HRO - Barry Williams

The Evolution of the National HRO and Its Contribution to Winning World War II

by Barry Williams, KD5VC

copyright 2004 Barry Williams

HRO Models
Table ModelsRack Model Equivalent
HRO - 1stHRO Senior - R 1st
HRO Senior - 2ndHRO Senior - R 2nd
HRO Senior - 3rdHRO Senior - R 3rd
HRO Senior - 4thHRO Senior - R 4th
HRO - Jr. 2nd
HRO - Jr. 3rdHRO Jr. - R 3rd
HRO - Jr. 4th
HRO-M JuniorHRO - MRR Junior
 RBJ (Could be Table also)
HRO - 5THRO - 5R
HRO - 5TA1HRO - 5RA1
HRO - 7THRO - 7R
HRO - 50THRO - 50R
HRO - 50T1HRO - 50R1
HRO - 60THRO - 60R
HRO - 500
HRO - 500P
HRO - 600


The National HRO in its many versions is extrremely popular with collectors of communications receivers. Because of its unique design features and its critical role in helping the Allies win World War II, it deserves a special place in radio history. Much has been written about the history of the Ntional Company and its memoriable receivers. John Nagle's article about the history of National in voume 1 of the AWA Review and Lawrence Ware's article about National's coil-catacomb receivers in volume 11 are two good examples. The AWA Old Timer's Bulletin and QST and other ham publications have also featured articles about National receivers from time to time, with the HRO series being a particularly popular topic. This article documents the evolution of this memorable receiver and should help collectors identify the subtle variations among the many different models.

National published one advertisement showing the following as dates that these receivers leading up to the HRO were first available:

SW-3 - 8, 1931
AGS - 7, 1932
FB-7 - 2, 1933
SRR - 4, 1933
HRO - 1, 1935

In addition the first ads for the following HRO models appeared on these dates in QST magazine:

HRO-Rack Mount - 7, 1935
HRO Junior - 2, 1936
SPC - 12, 1936

At the time of the HRO's introduction in 1934 National already offered the AGS, probably the world's best high performance shortwave super heterodyne at that time, and the reduced cost model FB-7. Hammarlund, the other radio manufacturer involved in high performance super heterodynes offered the Comet Pro. The FB-7 and Comet Pro did not have an RF amplifier or pre-selection stage, although external pre-selectors were available. A few years after the introduction of the HRO, Hammarlund introduced the Super Pro, another long lived design quite competitive with the HRO.

The National Company AGS was developed jointly by government and National engineers. Operating one of these receivers next to an HRO shows that the AGS has several shortcomings that were corrected in the HRO. The first of these was the coil plug in system. The AGS required the operator to find the matching three coils and put them individually in the receiver in the correct order. The lack of a B+ switch required the operator to either turn all power off to the receiver, or to try to change the coils with B+ applied. Under typical operating conditions this was probably not serious as quickly changing frequency bands was not usually required. The second problem was that the tuning system is very fast and quite touchy. The dial used a vernier scale allowing resetting to one part in 1500. The movement of the main tuning knob required to move the dial by one step is about .010 inches. The dial mechanism and the use of rack and pinions to drive the three variable capacitors meant that everything had to be freshly lubricated to allow smooth movement otherwise the system moves in small steps which are difficult to control. The last major problem was that the available improvement in performance due to electronic design, primarily driven by the new vacuum tubes and the interest in higher frequencies, was great enough that the AGS needed a major electronic design update after only 2 years of production. It is notable that the AGS went through at least 4 different tube line ups during these two years.

National determined that using different manufacturing techniques, state of the art electronics, and improvements in the mechanical design could best be addressed by a new receiver rather than an update of the AGS. National did attempt to address the coil system with a late modification which was called the AGU. The AGU attached all three coils to a single plate, forcing the matching coils to be inserted together in the correct order.

The First HRO Appears

The National Company announced the HRO in October 1934 with deliveries to be made in the later part of 1934, actual first shipments of the new receiver were made in March 1935. The stated reason for the late delivery was that the stability was not good enough given the precision allowed by the new dial mechanism. This was primarily corrected prior to shipment of the first units. Two groups working across the country from one another designed the radio. The mechanical group which included Graden Smith was lead by James Millen, and the electrical group which included Howard Morgan was directed by Herbert Hoover Jr. The impetus for the new design was to provide a high performance super heterodyne to meet the market niche previously filled by the AGS as well as a new requirement from the Bureau of Aeronautical Commerce, early predecessor of the CAA and FAA. The new specifications required a single module to change coils if such coil changes were required and improved performance at 14 MHz. and beyond. The HRO added a second RF stage, grouped the coils into a single plug in module, and was designed to not require an engineer to align each unit. The new electrical design broke no new ground, but remained state of the art for quite a few years. The mechanical design, however, was novel and excellent. The easier assembly along with a reduced cost metal cabinet allowed the HRO to be sold at a price similar to that of the AGS. The name of the new receiver came from a note Millen had put on the engineering drawing "Hell Of Rush". This model was known by the initials HOR until National decided that the mnemonic was not acceptable. The story was actually changed to state that the original note was "Hell of a Rush Order" to cover the original designation. Millen set the story straight in an interview many years later.

The HRO is a high quality super heterodyne with two tuned RF circuits, tuned mixer, electron coupled oscillators for the high frequency and beat frequency, two IF stages with six tuned circuits and a crystal filter. The electrical design was similar to James Lamb's 1932 "Single Signal Super Heterodyne", and the crystal filter was very much like the one he described in QST some time later. The controls on the front panel included AF and RF gain, crystal filter Selectivity and Phasing, BFO, AVC On/Off switch, S-Meter switch, B+ switch, and the wonderful micrometer dial and precision worm drive mechanism. The BFO was adjustable from the front panel and had markings of "Off", and "0" through "10". "0" on the knob indicated a BFO frequency about 5 KHz. above the IF frequency of 456 +/- 2 KHz. The IF was aligned to the actual crystal frequency, and National apparently allowed a variance of +/- 2 KHz. in the crystal frequency. Zero beat was approximately "8" on the knob and "10 indicated a couple of KHz. below the IF frequency. Moving the knob clockwise from "0" activated the BFO, by closing a switch operated by a pin on the BFO capacitor shaft. Because of this each time the BFO was turned off the operator had to reset the frequency the next time it was used. The B+ switch allowed changing of the coil modules without the possibility of shorting B+ to ground. In the standard HRO with A.C. power supply there was no fuse protection, so you could easily damage a resistor, choke, or power transformer by shorting the B+. The crystal filter did not allow placing the crystal in parallel like on the AGS-X, but only in series. In production a National manufactured plug in crystal was placed on top of the phasing unit. These crystals could be easily disassembled and a periodic cleaning with carbon tetrachloride was recommended by contemporary publications.

The micrometer type main tuning dial was an epicyclic unit with an outer dial marked with 50 graduations and an inner dial marked with the numbers from 0 to 500 in steps of 10. These numbers were visible through five windows in the outer dial. The inner dial was held off center and engaged the outer dial at the bottom position with a set of gear teeth. The inside of the outer dial had matching gear teeth, but a few percent more teeth so that the inner dial moved through a full set of steps for each ten turns of the outer dial. A micrometer dial was patented in 1934 by William Harding of London, an employee of Sperry Gyroscope Company (British patent number 419-002 issued 5/11/1934 titled "Improvements Relating to Angular Motion Indicators"). Graden Smith also received a US patent in 1936 (US Patent 2,060,537 issued 11/10/1936 titled "Indicating Device") for a micrometer dial which appears to be very similar to the Harding patent. National's Graden Smith definitely contributed the precision worm gearbox and capacitor mounting assembly, a work of engineering art and heart of the precision attributed to the HRO. The 20 to 1 ratio gearbox was designed by Smith to eliminate backlash. The end result of this combination is a smooth, large tuning knob with 500 widely spaced graduations allowing resetting the tuning capacitor to within about one part in 2000 or better. This precision was not surpassed for 20 years. The operator had to convert the reading on the knob into frequency by looking at a curve on the front of the coil module; this only gave a general indication of frequency. However, making a note that BBC on 5975 kHz. is at 334.5 on the dial (for example) allowed resetting to the BBC with far greater ease than any competing system. The later military versions shipped to the UK either came with individual calibration booklets from National, or booklets were created in the UK, giving the dial reading for each kilohertz. These were not graphic, but columnar and at least some of these had the National emblem.

