valve repair job I've ever done. OK, I thought. Lets start from scratch and see if the oscillator is working. Put the valves back in (heaters are in series so it wont work without all valves - BTW we don't have toobs in Australia), but no HT on the 807s. Turned a convenient receiver on, and there it was. OK, so the oscillator works, and now I'm stuck again because I don't know what to do next. In desperation I get the transmitter down on the floor so I can see right into the guts of it. After much peering, there it was. A resistor with rather charred looking plastic cover around it. Turns out that this was the screen resistor on the 807s in the finals that's used to drop output power when tuning. The problem, of course, was that the paper screen bypass capacitor had failed. I replaced the capacitor (or condensor as they were called back then) and the set started working. I suspect that resistor has gone a bit high because PA anode current is lower than it used to be when tuning, but its not used when operating so it doesn't really matter. This looks like perhaps a days work, but it actually took me three months to finally find the problem and fix it. I've had a lot of fun with the AT5/AR8. I love to get a bit of history on the air, and there are plenty of hams who are WW2 veterans who used this rig. I have been privelaged to work some of them. They get a real buz hearing their old rig on the air, and so do I. I've no idea how many AT5/AR8s are on the air in Australia. Mine is the only one I know of for sure, but there surely must be others. I just hope that rigs like this can stay on the air for many years to come. I'm doing my best to make sure this one does anyway. Steve Hill VK4CZT 39 Banbury St. Carina. 4152. Brisbane. Australia. -----------------------------------------

(Ed) is it possibly the TW-12 Steve mentions in the text is actually the Bendix TA-12? ********************************

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