National believed that a really top performing receiver had to have optimized plug in coils and an external power supply. The external power supply provided filament voltage of 2.5 or 6.3 volts as well as approximately 240 volts at 70 milliamperes for B+. The radio would work well with the correct filament voltage and a B+ of 160 volts or greater. We often measure the old power supplies producing 300 volts under no load as the power supply did not have a separate bleeder. The original "Dog House" power supply ran very warm. It is difficult to add much bleeder current as the transformer will overheat. When the B+ is turned on there is an immediate drop in voltage caused by the internal voltage dividers. National states in several documents that if you are using B+ batteries, the B+ switch should be turned "OFF" when the receiver is not being operated. If you turn off the filaments you will still drain power from the B+ battery through these voltage dividing strings.

The HRO Coil System

The coil modules used in the HRO were unique. They had two frequency ranges for each unit. In one mode the coil covered a wide frequency range (General Coverage), in the other mode (Band Spread), serial and parallel capacitors were added to reduce the coverage to approximately the width of an amateur band. The mode was changed by moving 4 shorting screws from one position to the other on each of the modules. The band change module consisted of 4 coils, the leftmost was the first RF coil, next came the second RF coil, then the mixer tuning coil, with the high frequency oscillator coil being on the right side. There were 4 coils supplied with the receiver designated as A, B, C, and D, which covered from 1.7 Mhz, to 30 Mhz, in the general coverage mode, and in the band spread mode they covered the 80, 40, 20 and 10 meter amateur bands. These modules slid into the bottom front of the receiver where temperature variations are minimal. Each coil cover had a molded mica-filled phenolic insulator that used the trademark R-39. Apparently this was developed in 1929, and National initially called it R-29, and then determined not to define it by the year it was introduced, so it would not later appear to be an old technology. The competing phenolic resin insulators available at that time were filled with hygroscopic material such as sawdust, cloth, or paper. This property caused the cheaper insulator's characteristics to change with changes in humidity. Each of the R-39 insulators on the National coil had 5 contact points. These insulators are stamped with the coil range and the position of that coil on the module. The designators were the letter of the coil followed by the position. For example the A3 insulator should be in the third position from the left (mixer tuning), and it should be in the A coil, which covers the 14 to 30 MHz. frequency range or the 10 meter amateur band. Later the RAS coils used the designation letter and a number between from 5 through 8, and the RBJ and possibly the RAW used the designations 9 through 12. The contacts on the insulators were placed into contact with matching brush blocks in the receiver when the coil module was pushed in by hand. There were matching handles on the front of the coil and the front of the receiver to ease this operation. The front plate of the coil was 3/16" thick aluminum, finished in either black wrinkle or gray paint. On this front panel were two frames with celluloid protected openings. The left frame contained the general coverage curve showing the frequency vs. dial reading, and the right frame contained a similar curve for the band-spread mode. These scales were placed on the aluminum front plate of the coil then a celluloid cover was placed on top of the scales, then a steel frame was screwed into place by four small sheet metal screws. Additional coils over the standard A, B, C, and D were available to cover the frequencies from 480 KHz. to 2.0 MHz. and 50 KHz. to 430 KHz. These were designated the E - .960 to 2.0 MHz. F - 480 to 960 KHz. G - 200 to 430 KHz., H - 100 to 200 KHz., and J - 50 to 100 KHz. The E and F coils are popular today since they cover the AM broadcast band, with the E coil also covering the 160 meter amateur band. The lower frequency coils were primarily for special or laboratory use. These additional coils had only one mode, that of general coverage, so the screws to change mode on the standard coils were missing, and the right frame fitted over a ground celluloid cover on which the names and dial readings of stations could be penciled in. The original documentation only mentions the A through D coils as standard and the E and F coils as being available, apparently the others being added later. The later military coils did not have the band spread capability of the early coils made for the HRO Senior. Most of the standard military receivers ordered for use in the UK came with coils A through E, although to indicate that they lacked the band spread capability they used the prefix J, so that the military coils were JA through JE. It is interesting that there is no difference between the E coil and the JE coil, some coils identified as JE coils were shipped with the post war HRO-5TA1.

Early Production HROs

We should note that National used two different pre-production models for pictures in their advertisements. The first of these had the same small knobs as used of the AGS, tube shields that we don't see in any other later National products, and the controls reflected the AGS lineage. This included the single gain control and the serial or parallel connection for the filter crystal. The second prototype is shown without a cover, and the front view shows a knob on the top of the BFO can, something like that used in the FB-7. In addition this model has an attachment for the cover similar to that used in the later rack models, which used a special cover with studs extending through the front panel to be secured by brass thumb nuts. The actual early production model used the two metal bar inserts to hold the cover in place, even on the rack models which included a full case with hinged top cover and a cover plate screwed to the bottom of the receiver.

The production HRO was available in two models, one receiving power from an A.C. power supply and the other for battery operation. At the time it was felt that the best performance from an A.C. supply required the use of 2.5 volt filaments. For 2.5 volt A.C. power the receiver used 3 type 57 tubes, 4 type 58 tubes, a 2B7, and a 2A5. The 6.3 volt versions for battery power used 3 type 6C6 tubes, 4 type 6D6 tubes, a 6B7 and a 42. The 57 or 6C6 sharp cut off pentode was used in the high frequency oscillator, the beat frequency oscillator, and the mixer. The 58 or 6D6 remote cut off pentode was used in the two RF amplifier stages and the two IF amplifier stages. The 2B7 or 6B7 was a dual diode pentode combination tube used for AVC and Audio detection and a first AF amplifier. The 2A5 or 42 pentode was used as the final AF amplifier. These tubes running in Class A produced a couple of watts of audio. The tube sockets were ceramic for the high frequency tubes including the RF amplifiers, HF Oscillator and mixer, and phenolic for the lower frequency tubes except the detector/first audio which was also ceramic. We should also note that National recommended 2.8 volts at the power supply for the 2.5 volt filaments as the wiring and power cable caused a voltage drop of about .3 volts.

Identifying the Early HROs

The best way to discriminate between the major changes, or models on the commercial pre World War II HROs is by serial number. The serial number is stamped into the chassis near the antenna connectors' insulator. National began production of the HRO with the serial number D1. The D was like the batch designation and the following number was the number of the particular radio in that batch. The D series may have been only 100 units; the following series were usually about 250 units. The batch designators A, B, C, I, O, appear not to have been used, giving a total of something over 5,000 receivers. When the end of this serial range was reached National began using the batch number as a suffix instead of a prefix. The last known of the early models is 169L.

Pre-World War II HRO Models

I have divided the early HRO into four models. Actually there were changes being gradually made throughout production of the HRO. So there will be many "transition" models including some features of one model and some of the next model. In general the first model was the unit first put in production, the second model accumulates the changes made up to the G or H production run. The third model includes those changes adopted by the P production run, and the fourth model goes from the X production run to the end of the plug in crystal models.

The first model HRO is different from the units generally seen in several ways. The main tuning knob was polished and the graduation dial (internal to the tuning knob) had black lettering on a bright metal background. The main tuning pointer had a red "NC" engraved on the diamond, and there was no "Pilot" light to indicate that filament power was being supplied to the radio. The S-Meter had 5 divisions, similar to the "R" readings that were used to represent signal strength in that period. The S-Meter was not in circuit all the time, so a momentary contact pushbutton with a white button operator was used. When the operator wanted to make a reading he would insure that the AVC was ON, press the button and take the reading. This was not popular, and the buttons were electrically noisy. The knobs on the early HRO were of the bar and skirt design, but the transition from the knob to the skirt was shorter than that used later. This model lacked an external means to disable the B+ for muting purposes when used with a transmitter. The vent openings were quite small; only four rows of 1/4" vent holes in the rear of the cabinet. The IF coils for these receivers were housed in round shield cans, and the chassis was painted black. The A.C. power supply marked "For FB-7" was shipped with some of the earlier units. Speakers were not advertised at this time, but the rack-mounted speaker was sometimes sold with the early HRO. These included an open 8" speaker in an 8 3/4" panel. There may have also been some 10 1/2" panels with 8" speakers supplied with the AGS and possibly the first HROs. There is a picture of a rack mount first model HRO as early as the July, 1935 issue of QST magazine. We also have a first model rack mount with serial E124. The ads indicate that an engraved front plate was available which mounted over the front panel. These actual units have a front plate that replaces the normal front panel. The serial numbers for these units consisted of a letter followed by 1 to 3 numbers. We believe that the first production model included serial numbers D1 through D100 and E1 through E250. These receivers generally used fixed capacitors from an outside supplier, including sealed electrolytic, paper and wax, and mica types, and resistors manufactured by National. These resistors had white bodies with hand-lettered values, unfortunately many of these have been replaced in the surviving receivers. Apparently the connection between the end cap and the resistance cartridge can become intermittent and the resistance values drift with age. The coil modules used in the first model HRO have scales with white backgrounds and black calibration numbers, grid and curves. By the F1 serial the changes to the second model were being incorporated, most notably the S-Meter switch was changed from the pushbutton to a push pull switch, and the scales were being changed to white lettering over black background. It has been stated that perhaps this was caused by glue stains through the light colored paper. We have checked quite a few early scales without finding any evidence of glue.

The second model replaced the push button with a push pull switch, the S-Meter had 9 divisions to mid scale, the case had much larger ventilation holes, a pilot light was added and a terminal block was added to the rear apron to allow the removal of B+ from the receiver. The two screws on this block need to be shorted for the receiver to operate. The remainder of the receiver was like the first model, with the change to the longer transition between the bar and skirt on the knob occurring during this model run. There were additional changes to the main tuning condenser insulators, and some changes in components suppliers, National no longer supplying the hand lettered resistors. These receivers include serials starting with F1 and ending around P1. We noted on a rack mount HRO with the serial N31 a combination of second and third model features, a gray painted chassis with round IF cans. Apparently the change to square IF cans being made by the P series.

The HRO Junior was introduced during this production run. It was a lower cost version with the crystal filter and S-Meter eliminated and including only one coil that worked only in the general coverage mode. At this point you will also notice the beginning of the designation HRO-Senior for the original HRO. One fairly sure way to determine if a model is a Junior is that the Junior tuning capacitors had tabs on the front of the capacitor bank which could be bent to adjust the receiver to the standard coil. These are visible when looking down just behind the front panel.

The third model of the HRO introduced the lighted S-Meter, the chassis was painted gray instead of the original black, and the IF coil shields were square. There were additional changes in the tuning capacitor insulators, and the cooling vents on the cabinet ends were added. The serial number for these radios range from P1 to W250.

The primary change in the fourth model was the inclusion of the ID tag on the front panel. The insulators on the main tuning capacitor have reached their final form, and on most HROs of this era, the plain rectangular knob replaced the round selectivity knob. These serial numbers extend from X1 to the end of production.

We noted above that there are several prototypes pictured which were quite different from any production models, and should also note that National did advertise at least one variation that was never produced, the HRO with internal power supply. Rack mount versions of the later receivers were available at a slightly higher cost (about $20 difference). The rack mounted receivers were very similar to the table mount HROs. The top cover of the cabinet did not have the raised spot in the middle to allow raising the lid without damaging a fingernail because this would interfere when sliding the unit into the rack. The front panel of the rack models was 3/16" thick aluminum plate with engraving to indicate the function of the B+ and AVC switches, engraved pointers for the other controls, and "Selectivity" engraved above that control. The finish on the rack panels was an alligator texture instead of the wrinkle finish used on the table models. We should note that a gray version of the rack mount system was available in the later models.

The additional equipment advertised at the time of introduction consisted of the HRO-P panel mentioned above for rack mounting, the RFSH speaker and panel, the RR relay rack, the LRR table mount light rack, and the E and F coils. The power supplies included the "Doghouse" style model 5897 along with the rack mount GRSPU. The 5897 was recommended, but the 5880, designed for the FB-7 could be used. The GRSPU was a special version for the HRO, providing 2.5 volts for the filaments as opposed to the standard version which provided 6 volts for the filaments in the AGS. This power supply is marked GRSPU for HRO. There were also available dual power supply models identified as GRDPU. These were apparently available with a selection of filament voltages, 2.5 and 2.5, 2.5 and 6.3, and 6.3 and 6.3. Shortly after this a rack mount coil holder panel was available. The early coil holders were available in two sizes, one that held 5 coils and one that held 10 coils. The 10 coil version is extremely rare, and may have been available only by special order. The 5 coil version had a simple round knob, whereas the 10 coil version had a lock as seen on the later military coil holders, such as the RAS coil holder and on the SPC.

In December of 1936 the SPC was advertised. This system allowed reducing the rack height of the HRO rack mount system by several inches as compared to the separate coil holder, power supply, and speaker. The original SPC advertisements showed a speaker without a grille and using a simple knob without locking for the five coil holder, and a power supply. The later SPCs included a speaker with grille and National cloisonné, a rack to hold five coils with a lock mechanism and a power supply mounted on the coil rack and behind the speaker. The National advertisements also indicated that the original power supply, coil holder, and power supply separates were available.

The third model rack mount HRO appears just like the second from the outside. A quick look inside will show that the round IF cans and black chassis have given way to the square IF cans and gray chassis. The fourth model is very difficult to differentiate other than having the HRO tag, the best being to look at the serial number.

The HRO Junior was introduced in February, 1936 to allow the purchase of HRO type performance at a reasonable price. In order to lower the price, as mentioned above, the S-Meter and Crystal Filter have been removed and the single coil that is provided has only the general coverage mode. The wider selectivity of the system without the crystal filter was actually almost required by the faster tuning of the general coverage only coil, and these receivers work quite well. Most users apparently bought the A and C coils sets. The A module giving 20 and 10 meters, the C giving 80 and 40 meters. These coils were not precisely tuned to the receiver with which they were shipped. Instead they were calibrated to a standard receiver. The front of the coil for the Junior has only a single frame located in the middle of the front plate. This contains the curve for the general coverage position of the coil in use. These coils do not include the shorting screws, but do share the 3/16" aluminum front used on the original HRO coils. All Juniors have tabs in front of the tuning capacitors which were bent to allow calibration. These can be seen by looking down just behind the front panel. The first Junior we have seen was serial number L63.

The 3rd model Junior has the gray chassis and square IF shield cans. Otherwise it is closely similar to the 2nd model, maintaining the tabs in front of the tuning capacitors.

There may have been some 4th model Juniors built, it would be difficult to determine which units are of this model without looking at the serial number. By this time the military personnel, primarily the US Navy, Coast Guard, and Great Britain, had assessed the HRO and the major thrust was in the military market.

HROs in World War II

There were at least a few rack mount HRO-Junior type receivers built by National, due to the lack of information on military versions it is possible that these may primarily be military produced after 1941, but there is at least one rack mount Junior type receiver that was made for American Airlines. We have example HRO Juniors in both rack and table mount with tags that identify them as HRO-M.

We should note that in the late 1940s National made an offer to realign and add an S-Meter to the receivers of those people who owned units without them. The cost was low (about $10), so many of the HRO Juniors we see now have S-Meters that were factory installed.

The military receivers were listed in one of the National full page ads in February of 1946. These include the HRO-M, and HRO-MX. These apparently had glass tubes and used the 456 kilohertz IF frequency. National also mentions the HRO-5 and HRO-W as having metal tubes and using the 456 kilohertz IF frequency. They also state that there were three receivers built strictly to US Navy requirements, although they are similar to the HRO. These include the RBJ, RAW, and RAS. These all had glass tubes, with the RAS using an IF frequency of 175 kilohertz, whereas the other two used the 456 kilohertz IF.

By September of 1939 Great Britain and France were at war with Germany and Italy. An interesting study can be made of cryptanalysis if this period. The British realized that the Marconi receivers that were available were too few in number and did not have the quality needed to provide intercept and communications receivers. They purchased receivers from several manufacturers including RCA, National, Hallicrafters, and Hammarlund. Undoubtedly the primary British receiver for the period of the entire war was the National HRO. Approximately 10,000 of these receivers were used by the British in intercept operation, diplomatic communications, aboard ships and at shore stations as well as for clandestine use. An estimated 1,000 standard HROs were purchased by Great Britain prior to the production of the military receivers.

The first military units were supplied to the US Coast Guard and Great Britain. The ones we have seen issued to the Coast Guard are standard HRO-Seniors with the marking "R-105". The British could not buy receivers from the US at the beginning of the war due to our neutrality. Many of the British personnel stationed in the US, such as those in the diplomatic service were asked to purchase HROs from the same stores used by US amateurs. These were then shipped to Great Britain. In Great Britain these units were designated as R106. One of the commercial HROs found at Bletchley Park has the serial number 259J near the antenna connector. When the US adopted "Cash and Carry" and later "Lend Lease", Great Britain contracted with National to buy large numbers of military receivers being split between the HRO-M and HRO-MX (R106), HRO-5 (R106 Mk II) and HRO-W (R106 Mk III). We estimate that there were probably about 3,000 HRO-M, about 3,500 HRO-MX and about 2500 HRO-5. There were very few of the HRO-W delivered prior to the end of World War II.

The HRO-M was also designated in some military manuals as the HRO-MTM for table mount, or HRO-MRR for relay rack mount. These were quite similar to the HRO Senior 4th model, but provided with general coverage only coils and also included special tags displaying the order number and construction information along with a few internal modifications, such as adding transformers to remove the possibility of having B+ on the headphones. The serial numbers on most of the HRO-M series were located behind the S-Meter gain adjust resistor, and the selectivity control was a rectangular knob instead of the round knob and pointer seen on some of the commercial HROs in the UK. Serial numbers for the HRO-M appear to be similar to those on the early HROs with a letter followed by a series of numbers. These numbers are normally higher than those seen with the original HRO, examples from Bletchley Park's communications museum being J457, N820, and E844. Most of the HRO-M and later military receivers were supplied with a set of 4 or 5 coils, although a minority had all 9 coils. For military use the band spread ability for the amateur bands was not needed, however the coils did continue having two frames, one with the calibration curve, and one with the ground celluloid cover to pencil in favorite frequencies. The front panel of these coils changes from 3/16" aluminum to .093" steel, due to the shortage of aluminum. The rack version is very similar to the rack mount HRO Senior. But included those changes required by the purchasing entity. The plug in IF filter crystals were last seen in these models. A few HRO-M units have been found with Bliley manufactured crystals. These are either replacements, or National may have purchased these units to make up for an internal manufacturing shortfall. According to a statement made by National just after the war the S-Meters on these units depended on what National had in stock. Some of the receivers found in Great Britain have 0-1 milliampere meters, some have the standard backlit meters, most had white unlighted meters.

The HRO-MX was the next major model produced by National. It appears that the HRO-MX designation was for the rack model, the table model being designated as HRO-MXTM. These units had a reworked crystal filter with the sealed crystal mounted inside the phasing enclosure. They are generally found with the 6-volt tubes, as are the HRO-M receivers. Other than the change in the crystal filter, and the adoption of more JAN (Joint Army Navy) parts these are very similar to the HRO-M. The serial numbers for these receivers consist again of a letter followed by 1 to 3 numbers with examples being F42, B93, D519, and E429. It is probable that some of the HRO-MX were Junior models, without the S-Meter and Crystal Filter. Since the crystal filter was the primary difference between the M and MX, it is difficult to determine if a military Junior is "Descended" from the M or MX. All of the Junior styles have the tabs on the tuning capacitors.

The HRO-5 was the next version of the HRO produced by National. The HRO-5 was modified to metal vacuum tubes with octal bases with the exception of the final audio amplifier that was a glass 6V6G. These vacuum tubes included four 6K7s, three 6J7s, a 6SQ7 and the 6V6G. The 6K7s were used as RF and IF amplifiers, the 6J7s were used as the HFO, BFO, and Mixer, the 6SQ7 was a dual diode-triode used for the Detector and AVC rectifier as well as the first Audio Amplifier. These tubes were more rugged and external metal shields required by the glass tubes are eliminated. Often the earlier versions with glass tubes when found have missing shields; this may allow feedback and reduce stability. The front panel of these receivers has an added bright metal ring around the AVC switch and the B+ switch which is silk-screened with the switch position and use. They also introduce the flanged knob with skirt for the selectivity control, giving a more pleasant appearance. The National tags usually indicate HRO-5T for table models or HRO-5R for rack models. The coils for the HRO-5 include both coils similar to those provided for the HRO-M and MX as described above, plus some coils with a silk screened bright panel on the front of the coil with the frequency to dial reading curve and the logging table. These are often referred to as "Silver Front Coils". As noted above, these were typically tagged as "R-106 Mark II". In conversation with most people in the United Kingdom you will find that if they acknowledge any difference in HRO models, they will normally mention the "Glass Valve HRO" and the "Metal Valve HRO".

The HRO-W was the last HRO model produced by National for military use by the U.K. These receivers were produced on contract near the end of World War II and are quite similar internally to the HRO5T. The primary difference on the units I have seen is that the S-Meters are typically the ones calibrated from 0-1 ma. These came with modified power supplies that were rectangular, not the "dog house" shaped supplies provided with earlier units and were "tropicalized". The production of these receivers began in mid 1945 and most were never shipped to the U.K. but sold to the public after the end of World War II. The dates of the application of fungicide were usually in the July to September 1945 range. After the war one of the National full-page technical advertisements in QST discusses the suitability of these units for radio amateurs. Most of these have six of the "Silver Front" coils as seen with some of the HRO-5s, and some have all nine coils. The 6 coils provide coverage from 480 kHz. to 30 MHz, the additional coils covering 50 kHz. to 430 kHz. The square power supplies are designated "697-W" and are of very high quality. The electrolytic capacitor is a plug in unit, the covers are attached with quarter turn fasteners and they run quite a bit cooler than the doghouse supplies.

HROs by Another Name

There were three special versions of the HRO built for the US Navy as mentioned above. National mentions that they appear similar to the HRO, but are built to US Navy specifications. The most common of these is the RAS. The RAS was built for the US Navy, but also widely used by the Marines and in some air communication services. The navy required that the receiver be able to cover the frequencies between 400 and 500 kHz. The standard HRO could not do this as the IF frequency was 456 kHz. This required that the IF be changed to 175 kHz, allowing full coverage from about 200 kHz. up to 30 MHz. The navy decided that the utility of a crystal filter and S-Meter were limited in a monitor receiver likely to be exposed to vibration so they had these units removed from the HRO making the RAS more similar to the HRO Junior. The low frequency IF would have given higher selectivity than required by the Navy so a 1 megohm swamping resistor was added to the secondary of the IF filters, and the secondary of the transformer was tapped down. The navy also required that there be a headphone coupling transformer installed, and that the coils be held in place in the receiver by a locking system. The navy further required that the extra coils be held in the coil holder so that the front panel of the coil was visible to the operator. This caused the RAS coil holder to be much taller than the standard coil holders produced by National. The RAS came with 7 coils that are not interchangeable with the HRO coils. These are differentiated from the standard coils by the two metal plates on the front of the coil that are engaged with the locking system, and by the stampings on the coil insulators. As described above the standard coil insulators were stamped with the coils designator and the position of the coil in the pack. On the RAS the leftmost coil is signified by 5 and the rightmost by 8. Therefore the highest frequency coil pack that covered 14 to 30 MHz. would have the stampings as follow: A5, leftmost 1st RF, A6, 2nd RF, A7, mixer, and A8, rightmost HF oscillator. It is possible to retune the higher frequency RAS coils to work in the HRO since the difference in the HFO frequency (279 KHz.) is small compared to the actual frequency (from 14 to 30 MHz. on the highest frequency coil). These receivers were used to monitor the 500 kHz. emergency frequency in Coast Guard stations until well into the 1970s. One option for the RAS was a noise limiter, this replaces one of the IF amplifier tubes, and is bolted in place and appears to be wired into the circuit, so is not a plug in option, but was installed at a service depot. These noise blankers were built by Mission Bell Radio Manufacturing Co. Inc. and are designated CKB50142. The standard setup of the RAS system required a 31" rack and consisted of the receiver, a power supply, the coil holder, and a speaker. The major difference in the circuitry of the RAS was that a 6F8 dual triode was used as the second detector and AGC amplifier. This replaced the dual diode/triode or dual diode/pentode found in the HRO. Due to the change in AF gain caused by elimination of the 1st audio amplifier and substitution of the triode detector for the diode detector, the final audio tube was changed to the 6V6G, which like the 6F8 used an octal socket. The RAS units also appear to be the point at which the rack mounted National HRO type receivers changed from having a full case with hinged lid to a dust cover. The dust cover was held in place by threaded fasteners extending through the panel and brass thumb nuts on the front and standard screws from the rear.

The second mentioned Navy model was the RBJ, also produced during World War II and not often seen. This was a receiver similar to the RAS, but using the standard 456 kHz. IF. Since the standard HRO had 9 available coils, the RBJ versions were stamped as 00, 0, and 1 through 7. With the 00 coil covering 50 to 100 kHz. and the 0 coil covering 100 to 200 kHz. This receiver is supposed to have the same tube line up as the HRO, and does have the coil locking mechanisms as used on the RAS. These appear to be quite rare.

The last variant for the Navy was the RAW. These used the 456 kilohertz IF and glass tubes as mentioned above. The only information we can find indicates that it is the same as the RBJ. These must also be quite rare.

Other Modifications of the HRO

One very rare variant was produced as a panoramic receiver for signals intelligence. This system used plug in coils like the HRO, but the characteristic tuning knob was gone. In its place there was a motor that was used to rotate the special tuning capacitor through its full range and back during each rotation of the motor. The output was fed to a panoramic display. This unit also had a built in power supply.

In addition the British modified the HRO-M and HRO-MX to better suit the requirement for a direction finding receiver. These units had a panel over the normal antenna inputs to accept inputs from a direction finding antenna (usually Babcock), and a sense antenna. Some of these models had a front panel switch to allow standard, DF and Sense antenna switching. The coils for these receivers were not supposed to be usable in the standard HRO, probably because they were modified for a low impedance antenna input.

The Signal Corp produced the R-140 by modifying the HRO-M, MX, and 5 for use in a frequency measuring system. The modifications included removing the B+ from the speaker terminals, and allowing an audio signal to be introduced to the second detector plate. In order to remove the B+ from the speaker terminals a resistor was placed between the lines normally located on the speaker terminals and one of the speaker terminals was grounded and the other connected to the audio amplifier plate by a .1 mfd capacitor. To provide a signal to the detector, the leads to the B+ terminals on the rear of the receiver were removed and tied together. One of these terminals was connected to ground and the other to the plate of the detector.

HROs During the 1945-1949 Era

The HRO5A was produced by National after the end of World War II, supposedly shipping in January of 1946. This was the original HRO5 receiver supplied with amateur band spread coils. Some of the first versions were supplied with the four standard HRO Senior coils covering the frequency range from 1.8 to 30 MHz. Very shortly after this unit was introduced to the civilian market National began producing the HRO5A-1. These are marked as HRO5TA-1 for the table unit and HRO5RA-1 for the rack mount unit. This receiver included a noise limiter, adding 6H6 and 6J5 metal vacuum tubes, but was otherwise very much like the HRO-5A. There were actually three versions of the HRO5TA-1. The first of these versions used the round S-Meter as seen on all the earlier HROs, and used the earlier crystal filter unit introduced on the HRO-MX. The early HRO5TA-1 crystal filter had two variable capacitors, one for the phasing and one for the bandwidth. The bandwidth control was on the top right corner, and the phasing control directly below it. This version has been seen with four standard HRO Senior coils, but more often with the later "Silver Front" coils that have the band spread capability added. These coils had the standard and band spread frequency curves on the bright metal plate. Additional coils were available as options. The later version used a new crystal filter unit with a five-position selectivity switch that eliminated the variable capacitor and instead switched fixed capacitors into the circuit. The phasing control was moved to the upper right corner with the selectivity switch directly under it. The late HRO-5TA1 used a different marking system and serial number series. These serial numbers were impressed into the chassis on the right side of the chassis top, opposite the antenna connectors. The chassis on the earlier models were painted gray, in the later models they are cadmium plated steel. The third version of the HRO5TA-1 was actually an HRO-W modified at National by adding a sub-chassis under the S-Meter which included the noise limiter and the provision of an S-Meter in place of the 0 - 1 ma. unit. From the advertisements it appears that the early version came out in 1945, with the modified HRO-W being produced in late 1945 or early 1946, and the Square S-Meter version being introduced in mid 1946.

There is some information that indicates National produced an HRO-6 receiver, which was similar to the HRO-5A1 with a different limiter circuit.

The HRO-7 was the first departure in styling from the black wrinkle finished rectangular box HRO that had been in production with minor changes for over 10 years. This HRO was built in a gray enclosure with rounded corners. We have seen a couple of HRO-7s with a dark blue finish, it is not yet determined if this was available from the factory, or if these were repainted professionally, these are seen with matching blue coils, speakers, and power supplies. The main tuning knob was changed from black to a matched gray knob with an inserted bright ring. The circuitry was changed by the incorporation of a voltage regulator and triode high frequency oscillator and swapping the location of the noise amplifier and first audio amplifier. The front of the coils now had a slide rule type scale, spreading the frequency curve over a large portion of the coil front. The handles were gone from the front panel and the coil, replaced by two coil locks that eased the insertion and removal of the coil pack. National introduced the Select O Ject, basically a high performance audio filter at about this time, and the HRO-7 had a plug in socket for the accessory. Another accessory, actually a complete receiver, was added to the product line at this time. The HFS receiver could be used as a stand-alone super regenerative receiver from 27 MHz. to 250 MHz, or it could be used as a converter to 10.7 MHz. For the amateur interested in VHF and UHF, the HFS used as a converter in front of an HRO was a very good choice.

The original HFO used an electron-coupled oscillator as the high frequency oscillator. If carefully designed this oscillator is relatively insensitive to voltage variations and load variations. In order to improve the performance of the HRO design the now available voltage regulator vacuum tube was used to reduce the voltage variations, and the triode oscillator was fitted with a frequency compensating capacitor. Many of the early HRO-7s were built on HRO5A-1 chassis. The factory simply riveted a plate over the opening for the original HFO and put the miniature sockets on the new plate. An original factory modified HFO should have the tube types stamped by the tubes and should be riveted. The new tubes were of the miniature type, a 0A2 VR tube and a 6C4 oscillator.

The HRO-7 was available in either table or rack mount versions. The rack mount version (HRO-7R) still had the 3/16" thick aluminum front panel and was finished in black wrinkle paint. The matching speakers for the table model were usually 8" in diameter and included a newly designed cabinet and a five-pin speaker plug. This plug change eliminated a problem with earlier HROs of damaging the audio output tube by removing the plate voltage while leaving the screen voltage on this tube. Many people don't realize that while running the earlier HROs without a speaker, the audio output tube should either be removed, or the output terminals should be jumpered. A new coil became available about this time to allow band spread only coverage of the new 15-meter amateur band; this was designated as the AC coil. National also produced these coils for earlier receivers, for example the HRO5A-1. These coils for the earlier receivers included the handles, but used the new front scales. Additional coils available included the AA coil that covered the 27 to 30 MHz range, and the AB coil that covered from 27 to 35 MHz. The HRO-7 was the first National HRO to allow reception of NBFM signals by the addition of the NFM-07 adapter instead of tuning off the signal and using "slope detection". The HRO-7 allowed an external audio input and also had an available tilt base.

National appears to have at least made manuals for a model HRO-12S. This appears to be similar to the circuit used in the RAS, including the 175 kHz IF frequency and the lack of a crystal filter. It did have an S-Meter and came with a 12 volts D.C. power supply. It should have come with A, B, C, D and G (230 to 515 kHz.) coils. It was also supplied with a speaker and shock mounts. This may have been intended to replace the RAS units that were in use well after this time. It is thought that these were sold primarily to Canada.

The HRO-50 and HRO-60

The HRO-50 followed the HRO-7 by 1949. This model added an internal power supply and added push pull audio output. It came with four coils, the AA, band spread coil for 11 and 10 meters (27 to 30 MHz.) along with the standard B, C, and D coils. The HRO could be used as a phonograph amplifier, and the available 10" table mount speaker and 5 or 6 watts of clean audio from the two 6V6Gs did a pretty good job. The primary change in this model was the addition of a direct reading frequency scale. All earlier HROs required that the operator read the setting of the dial and look at a curve to estimate the frequency. Estimating the frequency to within 25 kHz. was not really possible on some frequencies using the earlier system. Now the trouble of reading the dial and converting by use of the frequency curve was eliminated. The HRO-50 used the same IF system as used in the HRO-5 and HRO-7. The vacuum tubes in the front end of the HRO-50 were changed, however. The original 6D6s, or 6K7s were replaced with 6BA6s, and the mixer was changed to a 6BE6. Other minor changes were made, and there were added sockets for a crystal calibrator and Narrow Band FM adapter. At the end of the run, the AA coil was replaced by the A coil.

By mid 1951 the HRO-50-1 replaced the HRO-50. This receiver carried the conversion to modern tubes a bit further, replacing the BFO and IF amplifiers. The more important change was in the IF circuit. This system now included dual tuned transformers, giving greatly improved skirt selectivity and three stages of IF amplification using the 6SG7 vacuum tube. The twelve tuned circuits gave a pass band acceptable for SSB, although the detector and AVC required special procedures for listening to SSB. This was also available in two models, one for rack mount, the HRO-50R-1, and the standard tabletop unit, the HRO-50T-1. The rack version was the last of the black wrinkle finished HROs. These were often supplied to the US Coast Guard as the R-274. There were also a few HRO-50R1s with gray finish. The coils available for the HRO-50 and later HRO-60 were the four standard A, B, C, and D, the E, which covered the upper AM broadcast and 160 meter bands, the F that covered the lower AM broadcast band, the G, H, and J, low frequency coils covering 50 kHz. through 460 kHz. and four different high frequency coils. These were the AA, which covered 27 to 30 MHz, the AB which covered from 27 to 35 MHz, the AC which covered the 15 meter band only, and the AD coil which covered the 6 meter amateur band. The AD coil is very rare for the HRO-50 and may have been a special order only coil. Available accessories include the NBFM adapter, a crystal marker with output at 100 kHz., and 1.00 MHz, the Select-O-Ject, an external DC power supply for 6 volt input, an 8" rack mount speaker, a combination rack mount speaker and coil holder and 8" and 10" table speakers. One interesting note on the crystal marker oscillator was that the 100 kHz. output could be adjusted to WWV, whereas the 1.00 MHz. output was not adjustable. The speaker for the HRO-50 was the same as that used for the NC-173. There were National advertisements that stated that they were willing to build coils to cover any frequency as long as the requirement was large enough to justify the run.

In 1952 the HRO-60, last of the vacuum tube equipped HROs was produced. The HRO-60 was basically the same as the HRO-50-1 with the exception that above 7 MHz. the receiver was dual conversion. This means that the coils above 7 MHz. (A, B, AA, AB, AC, and AD) were not interchangeable with the earlier coils. The HRO-60 coils can not be used in earlier receivers, but it is possible to use HRO-50 coils in the HRO-60 by bypassing the first converter. This receiver also regulated the current on the HFO and Mixer filaments. The 4H-4C vacuum tube used for the filament regulation is now quite rare, and many owners are using the 6V6 indicated in a National Service Bulletin as a substitute. These receivers were available from National until 1964. The rack mount version of the HRO-60, the HRO-60R came with a front panel painted the same gray as the tabletop model. Accessories for the HRO-60 were the same as those for the HRO-50.

The HRO Goes Solid-State

The HRO-60 remained in production until 1964 as the top receiver of the National line. It was replaced by the solid state HRO-500, available in a portable version with a case, internal speaker and rechargeable Ni-Cad batteries designated HRO-500P or as a table model. There was also a rack mount adapter for the HRO-500. This new receiver covered from 0 to 30 MHz. without plug in coils, and with constant 1 kHz. divisions on the familiar micrometer dial. It used a phase locked loop, which was locked to the harmonics of a crystal oscillator and a linear VFO that covered 500 kHz. National wrote several one page advertisements for the 1964 QST issues describing the operation of the new phase locked loop. The HRO dial included an internal reduction system allowing either fast tuning (10 turns for 500 kHz.) or using an added vernier knob on the main tuning knob, about 30 turns for the full range of the VFO. Some of the main tuning knobs have a spinner attached and some do not. Selectivity for the HRO-500 was provided by cup core coils operating at the new 262 KHz IF frequency. These were variable from 500 Hz. to 6 kHz. in bandwidth. This receiver cause quite a sensation when it was introduced and is still popular today, though the number produced was not very great due to its cost. Many HRO-500s were used by communications companies. For example, Tropical Radio and Telegraph had several of these at each operating position. For very low frequency use National produced the LF-10 that included a tuner and amplifier for the very low frequencies as well as the ability to add an audio filter.

The HRO-500 was almost as large as the original HRO, and the weight of 26 pounds was not far away from that of the original. The semiconductors used were a combination of germanium and silicon transistors. Performance, compared to modern solid state receivers was fairly poor, especially in strong signal conditions. Compared to other receivers of similar vintage this unit shows up quite well. The frequency read out, pre selector and stability were very good.

The HRO-600 was the last of the HROs produced by National. These fully transistorized receivers are extremely rare today. We will defer to articles written for the AWA concerning this receiver, not having had the opportunity to access one of these units.

The HRO and its Special Role in Signals Intercept During World War II

The HRO had a long and illustrious career, with the greatest amount of use in World War II occurring in Great Britain. The estimate most normally quoted in references to signals intelligence during World War II was that signals intelligence and cryptanalysis shortened World War II by approximately 2 years. The scholarly book "Action This Day" acknowledges that without the availability of the excellent American receivers, primarily the HRO, the cryptanalysts and traffic analysts would have not been able to provide this intelligence. Two books that are highly recommended are "England Needs You" and "The Secret Wireless War". The first book was written by Joan Nichols, a young lady who became an intercept operator at Beaumanor during World War II, and includes a lot of information concerning the daily life of the operators along with the problems they faced. The second book by Geoffrey Pidgeon, describes in detail the equipment used for communication with the clandestine agents, at the intercept stations, and for distribution of the Enigma intelligence. Both of these books reference David White, the curator for the diplomatic wireless station and "Y" station at Bletchley Park. David has been kind enough to provide a large amount of information concerning the HRO at war. David worked for the Diplomatic Wireless Service after World War II until his retirement in 1992. The HROs continued in operation in the Diplomatic Wireless Service until 1957, and at some of the embassies until 1959. These were replaced by the Eddystone 730 which the operators thought were inferior to the HRO.

Bletchley has a collection of 20 or so HROs of which David keeps a dozen or so fully operational. Normally there are 5 or 6 HROs in the Diplomatic Wireless Hut and 6 more in the "Y" station. Most of the HROs used in Great Britain did not come with National power supplies or speakers. These were provided by local builders in the United Kingdom. The speakers are simple metal enclosures with perforated front grille. The power supplies were much more substantial than the "Doghouse" supplies normally seen with the HRO; these were produced by two different companies. These power supplies are marked G5NI, Birmingham, and PSEI Limited. With the limited shipping space available and the requirement for the 240 volt "mains" power, the local construction of these accessories was a very good option.

Early in the war the government of Great Britain contacted many of the amateurs licensed within the country to watch for clandestine transmissions from inside the country. These were called Voluntary Interceptors or VI; later in 1939 they were recognized as the Radio Security Service, or RSS. The VI and RSS operator had to make do with what was available until the HROs arrived in large enough quantity for this vital work. After the war was over these surplus receivers were sold to the VI/RSS operators for 5 pounds each. Some VI operators recalled using two tube regenerative receivers until being issued an HRO. Apparently many of the HROs purchased for the VI/RSS operators were commercial units with nine coils including the standard band spread coils.

The largest use of receivers in Great Britain during World War II were the intercept stations which monitored German and Italian traffic outside Great Britain. There were a few intercept stations that used the Marconi and Eddystone receivers, but most used HROs. Examples include the navy stations at Scarborough which used 80 HROs, and Flowerdown, which used 40 of them. Hanslope used 66 HROs in 32 listening positions, 30 having two HROs each and two positions having three HROs each. Forfar used 60 HROs, Whaddon had 30, Nash and Weald each had 8 units Thurso had 14, Gilnahirk had 12, and St. Erth had 12. We should also note that Nash and Weald used DC power supplies for the receivers and had a diesel generator to charge the batteries in order to eliminate power line noise; the other stations used "Mains" power. There were some listening operations in India, Ceylon, and Australia during WWII, some of these used HROs and some used the AR88 from RCA. The first listening station at Bletchley (Station X) used a pair of RCA AR-77s. One of the original receivers has been returned to the small room in the attic of the mansion.

The listening conditions for these intercept operators were normally poor. The German army did transmit noise or music very near the frequency of the Wireless Telegraphy (WT) transmissions in order to reduce the probability of intercept outside the intended communications radius. The signals were generally weak and drifting in frequency, and were normally keyed manually. The difference in manual keying or "Fist" did allow the intercept operators to follow the transmitting station and operator when they changed call signs and frequencies, sometimes the call signs were changed daily and the transmitting frequencies changed even more often. The transmissions were coded groups and had to be copied perfectly. At times more than one operator would be assigned to insure perfect copy. If the operator missed part of the transmission they would write in probable letters with a question mark, and make dashes to indicate the number of letters missed. This allowed the combination of two or more partial intercepts to often make a perfect one.

A typical intercept station such as Beaumanor might have several receiving set huts, or "Set Rooms", the typical set room would look something like a school room with the operators facing a control operator who would be seated at the front of the room facing the operators. There would be a central aisle with operators and sets on each side of the aisle. The control operator would assign most sets to guard particular frequencies and would connect those sets to antennas which were deemed preferable for that frequency and distance. The antennas were primarily V-Beams and Rhombics, the heights being 50 to 75 feet. There would be multiple antennas for each band of frequencies pointed toward different locations from Africa to the Arctic. Each of these antennas was fed to a wide band amplifier which distributed the signal to as many as 20 HROs. These amplifiers were designed by the General Post Office and used type 807 vacuum tubes as class A RF amplifiers. Once the operator is assigned a frequency by the control operator they would then use the calibration book for their receiver, or a "cheat sheet" attached to the front of the receiver and marked with something like "remove under threat of death" to tune the correct frequency. The operator might then wait for hours until some action occurred. Unless they were quite familiar with the station, they would then have to determine if the station was part of a network and the type of network, Star or Circle. They would copy the preamble at the top of the "Red Sheet" and the coded groups below. The Circle network had everyone transmit on the same frequency; the Star had two frequencies, one for the control station to transmit and one for everyone else. The Star network would either require a dual receiver setup, or two operators. We should also note that for some coded messages such as Enigma, the cryptanalysis required approximately 100 messages per day, when following the dozens of codes being used you can see that the volume of coded messages intercepted was huge at times several hundred thousand coded groups per day. Several good books concerning the breaking of the German and Italian codes are available, including Winterbotham's "The Ultra Secret" the first to break the secret of the success of cryptanalysis in Great Britain during WW II, and David Kahn's "The Codebreakers". I recommend a visit to the Bletchley Park internet site as well as Tony Sale's Codes and Ciphers internet site . If you get a chance to visit the United Kingdom, it is well worth the time and the 10 Pound entry fee to visit Bletchley Park. Expect to spend a full Saturday or Sunday there, and you will wish for additional time.

There were thousands of W-T intercept operators in the United Kingdom during World War II. Most of these were young women who knew nothing about radio or W-T until after induction in their particular branch of the service. The stress on the intercept operator was severe. At times a stressed operator may be taken off intercept service and assigned search operations where they would tune over a band of frequencies looking for new stations; this was thought less stressful than intercept operation. In the worst case the operator would be reassigned to radio direction finding or other duties.

The operators mention a couple of problems, one is the possibility of contacting the uncovered B+ terminals on the back of the receiver behind you, the other was that the power supplies were located on the floor, and periodically someone would accidentally turn off the power supply with their foot. Joan mentions that the men who were operators never accidentally turned off the supply, so "what did they do with their feet?"

With an idea of the operating conditions you can see why the HRO was referred to as "the Remarkable HRO, "the Cadillac of Receivers", and "the Best Receiver Money Could Buy", by many of the operators quoted in "The Secret Wireless War". The stability and excellent capability of returning to a particular frequency were better than any other receiver available in the United Kingdom until the early 1960s.

In order to distribute "Ultra" intelligence Special Liaison Units and Special Communication Units were formed. The Special Communications Units were equipped with a standard British Mark III transmitter and an HRO. These were mounted in various vehicles until the Dodge ambulance chassis (Power Wagon) was found to be the best available unit. The Mark III transmitter was a 2 tube crystal controlled unit of about 30 watts output power. These units were assigned to the various commanders and were required to keep in contact with the home units in Great Britain in order to pass new intelligence to the commanders as quickly as possible. These units were very successful, and ingenious in finding locations for their wire antennas which allowed reliable communications while having to move sometimes up to 3 times a day. One illustration of their success occurred at the end of the war when there were several communications systems located within one of the stadiums in Germany. The SCU offered to send messages for a General whose own communications unit had not been successful. The General visited the SCU and asked to see their transmitter, when they showed him the 30 watt Mark III he became enraged as he could not understand how they could stay in communications when his own unit with 500 watt transmitters could not get his messages through. Several of the SCU operators commented that their HROs were perfectly reliable. Some of the operators reported not having to do any maintenance; even replacing a vacuum tube during the period spent traveling through Europe between D-Day and VE-Day.

HROs in the U.S. Military During WWII

In the United States the HRO and its variants were also widely used during World War II, primarily by the Navy, Coast Guard and Marines. These sets include the RAS, and the "reverse lend lease" version of the HRO built by Amalgamated Wireless of Australasia Ltd., the AMR-100 (table mount) and AMR-101 (rack mount). You will find pictures of the control station for the Coast Watchers in the Solomon Islands (call sign KEN) prominently showing an RAS, the AMR-100 can be seen in a late 1945 QST article concerning the USS Apache, a communications ship used by Mc Arthur in the Philippine Islands. You will also see banks of RAS receivers in pictures taken at locations such as Albro Lake as well as other communications and training stations. Although seldom seen in Army and Army Air Corp service we can note that the RAS was used to guard the 500 kHz emergency frequency up through the early 1970s.

At the beginning of WWII you will find references to owners turning in their HROs for use during the period that the military demand for receivers outstripped supply. We have seen several early HROs including HRO Juniors which were built in 1937 or 1938 that have military acceptance markings. We can be fairly sure that at least some of these were privately owned before finding a place to serve during the war years. National supplied a much larger number of NC-100/NC-200 variants than HROs to the US military during WWII.

Modifications in HROs

The wide availability of the HRO, and the fact that it was often "War Surplus", led to these receivers being highly modified. Not many people thought that the HRO they picked up as surplus might be worth twenty times as much as they paid for it if they had not modified it. The modification most seen on the early HROs is the change of the High Frequency Oscillator (6C6 or 6J7) for a miniature 6C4 triode oscillator and the addition of a voltage regulator such as the 0A2, the same change National adopted with the HRO-7. The next most often seen modification is the addition of the noise blanker as adopted by National on the HRO-5A1. Normally this modification consists of the addition of a 6H6 and a 6J5. Typically if you see a 6H6 on an HRO earlier than the 5TA1, then a limiter has been added. Usually there will also be an additional control between the S-Meter and the main tuning dial. The third most popular change was to replace the earlier vacuum tubes with later versions. Depending on when the modification was performed, this might be octal vacuum tubes in place of the 6 pin glass tubes, or miniature vacuum tubes in place of either the octal or 6 pin glass vacuum tubes. Normally the modifications will be in the RF amplifiers, mixer, and oscillator. The miniature tube modifications usually upset the gain distribution in the receiver. Normally when we find one of these receivers we restore it to its original circuit, however the voltage regulated HFO and noise limiters were improvements in these receivers. There are some modifications to the early receivers that were performed by National. One 1st model HRO was returned to the factory and a noise limiter added along with new small knobs, so the Limiter knob would match the rest. It is possible that the change to the voltage regulated HFO was sometimes done at the factory. We see evidence of the factory performing these changes in the HRO-W to HRO-5TA1 and in the HRO-7s that were built on HRO-5TA1 chassis. If the modifications look like those done in these two instances, then we should probably not try to return the radio to original. On the other hand, the miniature tube replacements for the RF amplifiers, etc. should be returned to original.

Reverse Engineered HROs

During and after the period spanned by World War II the HRO received sincere flattery by being copied by the Germans and the Japanese. There are two German copies of the HRO, one made during the war, and one made after the war in East Germany for supply to the Chinese. The Japanese made a copy of the dial and drive mechanism, complete to the emblem on the top and also supplied a number of receivers with a different dial mechanism, but with the general layout and plug in coils introduced by the HRO. There were also three versions made in Allied countries, one in New Zealand, and two in Australia. Probably the best of these was the extremely well made AWA AMR-100 and AMR-101. These and the Kinglsey AR-7 were manufactured in Australia. The Kingsley used a single tube for the HF converter and would have suffered from pulling of the HFO. The Collier and Beale 941 SWB built in New Zealand is a very close copy of the HRO, actually using imported dial mechanisms manufactured by National. The information available on these copies indicated that the Australian copies operated very well, and the German copies met the specs of the original HRO, but were easier to overload. There is an operational Japanese HRO at Bletchley Park, but we have not had time to compare it side by side with the original HROs. We have also been apprised of a pre-war Russian copy of the HRO designated as the 45PK-1 and a radio direction finding version 45PK-3. These were used at Kronstadt, the naval base of the Baltic fleet and at Oranienbaum, a suburb of Leningrad, the location of the Russian armies that were surrounded. Apparently none of these receivers have survived. They appear to use the grouped plug-in coil bank, but a different dial mechanism.

HRO Power Supplies

The power supplies available for the National receivers are somewhat difficult to follow. Prior to the AGS National built the SW-3, the AC version of this receiver requiring 2.5 volts for 3 vacuum tubes and 180 volts at about 40 ma. This was provided by the 5880AB. This power supply is really too light for the FB-7, even if the final audio amplifier is removed, National ads indicate that it can be used with the FB-7, though it is not recommended. There were two correct power supplies for the FB-7, the 5887AB provided the 2.5 volt filament current for the 7 tube FB-7 along with about 180 volts at something like 60 ma. This allows operation of the FB-7 at lower listening levels where "room filling loudspeaker sound" is not required. The second power supply provided for the FB-7 was the 5897AB. This produced the 2.5 volt filament voltage at a current level which could provide for the 9 vacuum tubes in the HRO, and provided about 240 volts at 70 ma. to provide full output from the audio output tubes in the HRO or FB-7. For the AGS, National supplied the 5886AB which was also supplied with the National SRR receiver. This power supply provided filament voltage at 6.3 volts, adequate for 9 vacuum tubes, along with the 240 volts at 70 ma. National also supplied a single rack mount supply designated GRSPU, and available with either 2.5 or 6.3 volt filaments, and adequate output for several watts from the audio amplifier. These may have been designated as GRSPU-2 and GRSPU-6, although we have seen one GRSPU marked "for HRO". The second National rack mount supply was capable of providing power for two receivers, basically two GRSPU internal units in a single rack panel. These units were designated GRDPU, and were available for 6.3 or 2.5 volt filaments, or a mixture of one of each. By late 1940 National introduced the 697 power supply; this provided the proper current for an HRO with 6.3 volt filaments. We have seen a few rack mount power supplies marked with the type 697. The power supplies mentioned above with the exception of the rack power supplies were built into a small enclosure normally described as a "Doghouse". In early 1945, National produced the 697-W, a heavier duty external supply with a plug in electrolytic capacitor, and was treated with fungicide. These were usually treated with fungicide between early 1945 and September of 1945. The AC supplies described above used capacitor input dual section filters following a transformer and type 80 full wave rectifier. The filter system normally used a triple section 8 mfd. Electrolytic capacitor and a choke of approximately 10 henrys. One of the capacitor sections was connected across the output from the rectifier, this current was passed through the choke, and the other two sections of the capacitor were connected in parallel across the output. In the 697 supply the rectifier is normally changed to a type 5Y3. In addition the early supplies included a pair of small capacitors connected between the output and the AC input to reduce hum. They may also contain a variable resistor with the tap grounded and the two ends connected to the filament lines. The HRO moved this resistor to the receiver chassis, making it more effective as the power supply chassis is not normally connected to the receiver chassis. National also announced a vibrator type external DC supply for the HRO in November of 1939 eventually consisting of the 686 for 6 volt input and 1286 for 12 volt input. This fed the battery voltage to the filaments and used a vibrator pack to provide approximately 165 volts at 45 ma. The vibrator pack was normally supplied by Mallory. The full wave rectifiers were 6X5 for the model 686, and the 0Z4 for use in either unit.


If you are lucky enough to own an HRO receiver, perhaps this article will allow you to see it in a different light. It was a landmark in the development of the superhetrodyne receiver, and a great contributor to the Allied war effort during World War II. Many intercept operators will state that it was the best intercept receiver available during World War II, and for quite some time after the end of the war. It was stable, rugged, simple to operate, and the dial mechanism allowed resetting to a frequency much better than any other available receiver.


Repeating after Charles Fisher, I can't believe that there are no mistakes in this paper. I take full credit for all errors, and commend all the people who provided help with all the facts. Please feel free to contact me concerning errors, additional facts, and particularly observations which do not agree with this paper.

I want to thank a large number of people for providing information for this article. David White has provided a wealth of information and guidance concerning the military use of the HRO in the United Kingdom as well as the view of an intercept and point to point W-T operator using the HRO. He not only helped me, but also was instrumental in getting Joan Nichols and Geoffrey Pidgeon to write their excellent books. I also appreciate the help of my friends at the RAF museum in Henlow including Dave Thompson, Vic and Mike who help keep the museum in great condition. Colin Mackinnon for guidance on the HRO copies in his collection in Australia. Closer to home, John Orohood, Greg Gore, Rob West, Bill Fizette, and Henry Hand at the Western Historic Radio Museum have all produced excellent information and guidance. Most of all, I must not forget my mentor on all things electronic Max Busick (SK), W5GJ until his passing, and his son Don Busick K5AAD, last of the line.

e-mail Barry Williams

Back to Radio Bay.